Next-Generation Thermal/Environmental Barrier Coatings for

Next-Generation ThermalEnvironmental Barrier Coatings for Ceramic-Matrix

Composites

Dissertation

Presented in Partial Fulfillment of the Requirements of the Degree Doctor of Philosophy

in the

Graduate School of Brown University

By

Laura Ruth Turcer MS

Graduate Program Engineering

Brown University

2020

Dissertation Committee

Dr Nitin P Padture (Advisor)

Dr Reid F Cooper

Dr Brian W Sheldon

ii

copy Copyright 2020 by Laura R Turcer

iii

This dissertation by Laura R Turcer is accepted in its present form by the School of Engineering

as satisfying the dissertation requirement of Doctor of Philosophy

Date ________________________ _______________________________________

Nitin P Padture Advisor

Recommended to the Graduate Council

Date ________________________ _______________________________________

Reid F Cooper Reader

Date ________________________ _______________________________________

Brian W Sheldon Reader

Approved by the Graduate Council

Date ________________________ _______________________________________

Andrew G Campbell Dean of the Graduate

School

iv

CURRICULUM VITAE

2015 to presenthelliphelliphelliphelliphelliphelliphelliphelliphelliphellipGraduate Research Associate School of Engineering

Brown University

2017helliphelliphelliphelliphelliphelliphelliphelliphelliphellipMS Materials Science and Engineering School of Engineering

Brown University

2014helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBS Materials Science and Engineering

The Ohio State University

2010helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipDublin Scioto High School

1992helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellipBorn Youngstown Ohio

v

PUBLICATIONS

1 LR Turcer NP Padture ldquoRare-earth solid-solution environmental-barrier coating

ceramics for Resistance Against Attack by Molten Calcia-Magnesia-Aluminosilicate

(CMAS) Glassrdquo Journal of Materials Research Invited Submitted

2 LR Turcer NP Padture ldquoTowards thermal environmental barrier coatings (TEBCs)

based on rare-earth pyrosilicate solid-solution ceramicsrdquo Scripta Materialia 154 111-117

(2018) Invited Viewpoint Article

3 LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-

Barrier Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-

Aluminosilicate (CMAS) Glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European

Ceramic Society 38 3905-3913 (2018)



Aluminosilicate (CMAS) Glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the

European Ceramic Society 38 3914-3924 (2018)

These authors contributed equally

vi

DEDICATION

Dedicated to my family

vii

ACKNOWLEDGEMENTS

I would like to thank Professor Nitin Padture my advisor for his support and supervision

His mentorship has helped me grow as a researcher and as an individual I really appreciate how

much he cares about his graduate students He not only focuses on supporting my research goals

but has supported me through my experimentsrsquo successes and failures papers and presentations

Thank you to Professor Reid Cooper for his support and guidance I really enjoyed our

discussions and I am grateful for his encouragement I appreciate Professor Brian Sheldonrsquos

support and advice Both Professors Cooper and Sheldon are wonderful teachers and I am so

grateful I was able to take their classes and that they made time for my defense

My lab mates were also supportive I would first like to thank Professor Amanda (Mandie)

Krause When I first started at Brown University she was concluding work on her PhD Mandie

mentored me in many ways She trained me on how to use lab equipment furnaces CMAS testing

FIB lift-out TEM etc She helped me conceptualize and organize my research She also helped

me select classes to achieve my research goals Overall Mandie made my transition into grad

school a smooth one Hector Garces was also very helpful as I began graduate work He taught me

ceramic processing and XRD and has continued to help me when equipment isnrsquot functioning I

would like to thank Mollie Koval Connor Watts Hadas Sternlicht Anh Tran and Arundhati

Sengupta who all contributed significantly to this project My lab mates Dr Lin Zhang Dr

Yuanyuan Zhou Qizhong Wang Min Chen Srinivas Yadavalli and Zhenghong Dai Dr Christos

Athanasiou and Dr Cristina Ramiacuterez have been supportive I would like to give a special thanks

to Qizhong Wang who helped me talk through problems and checked my math I would like to

thank Yoojin Kim Helena Liu Steven Ahn Selda Buumlyuumlkoumlztuumlrk Juny Cho Nupur Jain Sayan

viii

Samanta Gali Alon Tzenzana Ana Oliveira Ally MacInnis and Cintia J B de Castilho for their

support and friendship

I would like to thank Tony McCormick for his help He taught me how to use the

characterization tools necessary for most of this work and was always friendly and willing to help

I appreciate Indrek Kulaots and Zack Saleeba for their help in DTA analysis I would also like to

thank John Shilko and Brian Corkum for their assistance Much thanks to Peggy Mercurio Cathy

McElroy and Diane Felber for their friendly assistance and administrative expertise Although my

defense will now be held on Zoom I would like to thank Kathy Diorio Beth James Amy Simmons

and Paul Waltz for their assistance navigating arrangements and helping me find a room for my

defense

All of this work would not have been completed without the contributions of Professor

Sanjay Sampath and Dr Eugenio Garcia at the State University of New York at Stony Brook

University I am grateful for their collaboration and ability to produce APS coatings Thanks to

Dr Gopal Dwivedi at Oerlikon Metco for providing materials I would also like to thank Professor

Martin Harmer at Lehigh University for allowing me use of his SPS while ours was down Thanks

to Professor Elizabeth Opila of the University of Virginia and her students Dr Bekah Webster

and Mackenzie Ridley for their help with water vapor corrosion studies

Last but not least I would like to thank my family and friends for their support and love

A special thanks to my parents Joe and Catherine I really grateful for my mom my Aunt Elizabeth

(Zee) Enke and my friend Ally MacInnis They took time out of busy schedules to review my

thesis They sent care packages and listened to my whining

ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi

ACKNOWLEDGEMENTS vii

TABLE OF CONTENTS ix

TABLE OF TABLES xiii

TABLE OF FIGURES xv

CHAPTER 1 INTRODUCTION 1

11 Gas-Turbine Engine Materials 1

12 Environmental Barrier Coatings 3

121 EBC Requirements 4

122 EBC Materials and Processing 5

123 EBC Failure 7

13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits 8

131 CMAS Induced Failure 10

132 Approaches for CMAS Mitigation 12

14 Approach 13

141 Materials SelectionOptical Basicity 13

142 Objectives 16

CHAPTER 2 Y-CONTAINING EBC CERAMICS FOR RESISTANCE AGAINST

ATTACK BY MOLTEN CMAS 18

21 Introduction 18

22 Experimental Procedure 19

221 Processing 19

222 CMAS interactions 20

223 Characterization 21

23 Results 22

231 Polycrystalline Pellets 22

x

232 YAlO3-CMAS Interactions 24

233 Y2Si2O7-CMAS Interactions 30

24 Discussion 34

25 Summary 36

CHAPTER 3 Y-FREE EBC CERAMICS FOR RESISTANCE AGAINST ATTACK BY

MOLTEN CMAS 38

31 Introduction 38


321 Processing 40

322 CMAS Interactions 41


33 Results 42


332 Yb2Si2O7-CMAs Interactions 44

333 Sc2Si2O7-CMAS Interactions 51

334 Lu2Si2O7-CMAS Interactions 55

34 Discussion 60

35 Summary 65

CHAPTER 4 RARE-EARTH SOLID-SOLUTION ENVIRONMENTAL-BARRIER

COATING CERAMICS FOR RESISTANCE AGAINST ATTACK BY MOLTEN

CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 67

41 Introduction 67

42 Experimental Procedures 69

421 Powders 69

422 CMAS Interaction 70


43 Results 71

431 Powder and Polycrystalline Pellets 71

432 NAVAIR CMAS Interactions 75

433 NASA CMAS Interactions 78

434 Icelandic Volcanic Ash CMAS Interactions 80

44 Discussion 82

45 Summary 84

xi

CHAPTER 5 THERMAL CONDUCTIVITY 85

51 Introduction 85

511 Coefficient of Thermal Expansion 86

512 Phase Stability 87

513 Solid solutions 88

52 Calculated Thermal Conductivity of Binary Solid-Solutions 89


522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity 90

523 Thermal Conductivity Calculations for Binary Solid-Solutions 91

53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity 96


532 Comparison of Experimental and Calculated Thermal Conductivity 97

54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution 100

541 Introduction to High-Entropy Ceramics 100


543 Solid Solution Confirmation 103

544 Experimental Thermal Conductivity Results 106

55 Summary 107

CHAPTER 6 RARE-EARTH SOLID-SOLUTION AIR PLASMA SPRAYED

ENVIRONMENTAL-BARRIER COATINGS FOR RESISTANCE AGAINST ATTACK

BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS 109

61 Introduction 109


621 Air Plasma Sprayed Coatings 111

622 Heat Treatments 111



63 Results 113

631 As-sprayed and Heat-Treated Coatings 113


64 Discussion 122

65 Future Work 124

66 Summary 124

xii

CHAPTER 7 CONCLUSIONS AND FUTURE WORK 126

71 Summary and Conclusions 126

72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES

Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78] 14

Table 2 Calculated Optical Basicities of various potential EBC compositions that have been tested

with CMASs Based off Ref [78] 15

Table 3 Average EDS elemental composition (at cation basis) from the regions indicated in the

SEM micrograph in Figure 11 of YAlO3 interaction with CMAS at 1500 degC for 1 min and 1 h The

ideal compositions of the three main phases and CMAS are also included 25


TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h 26


SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h 29


SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h 31


SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h 33


SEM micrograph in Figure 23B of β-Yb2Si2O7 interaction with CMAS at 1500 degC for 1 h The

ideal compositions of the two main phases and the CMAS are also included 46

Table 9 Average EDS elemental composition (at cation basis) from the regions indicated in

SEM and TEM micrographs in Figures 25 and 27 respectively of β-Yb2Si2O7 interaction with

CMAS at 1500 degC for 24 h 49


the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h 52


the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h 55


the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h 57

Table 13 Original CMAS compositions used in this study (mol) and the CaSi (at) ratio for

each 69

Table 14 Average EDS elemental composition (at cation basis) from the regions numbered in

the TEM micrograph in Figure 44B of as-SPSed Yb1Y1Si2O7 EBC ceramic The ideal composition

is also included 75

xiv


the SEM micrographs in Figures 45E and 45G of interaction of Yb18Y02Si2O7 and Yb1Y1Si2O7

respectively EBC ceramics with NAVAIR CMAS at 1500 ˚C for 24 h The ideal compositions

are also included 78


the SEM micrographs in Figures 46E 46F 46G and 46H of interactions of Yb2Si2O7

Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with NASA CMAS at 1500

˚C for 24 h 80



Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 EBC ceramics respectively with Icelandic Volcanic

Ash CMAS at 1500 ˚C for 24 h 82

Table 18 Properties and parameters for pure β-RE-pyrosilicates 93

Table 19 Rule-of-mixture values of properties for RE-pyrosilicate solid-solutions at x where the

calculated thermal conductivities in Figure 51 are the lowest kMin calculated using Equation 10

96

Table 20 Thermal conductivities of YxYb(2-x)Si2O7 solid-solutions both experimental data and

rule-of-mixture calculations 99


the TEM micrographs in Figures 56B and 56C of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7

EBC ceramic pellet 106

Table 22 Density measurements relative density and open porosity for the as-sprayed and heat-

treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings 116


the SEM micrographs in Figures 64B and 64D of the Yb2Si2O7 APS coating interaction with



the SEM micrographs in Figures 66B and 66D of the Yb1Y1Si2O7 APS coating interaction with


xv

TABLE OF FIGURES

Figure 1 Schematic illustration of a TBC-coated turbine blade The blue line is the relative thermal

gradient through the TBC layers From Ref [1] 1

Figure 2 Operational temperatures of gas-turbine engines over the past five decades redrawn from

Ref [3] The orange region denotes the temperature at which calcia-magnesia-aluminosilicate

(CMAS) deposits melt interact and degrade coatings 2

Figure 3 A schematic illustration of the (A) oxidation of SiC in the presence of oxygen and (B)

volatilization of the SiO2 layer in the presence of water vapor leading to recession of the SiC-

based CMC material [12] 4

Figure 4 (A) Cross-sectional SEM image of BSASBSAS+mulliteSi on melt-infiltrated (MI)

CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13] 5

Figure 5 Schematic illustrations of key EBC failure modes (A) Recession by water vapor (B)

Steam oxidation (C) Thermo-mechanical fatigue (D) CMAS ingestion (E) Erosion and (F)

Foreign object damage [51] 8

Figure 6 Compositions of major components of three different classes of CMAS (mineral sources

engine deposits and simulated CMAS sand) from the literature (name of authorcompany on the

x-axis) and their calculated optical basicities (OB or Λ discussed in Section 141) modified from

References [5978] Mineral sources FordAverage Earthrsquos Crust [6279] SmialekSaudi Sand

[61] PTIAirport Runway Sand [80] TaylorMt St Helenrsquos Volcanic Ash [81]

DrexlerEyjafjallajoumlkull Volcanic Ash [71] ChesnerSubbituminous Fly Ash [82]

ChesnerBituminous Fly Ash [82] and KrauseLignite Fly Ash [78] Engine deposits Smialek

[61] Borom [62] Bacos [83] and Braue [84] Simulated CMAS Sand Steinke [85] Aygun

[7086] Kraumlmer [65] Wu [87] and Rai [88] 9

Figure 7 (A) Cross-sectional SEM image showing a CMAS-interacted Yb2Si2O7 top-coat

EBCMulliteSi bond-coatSiC-CMC after just 1 minute at 1300 degC [33] (B-C) Cross-sectional

SEM images showing the CMAS-interacted Yb2Si2O7 (containing Yb2SiO5 and Yb2O3 lighter

streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36] 11

Figure 8 Cross-sectional SEM images showing the CMAS-interacted Yb2Si2O7 (containing

Yb2SiO5 and Yb2O3 lighter streaks) EBCs that have interacted with CMAS at 1300 degC for (A)

100 h and (B) 200 h [36] 11

Figure 9 (A) A cross-sectional SEM image of a thermally-etched YAlO3 pellet and (B) indexed

XRD pattern of the YAlO3 pellet (some Y3Al5O12 (YAG) and Y4Al2O9 (YAM) impurities are

present) 23

Figure 10 (A) Cross-sectional SEM image of a thermally-etched γ-Y2Si2O7 pellet and (B) indexed

XRD pattern showing phase-pure γ-Y2Si2O7 23

xvi

Figure 11 Cross-sectional SEM images of YAlO3 pellets that have interacted with the CMAS at

1500 degC in air for (A) 1 min and (B) 1 h The circled numbers correspond to locations where

elemental compositions were obtained using EDS and they are reported in Table 3 The dashed

boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB 24

Figure 12 Bright-field TEM micrographs of CMAS-interacted YAlO3 pellet (1500 degC 1 h) from

regions within the interaction zone similar to those indicated in Figure 11B (A) near-top and (B)

near-bottom Y-Ca-Si apatite (ss) and YAG (ss) grains are marked with circled numbers and their

elemental compositions (EDS) are reported in Table 4 The inset in Figure 12A is indexed SAEDP

from a Y-Ca-Si apatite (ss) grain Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo

respectively 26

Figure 13 Cross-sectional SEM images of CMAS-interacted YAlO3 pellet (1500 degC 24 h) at (A)

low and (B) high magnification Corresponding EDS elemental maps (C) Ca and (D) Si The

dashed boxes in (B) indicate regions from where higher-magnification SEM images in Figure 14

were collected 28

Figure 14 Higher-magnification SEM images of the cross-section of CMAS-interacted YAlO3

pellet (1500 degC 24 h) from regions within the interaction zone similar to those indicated in Figure

13B (A) near-top and (B) near-bottom The circled numbers correspond to locations where

elemental compositions were obtained using EDS and they are reported in Table 5 29

Figure 15 Indexed XRD pattern from YAlO3-CMAS powders mixture that was heat-treated at

1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) Y3Al5O12 (YAG) and Y4Al2O9

(YAM) in addition to unreacted YAlO3 30

Figure 16 Cross-sectional SEM image of a γ-Y2Si2O7 pellet that has interacted with CMAS at

1500 degC for 1 h The circled numbers correspond to regions where the elemental compositions

were measured by EDS and they are reported in Table 6 31

Figure 17 Cross-sectional SEM images of CMAS-interacted γ-Y2Si2O7 pellet (1500 degC 24 h) (A)


dashed boxed in (B) indicate regions from where higher-magnification SEM images in Figure 18

were collected 32

Figure 18 Higher-magnification SEM images of the cross-section of CMAS-interacted γ-Y2Si2O7




Figure 19 Indexed XRD pattern from γ-Y2Si2O7-CMAS powders mixture that was heat-treated at

1500 degC for 24 h showing the presence of Y-Ca-Si apatite (ss) and some un-reacted γ-Y2Si2O7

34

xvii

Figure 20 (A) Cross-sectional SEM image of a thermally-etched β-Yb2Si2O7 pellet and (B)

indexed XRD pattern showing phase-pure β-Yb2Si2O7 42

Figure 21 (A) Cross-sectional SEM image of a thermally-etched β-Sc2Si2O7 pellet and (B) indexed

XRD pattern showing phase-pure β-Sc2Si2O7 43

Figure 22 (A) Cross-sectional SEM image of a thermally-etched β-Lu2Si2O7 pellet and (B)

indexed XRD pattern showing phase-pure β-Lu2Si2O7 44

Figure 23 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 1 h) at

(A) low and (B) high magnifications (C) EDS elemental Ca map corresponding to (B) The dashed

box in (A) indicates the region from where higher-magnification SEM image in (B) was collected

The circled numbers correspond to locations where elemental compositions were obtained using

EDS and they are reported in Table 8 The dashed boxes in (B) indicate the regions from where

the TEM specimens were extracted using the FIB 45

Figure 24 Bright-field TEM micrographs and indexed SAEDPs of CMAS-interacted β-Yb2Si2O7


23B (A) near-top and (B) middle Yb-Ca-Si apatite (ss) grain β-Yb2Si2O7 grain and CMAS glass

are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively 46

Figure 25 Cross-sectional SEM images of CMAS-interacted β-Yb2Si2O7 pellet (1500 degC 24 h)

(A) low (whole pellet) and (BD) high magnification The dashed boxes in (A) indicate regions

from where higher-magnification SEM images in (B) and (D) were collected The circled numbers

in (B) correspond to locations where elemental compositions were obtained using EDS and they

are reported in Table 9 The dashed box in (B) indicates the region from where the TEM specimen

was extracted using the FIB 48

Figure 26 Indexed XRD pattern from β-Yb2Si2O7-CMAS powders mixture that was heat-treated

at 1500 degC for 24 h showing the presence of some Yb-Ca-Si apatite (ss) and unreacted β-Yb2Si2O7

49

Figure 27 Bright-field TEM image of CMAS-interacted β-Yb2Si2O7 (1500 degC 24 h) from regions

within the interaction zone similar to that indicated in Figure 25B β-Yb2Si2O7 grains and CMAS

glass are marked The circled number corresponds to a location where elemental composition was

obtained using EDS and it is reported in Table 9 49

Figure 28 Collages of cross-sectional optical micrographs of β-Yb2Si2O7 pellets that have

interacted with CMAS at 1500 degC for (A) 1 h (B) 3 h (C) 12 h (D) 24 h and (E) 24 h The pellets

in (A)-(D) are ~2 mm thick and the pellet in (E) is ~1 mm thick The region between the arrows

is where the CMAS was applied The gray contrast in the lsquoblisterrsquo cracks in some of the

micrographs is epoxy from the sample mounting 50

xviii

Figure 29 SEM images of polished and HF-etched cross-sections of β-Yb2Si2O7 pellet (2 mm

thickness) that has interacted with CMAS at 1500 degC for 6 h (A) top region and (B) bottom region

51

Figure 30 (A) Cross-sectional SEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 1 h)

and (B) corresponding EDS elemental Ca map The circled numbers in (A) correspond to locations

where elemental compositions were obtained using EDS and they are reported in Table 10 52

Figure 31 Cross-sectional SEM images of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h) at

(A) low (whole pellet) and (BC) high magnifications The dashed boxes in (A) indicate regions

from where higher-magnification SEM images in (B) and (C) were collected and the region from

where the TEM specimen was extracted using the FIB 53

Figure 32 (A) Bright-field TEM image of CMAS-interacted β-Sc2Si2O7 pellet (1500 degC 24 h)

from region within the interaction zone similar to that indicated in Figure 31A Indexed SAEDP

is from the grain marked β-Sc2Si2O7 (B) Higher-magnification bright-field TEM image from

region indicated by the dashed box in (A) (C) EDS elemental Ca map corresponding to (B)

Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively The circled numbers in

(B) correspond to locations where elemental compositions were obtained using EDS and they are

reported in Table 11 54

Figure 33 Indexed XRD pattern from β-Sc2Si2O7-CMAS powder mixture that was heat-treated at

1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7 55

Figure 34 Cross-sectional SEM images of CMAS-interacted β-Lu2Si2O7 pellet (1500 degC 1 h) at

(A) low (entire CMAS-interacted zone) (B) low (whole pellet thickness) and (C) higher

magnification The dashed boxes in (A) indicate regions from where higher-magnification images

in (B) and (C) were collected (D E) Higher magnification images represented in (C) as dashed

boxes and (F G) their corresponding EDS Ca maps respectively The circled numbers in (D F)

correspond to locations where elemental compositions were obtained using EDS and they are



(A) low (whole pellet thickness) and (B) high magnifications The dashed box in (A) indicates the

region from where (B) was collected (C) EDS elemental Ca map corresponding to (B) 58


(A) low and (B C) higher magnifications (B) was obtained from a region near the bottom of the

CMAS-interaction zone and (C) was obtained from a region away from the CMAS-interaction

zone close to the edge of the pellet 59

Figure 37 Indexed XRD pattern from β-Lu2Si2O7-CMAS powders mixture that was heat-treated

at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7 59

xix

Figure 38 (A) Schematic illustration of molten CMAS-glass penetration into ceramic grain

boundaries causing a dilatation gradient (B) resulting in a lsquoblisterrsquo crack due to buckling of the

top dilated layer 61

Figure 39 (A) Cross-sectional SEM image of a thermally-etched pellet of as-sintered β-

Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map 62

Figure 40 (A) Collage of cross-sectional optical micrographs of β-Yb2Si2O71 vol CMAS pellet

that have interacted with CMAS at 1500 degC for 24 h The region between the arrows is where the

CMAS was applied (B) Cross-sectional SEM image of the whole pellet from the region marked

by the dashed box in (A) (C) Higher-magnification cross-sectional SEM image of the region

marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map 63

Figure 41Cross-section SEM images of dense polycrystalline RE2Si2O7 pyrosilicate ceramic

pellets that have interacted with the CMAS glass under identical conditions (1500 degC 24 h) (A)

Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7 65

Figure 42 (A) Phase stability diagram of the various RE2Si2O7 pyrosilicate polymorphs Redrawn

and adapted from Ref [37] (B) Binary phase diagram showing complete solid-solubility of the

Yb(2-x)YbxSi2O7 system with different polymorphs The dashed lines represent the compositions

chosen in this chapter Adapted from Ref [38] 68

Figure 43 SEM images of powders (A) Yb18Y02Si2O7 and (B) Yb1Y1Si2O7 Cross-sectional SEM

images of the thermally-etched EBC ceramics (C) Yb18Y02Si2O7 and (D) Yb1Y1Si2O7 (E) XRD

pattern of Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics showing β-phase (F) Higher

resolution XRD patterns 72

Figure 44 (A) Bright-field TEM image of as-SPSed Yb1Y1Si2O7 EBC ceramic (B) Higher

magnification bright-field TEM image of the region marked in (A) The circled numbers

correspond to regions from where EDS elemental compositions are obtained (see Table 14) (C)

High-magnification bright-field TEM image showing a grain boundary (D) EDS line scan along

L-R in (C) 74

Figure 45 Cross-sectional SEM images of NAVAIR CMAS-interacted (1500 ˚C 24 h) EBC

ceramics (A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes

indicate from where the corresponding higher-magnification SEM images are collected (E)

Yb18Y02Si2O7 and (G) Yb1Y1Si2O7 Corresponding EDS Ca elemental maps (F) Yb18Y02Si2O7

and (H) Yb1Y1Si2O7 The circled numbers in (E) and (G) correspond to regions from where EDS

elemental compositions are obtained (see Table 15) (A) and (D) adapted from Refs [117] and

[116] respectively 77

Figure 46 Cross-sectional SEM images of NASA CMAS-interacted (1500 ˚C 24 h) EBC



Yb2Si2O7 (F) Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca

xx

elemental maps (I) Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled

numbers in (E) through (G) correspond to regions from where EDS elemental compositions are

obtained (see Table 16) 79

Figure 47 Cross-sectional SEM images of IVA CMAS-interacted (1500 ˚C 24 h) EBC ceramics

(A) Yb2Si2O7 (B) Yb18Y02Si2O7 (C) Yb1Y1Si2O7 and (D) Y2Si2O7 Dashed boxes indicate from

where the corresponding higher-magnification SEM images are collected (E) Yb2Si2O7 (F)

Yb18Y02Si2O7 (G) Yb1Y1Si2O7 and (H) Y2Si2O7 Corresponding EDS Ca elemental maps (I)

Yb2Si2O7 (J) Yb18Y02Si2O7 (K) Yb1Y1Si2O7 and (L) Y2Si2O7 The circled numbers in (E)

through (G) correspond to regions from where EDS elemental compositions are obtained (see

Table 17) 81

Figure 48 (A) Cross-sectional SEM micrograph of a TEBC on a CMC [13] (B) Schematic

illustration of the TEBC concept adapted from [4] (C) Schematic Illustration of the TEBC

concept 85

Figure 49 (A) Average CTEs of various RE2Si2O7 pyrosilicate polymorphs which is adapted from

Ref [137] The horizontal band represents the CTE of SiC-based CMCs (B) Stability diagram of

the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37] 87

Figure 50 Thermal conductivities of dense polycrystalline RE2Si2O7 pyrosilicate ceramic pellets

as a function of temperature The data for Lu2Si2O7 is from Ref [142] 91

Figure 51 The calculated thermal conductivities of various RE2Si2O7 pyrosilicate solid-solutions

at 27 200 400 600 800 and 1000 degC (A) YxYb(2-x)Si2O7 (B) YxLu(2-x)Si2O7 (C) YxSc(2-x)Si2O7

(D) YbxSc(2-x)Si2O7 (E) LuxSc(2-x)Si2O7 and (F) LuxYb(2-x)Si2O7 The thermal conductivities of the

pure dense RE2Si2O7 pyrosilicates from Figure 50 are included as solid symbols on the y-axes

The dashed lines represent 1 Wmiddotm-1middotK-1 94

Figure 52 Thermal conductivities of dense polycrystalline Yb2Si2O7 Y2Si2O7 Y02Yb18Si2O7

and Y1Yb1Si2O7 pyrosilicate ceramic pellets as a function of temperature The dashed line

represents 1 Wmiddotm-1middotK-1 97

Figure 53 The calculated thermal conductivities of the YxYb(2-x)Si2O7 system at 27 200 400 600

800 and 1000 degC (solid lines) compared to the experimentally collected thermal conductivities

which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1 98

Figure 54 Indexed XRD pattern from an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet

compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets 103

Figure 55 Cross-sectional SEM image of an as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet and

the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si 104

Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-

(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone

xxi

axis are denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing

grain boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The

circled regions are where EDS elemental compositions were obtained and can be found in Table

21 105


Y1Yb1Si2O7 and β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pyrosilicate ceramic pellets as a function of

temperature The dashed line represents 1 Wmiddotm-1middotK-1 107

Figure 58 Cross-sectional SEM micrographs of the as-sprayed Yb2Si2O7 APS coating at (A) low

and (B) high magnification The lighter gray regions in these images contain less silica (C)

Indexed XRD patterns from the as-sprayed Yb2Si2O7 APS coating on the top and bottom sides

showing a mostly amorphous coating 113

Figure 59 Cross-sectional SEM micrographs of the as-sprayed Yb1Y1Si2O7 APS coating at (A)

low and (B) high magnification The lighter gray regions in these images contain less silica (C)

Indexed XRD patterns from the as-sprayed Yb1Y1Si2O7 APS coating on the top and bottom sides


Figure 60 In-situ high-temperature XRD of the as-sprayed Yb1Y1Si2O7 APS coating starting from

room temperature (25 degC) XRD patterns were collected also collected at 800 900 1000 1100

1200 1300 and 1350 degC The circle markers and the solid lines index the Yb1Y1Si2O7 phase and

the square markers and dashed line index the Yb1Y1SiO5 phase 115

Figure 61 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb2Si2O7 APS

coating at (A) low and (B) high magnification The lighter gray regions in these images are

Yb2SiO5 the darker gray regions are Yb2Si2O7 and the black regions are pores (C) Indexed XRD

patterns from the heat-treated (1300 degC 4 h) Yb2Si2O7 APS coating on the top and bottom sides

showing both Yb2Si2O7 and Yb2SiO5 are present 116

Figure 62 Cross-sectional SEM micrographs of the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS


Yb1Y1SiO5 the darker gray regions are Yb1Y1Si2O7 and the black regions are pores (C) Indexed

XRD patterns from the heat-treated (1300 degC 4 h) Yb1Y1Si2O7 APS coating on the top and bottom

sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present 117

Figure 63 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h) Yb2Si2O7

APS Coating The dashed line indicates the depth of the CMAS interaction zone The dashed box

indicates the region where (B) was collected (B) A higher magnification image and its

corresponding Si Ca and Yb elemental EDS maps 118

Figure 64 (A) A low magnification cross-sectional SEM micrograph of the CMAS-interacted

(1500 degC 24 h) Yb2Si2O7 APS coating The dashed boxes in (A) indicate where higher

magnification images were obtained (B D) The higher magnification SEM micrographs and (C

E) their corresponding elemental Ca EDS maps respectively The circled numbers in (B D)

xxii



Figure 65 (A) Cross-sectional SEM micrograph of the CMAS-interacted (1500 degC 24 h)

Yb1Y1Si2O7 APS Coating The dashed line indicates the depth of the CMAS interaction zone The

dashed box indicates the region where (B) was collected (B) A higher magnification image and

its corresponding Si Ca Y and Yb elemental EDS maps 120


(1500 degC 24 h) Yb1Y1Si2O7 APS coating The dashed boxes in (A) indicate where higher





Figure 67 (A-D) Plan view SEM images of the impingement site for Yb2Si2O7 Yb18Y02Si2O7

Yb1Y1Si2O7 and Yb2Si2O7 respectively (E-H) Cross-sectional SEM images of the impingement

zone for Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Yb2Si2O7 respectively (I-L) The

corresponding Si elemental EDS maps to (E-H) respectively 130

1

CHAPTER 1 INTRODUCTION

11 Gas-Turbine Engine Materials

The use of ceramic thermal barrier coatings (TBCs) on Ni-based superalloy components

in conjunction with air-cooling has resulted in the hot-section of gas-turbine engines ability to

operate at maximum temperatures above 1500 degC [1ndash4] Figure 1 is a schematic illustration of a

TBC-coated turbine blade allowing for higher operating temperatures and the relative thermal

gradient through the TBC layers This has resulted in outstanding power and efficiency gains in

gas-turbine engines used for aircraft propulsion and land-based power generation


gradient through the TBC layers From Ref [1]

TBC microstructures usually contain cracks and pores which are deliberate to reduce TBC

thermal conductivity and to provide strain-tolerance against residual stresses that buildup due to

the thermal expansion coefficient (CTE) mismatch with the base metal substrate TBCs with even

2

higher temperature capabilities and lower thermal conductivities are being developed [3ndash5] Figure

2 shows the progress over decades for the temperature capabilities of Ni-based superalloys TBCs

and Ceramic-Matrix Composites (CMCs) along with the allowable gas temperature in a gas-

turbine engine However TBC developments have outpaced those of the Ni-based superalloys

which has led to more aggressive cooling requirements Unfortunately this results in an increase

of inefficiency losses or the difference in ideal and actual specific core power for a gas-inlet

temperature [46]



(CMAS) deposits melt interact and degrade coatings

3

Therefore hot-section materials with inherently higher temperature capabilities are

needed In this context CMCs typically comprising of silicon carbide (SiC) fibers in a SiC matrix

are showing promise to replace Ni-based superalloys in the engine hot-section [46ndash8] CMCs have

already replaced some Ni-based superalloy hot-section stationary components in gas-turbine

engines that are in-service commercially both for aircraft propulsion and power generation

12 Environmental Barrier Coatings

CMCs for gas-turbine applications both aerospace and power generation are primarily

SiC-based continuous SiC fibers in a SiC matrix SiC-based CMCs are lightweight damage

tolerant resistant to thermal shock and impact and display better resistance to high temperatures

and aggressive environments than metals [9] SiC-based CMCs have excellent high temperature

capabilities they maintain mechanical properties at temperatures up to 3000 degC [10]

Unfortunately SiC-based CMCs undergo active oxidation and recession in the high-velocity hot-

gas stream containing both oxygen and water vapor [411ndash13] In the presence of oxygen SiC

forms a passive SiO2 layer on the surface using the chemical reaction below [14] and shown as a

schematic illustration in Figure 3A

119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)

However in the gas-turbine engine combustion environment ~ 10 water vapor is also present

This leads to the volatilization of the SiO2 layer and active recession of the base layer according

to the reaction below [15] which can also be seen as a schematic illustration in Figure 3B

1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4



based CMC material [12]

Therefore SiC-based CMCs need to be protected by ceramic environmental barrier

coatings (EBCs) [47131617]

121 EBC Requirements

Along with the need to protect SiC-based CMCs from oxygen and water vapor due to active

oxidation and recession there are many other requirements on EBCs EBCs should have low

permeability of oxygen and water vapor Therefore they should also be dense and crack-free to

prevent recession of the SiC-based CMC Consequently they must have a good coefficient of

thermal expansion (CTE) match with the SiC-based CMCs [78] EBCs must also have low silica

activityvolatility so that they do not show major recession like the SiC-based CMCs EBCs will

be operating at temperatures around 1500 degC so they should have high-temperature capability

phase stability and robust mechanical properties They need to have chemical compatibility with

the bond-coat material And lastly they must be resistant to molten calcia-magnesia-

aluminosilicate (CMAS) deposits which will be discussed in more detail is Section 13

A B

5

122 EBC Materials and Processing

In the late 1990s EBCs comprised of a silicon bond-coat on a CMC an interlayer of barium

strontium aluminum silicate (BSAS (1 - x)BaOxSrOAl2O32SiO2 with 0 lt x lt 1) and mullite

(3Al2O32SiO2) mixture and a top coat of BSAS called Gen I were early successful EBC

architectures [71318] This Gen I EBC system is shown in Figure 4A All layers were deposited

by thermal spray [18] The Si bond-coat enhances the adherence between the CMC and the mullite

layer and promotes the formation of a dense and protective SiO2 thermally grown oxide (TGO)

which adds additional protection to the CMC [131718] Mullite was promising due to its low

CTE Unfortunately crystalline mullite coatings experience silica volatility and phase instability

in water vapor environments [1719] An Al2O3 layer remains but it is porous and brittle Adding

a topcoat of BSAS which has a lower silica activity than mullite and a CTE of ~43 x 10-6 degC-1 in

the celsian phase closely matching that of SiC (~45 x 10-6 degC-1) has been found to provide

adequate high-pressure protection at temperatures below 1300 degC [18]


CMC [18] (B) Cross-sectional SEM image of a later generation TEBC [13]

The next generation EBCs or Gen II to VI were developed for higher temperature

applications These are based on rare earth (RE) silicates with several variations such as the

A B

6

additions of oxides (ie HfO2 mullite etc) [13] The most studied EBCs have been Y-silicates

(Y2SiO5 [20ndash22] and Y2Si2O7 [22ndash27]) and Yb-silicates (Yb2SiO5 [28ndash32] and Yb2Si2O7

[23252633ndash36]) The monosilicates Y2SiO5 and Yb2SiO5 have low silica activity and high

melting points but they have higher CTEs than SiC The disilicates Y2Si2O7 and Yb2Si2O7 have

a better CTE match to SiC but a higher silica activity [7] However EBCs tend to fail

mechanically therefore disilicate EBCs are being used Yb2Si2O7 has been a focus due to its phase

stability as it does not experience a phase transition up to 1700 degC [3738]

Bond coat replacements are also being studied due to the low melting point of Si (1410 degC)

[13] Oxide bond-coats containing rare earths (ie Hf Zr Y) could improve oxidation resistance

and thermal cycling durability [13] EBC systems that also include thermal barrier coatings (TBCs)

on top of the EBC system described called TEBC have also been studied The TBC has a lower

thermal conductivity to help with high temperatures experienced in a gas-turbine engine However

the CTE difference of the TBC (9-10 x 10-6 degC-1) and the EBC (4-5 x 10-6 degC-1) in TEBC systems

is large which means a graded CTE interlayer is needed between the two coatings to alleviate

stress concentrations that occur at interfaces [413] An example of this TEBC system can be seen

in Figure 4B

EBC deposition is still a significant challenge [3940] Conventional air plasma spray

(APS) is preferred but the EBCs typically deposit as an amorphous coating [41] Many have

performed APS inside a box furnace so that the substate is heated to temperatures around 1000 degC

so that the coating can crystalize during spraying [1733364243] but this is difficult in a

manufacturing setting Post-deposition heat treatment has also been done on APS Yb2Si2O7 EBC

coatings [41] however crystallization has a significant volume change which leads to porous

coatings and undesirable phases can form during crystallization Other methods being studied are

7

plasma spray physical vapor deposition (PS-PVD) [39] high-velocity oxygen fuel spraying

(HVOF) [40] slurry dipping [4445] electron beam physical vapor deposition (EB-PVD) [4647]

chemical vapor deposition (CVD) [48] magnetron sputtering [49] and sol-gel nanoparticle

application [50]

123 EBC Failure

EBCs are subjected to hostile operating conditions in the hot-section of gas-turbine

engines The typical environment is ~10 atm of pressure with a ~300 ms-1 velocity of gas-stream

that contains a water vapor partial pressure of ~01 atm and an oxygen partial pressure of ~02 atm

[9] Below in Figure 5 Lee [51] shows schematic illustrations of the different failure mechanisms

EBCs face As seen earlier in Section 121 SiC volatilization occurs in the presence of water

vapor Like CMCs EBCs usually contain Si (ie RE2SiO5 or RE2Si2O7) therefore they have a

non-zero silica activity [5253] (less than that of SiO2) which will lead to recession of the EBC

which is shown schematically in Figure 5A [51] Figure 5B shows a schematic illustration of steam

oxidation This occurs when water vapor permeates through the EBC and reacts with the Si bond

coat forming a SiO2 scale or thermally grown oxide (TGO) [174254] As the Si bond-coat

becomes the SiO2 TGO many factors increase the stresses in the EBC system including (i) ~22-

fold volume expansion as the SiO2 TGO forms [42] (ii) phase transformation (β rarr α cristobalite)

of SiO2 [55] and (iii) mismatch in the CTE between the α cristobalite SiO2 (103 x 10-6 degC-1 [56])

and the EBC (4-5 x 10-6 degC-1 [1757]) As the thickness of the SiO2 TGO increases stresses build

up and once a critical thickness is reached spallation of the EBC occurs [5158]

EBCs must also withstand thermo-mechanical cycling (up to 1700 degC) (see Figure 5C) and

degradation due to molten calcia-magnesia-aluminosilicate (CMAS discussed further is Section

8

13) at high temperatures above 1200 degC (see Figure 5D) Particle damage can occur by erosion

(see Figure 5E) or foreign object damage (FOD) (see Figure 5F) which decreases EBC lifetimes

significantly [51] And in the case of rotating parts they will need to carry loads that may cause

creep and rupture EBCs are expected to be lsquoprime reliantrsquo or last for the lifetime of the

components which can be several 10000s of hours of operation [9]



Foreign object damage [51]

13 Calcia-Magnesia-Aluminosilicate (CMAS) Deposits

As the coating-surface temperatures in gas-turbine engines reached 1200 degC a new damage

mechanism has become important the degradation of TBCs [59ndash68] and EBCs [2325ndash

2733343669] from the melting and adhesion of calcia-magnesia-aluminosilicate (CMAS)

A

B

C

D

E

F

9

deposits In aircraft engines CMAS is introduced in the form of ingested airborne sand [61ndash

656970] or volcanic ash [24606771ndash73] In power-generation engines CMAS is introduced in

the form of lsquofly ashrsquo an impurity in alternative fuels such as syngas [6874ndash77] Figure 6 shows

the composition of various CMASs including mineral sources like volcanic ash deposits found in

engines and synthetic CMASs used in laboratory experiments The compositional differences lead

to differences in the melt temperature viscosity and wetting of the CMAS which all play a role

in how the CMAS will interact with EBCs









[7086] Kraumlmer [65] Wu [87] and Rai [88]

10

131 CMAS Induced Failure

The most prevalent failure mode in EBCs is caused by the CTE mismatch between the

CMAS glass and the EBC CMAS has a CTE of 9-10 x 10-6 degC-1 [89] while most potential EBCs

have CTEs of ~4-5 x 10-6 degC-1 [1757] Upon cooling to room temperature this can lead to through

cracks which originate in the glass and travel all the way to the bond coat [33] Stolzenburg et al

[33] showed an example with a multi-layer EBC system substrate Si bond-coat mullite and

Yb2Si2O7 as the top-coat EBC After just one minute at 1300 degC the stresses in the coating caused

cracking through the coating which can be seen in Figure 7A In Figures 7B and 7C Zhao et al

[36] also saw similar cracking The coatings in this study were majority Yb2Si2O7 with Yb2SiO5

and Yb2O3 impurities These tests were also conducted at 1300 degC but for longer times of (B) 4 h

and (C) 24 h Sharp cracks are observed coming from the surface of the CMAS and through the

apatite (Ca2RE8(SiO4)6O2) layer Once the cracks hit the Yb2Si2O7 a lower CTE material they

seem to deflect or turn left or right This cracking mechanism has also been seen in TBCs that have

interacted with CMAS In TBCs and EBCS during cooling vertically aligned or lsquochannelrsquo cracks

form near the surface Delamination between lsquochannelrsquo cracks can occur leading to spallation of

the coating due to crack propagation and coalescence [64]

If spallation occurs the base materials are exposed and silica volatilization will proceed

If spallation does not occur these cracks are still fast channels to the CMC for oxygen and water

vapor or molten CMAS Lee [51] has showed that even without cracks the Si bond-coat forms a

TGO and after a critical thickness EBC spallation can occur If cracks are present the Si bond-

coat has a direct path for oxygen and water vapor so localized silica volatilization can occur

leading to premature spallation of the coatings

11




streaks) EBCs that have interacted with CMAS at 1300 degC for (B) 4 h and (C) 24 h [36]

Another CMAS-induced failure mechanism observed in EBCs has been the formation of a

reaction-crystallization product apatite (Ca2RE8(SiO4)6O2) which can be seen in Figure 8 Zhao

et al [36] found that after 200 h at 1300 degC almost half of the coating thickness has either been

incorporated into the CMAS melt or has formed an apatite reaction phase It has been seen that

apatite formation in Y-containing materials is faster than ytterbium silicates [2427]



100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12

132 Approaches for CMAS Mitigation

CMAS-attack of EBCs is a relatively new issue and there is a paucity of approaches for

CMAS mitigation EBCs that react heavily with CMAS have been shown to lose coating thickness

and have additional reaction products form [3336] The CTE of potential reaction products are

unknown If they have a CTE mismatch with the EBC through-cracks can occur (more detail can

be found in 131) An example of a reaction product with a mismatched CTE can be seen in

Figures 7 and 8 Due to EBC requirements of dense and crack-free coatings the concept of optical

basicity (OB see Section 141 for more detail) has been used Briefly OB quantifies the chemical

reactivity of oxides and glasses OB was used to select potential EBC ceramics that would not

react heavily with CMAS [78] Materials selection of EBCs with low reactivity with CMAS is a

major focus because dissolution of the EBC would be stopped after the solubility limit of the EBC

in CMAS was reached

Coating systems for gas-turbine engines tend to include a porous TBC top-coat on the EBC

system Significant amount of research has gone into improving TBC resistance to CMAS

Sacrificial non-wetting and impermeable layers have been applied to the surface of TBCs to stop

CMAS penetration or sticking [9091] These coatings increase the CMAS melt temperature or

viscosity upon dissolution [909293] However once consumed CMAS can then attack the

coating system Therefore TBCs that react heavily with CMAS so that CMAS is consumed by

the formation of a reaction-crystallization product have been shown to provide better protection

[7894] Crystallization of reaction products of unknown CTEs works with the TBC because TBCs

are porous However TBCs are not the focus of this study

13

14 Approach

First the concept of optical basicity (OB Λ) was used as a first order screening for potential

EBCs (see Section 141 for more details) Then the selected materials were made through powder

processing and spark plasma sintering (SPS) to obtain dense polycrystalline lsquomodelrsquo EBC ceramic

pellets for lsquomodelrsquo CMAS experiments Their high-temperature interactions were studied (see

Section 142 for more details)

141 Materials SelectionOptical Basicity

As a first order screening optical basicity (OB Λ) was used to determine potential EBC

materials EBC must be dense impervious and crack-free therefore a limited reaction with CMAS

is desired so that the EBC is not consumed by the CMAS or a reaction-crystallization product with

unknown or different CTEs Duffy et al [95] first used the concept of OB to quantify the chemical

activity of oxides and glasses The OB concept is based on the Lewis acid-base theory which

defines acids as electron acceptors and bases as electron donors OB of a single metal oxide is

defined as the measure of the oxygen anionrsquos ability to donate electrons which depends on the

polarizability of the metal cation [9596]

Cations with high polarizability draw the electrons away from the oxygen which does not

allow the oxygen to donate electrons to other cations which is more lsquoacidicrsquo or a low OB value

On the other end of the scale the lsquobasicrsquo or high OB values oxygen can donate electrons to other

cations due to the low polarizability of the cation [97] OBs of relevant single cation oxides for

EBCs are seen below in Table 1 Ultraviolet spectroscopy [969899] X-ray photoelectron

spectroscopy [97] and mathematical relationships between refractivity and electronegativity

[100ndash102] have been used to measure or estimate the OBs for single cation oxides

14

Table 1 Optical Basicities of relevant single cation oxides for EBCs Based off Ref [78]

Single Cation Oxide Λ Ref

CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]

Lu2O3 0886 [106] Based on Al3+ CN = 4 For CN = 6 OB = 040

Duffy [96] found that the OB (Λ) for an oxide or glass composed of several single cation

oxides can be calculated using the equation below

Λ119872119906119897119905119894minus119888119886119905119894119900119899 119874119909119894119889119890119866119897119886119904119904 = 119883119860 times Λ119860 + 119883119861 times Λ119861 + 119883119862 times Λ119862 + ⋯ (Equation 3)

where ΛA ΛB and ΛC are the OB values of the single cation components and XA XB and XC are

the fraction of oxygen ions each single cation oxide donates Although this model was used to

determine the chemical reactivity of glasses it has also been used to access crystalline materials

as well [104107] However for crystalline materials coordination states need to be considered

OB values change based on the coordination number (CN) in glasses with an intermediate oxide

Al2O3 [104]

The difference in OB values of products in a reaction tend to be less than that of the

reactants ie there is a lsquosmooth[ing] outrsquo the overall electron density of the oxygen atoms [96]

Therefore the reactivity is proportional to the change in OB

119877119890119886119888119905119894119907119894119905119910 prop ΔΛ (= Λ119879119861119862119864119861119862 minus Λ119862119872119860119878) (Equation 4)

This has been used to describe high-temperature reactivity in metallurgical slags [108109] glasses

[100105] and oxide catalysts [110] Acidity a variation of the OB concept has also been to

15

explain the hot corrosion behavior of TBCs interaction with sodium vanadates [111] They found

that TBCs (basic OB values) readily react with corrosive agents (acidic OB values) Krause et al

[78] showed that OB difference calculations are a quantitative chemical basis for screening

CMAS-resistant TBC and EBC compositions TBC are porous and a reaction is desired (ie high

reactivity with CMAS) so that the CMAS is consumed by a reaction-crystallization product which

will stop the progression of CMAS into the base material The OBs of a wide range of CMAS

compositions which can be seen in Figure 6 fall within a narrow OB range of 049 to 075 which

is acidic Unlike TBCs EBCs need to be dense so a limited reaction with CMAS is desired [78]

Below is a table of EBC ceramics that have been studied to determine their resistance to CMAS

(Table 2) There is a column in Table 2 that is the change in OB (ΔΛ) between a common CMAS

sand with an OB of 064 and the chosen EBC ceramics


with CMASs Based off Ref [78]

Multi-Cation Oxide Ref Λ ΔΛ wrt Sand

(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004

Based off Krause et al [78] For Al3+ CN = 4 CN = 6

16

As stated earlier the focus of EBCs has been primarily on RE2Si2O7 which can be seen to

have small OB difference with CMAS glass There have been a few experiments conducted with

these ceramics and their interactions with CMAS glass [23252633ndash36] However a systematic

study and understanding of CMAS interactions at 1500 degC with dense EBC ceramics had yet to be

done The preliminary lsquomodelrsquo EBCs chosen for this study are Yb2Si2O7 Y2Si2O7 Sc2Si2O7 and

Lu2Si2O7 YAlO3 was also chosen because it is Si-free and has been included in a patent as a

potential EBC ceramic [115]

142 Objectives

This work is focused on exploring potential EBC ceramics First lsquomodelrsquo CMAS

interaction studies at 1500 degC for varying amounts of time were conducted on lsquomodelrsquo EBC

ceramics or dense polycrystalline spark plasma sintered (SPSed) pellets This was done with the

overall goal of providing insights into the chemo-thermal-mechanical mechanisms of these

interactions and to use this understanding to guide the design and development of CMAS-resistant

EBCs A comparison between Y-containing EBC ceramics viz YAlO3 and Y2Si2O7 and Y-free

EBC ceramics viz Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 and their high-temperature interactions with

CMAS are seen in Chapter 2 and 3 respectively [116117]

Chapter 4 uses the insights learned in Chapters 2 and 3 to explore lsquomodelrsquo EBC ceramics

of solid-solutions of Yb2Si2O7 and Y2Si2O7 or Yb(2-x)YxSi2O7 Two solid solutions Yb18Y02Si2O7

and Yb1Y1Si2O7 and their pure end components Yb2Si2O7 and Y2Si2O7 have been chosen to

explore their high temperature interactions with CMAS In this section three different CMAS

compositions are chosen with varying amounts of Ca and Si (CaSi of 076 044 and 010) to

determine how different compositions change the interaction with the same EBC ceramics The

17

thermal conductivity of these solid solution ceramics and the concept of low-thermal conductivity

thermal environmental barrier coatings (TEBCs) are explored in Chapter 5 [118119]

After completing lsquomodelrsquo experiments on dense polycrystalline EBC ceramic pellets a

few ceramics were air plasma sprayed (APS) as EBC coatings These APS EBCs were made at

Stony Brook University in collaboration with Professor Sanjay Sampathrsquos group In Chapter 6 the

focus will be on the coating interactions with CMAS and understanding the effect of the APS

coating microstructure (ie grain size porosity and splat boundaries)

18


ATTACK BY MOLTEN CMAS

This chapter was reproduced from a previously published article LR Turcer AR Krause

HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier coating ceramics for resistance

against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass Part I YAlO3 and γ-

Y2Si2O7rdquo Journal of the European Ceramic Society 38 3095-3913 (2018) [116]

21 Introduction

Based on the optical basicity (OB) concept (for more detail see Section 141) YAlO3 γ-

Y2Si2O7 β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 have been identified as promising CMAS-

resistant EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a

rough screening criterion based on purely chemical considerations and that the actual reactivity

will depend on various other factors including the nature of the cations in the EBC ceramics and

the CMAS composition Interactions of these five promising lsquomodelrsquo EBC ceramics (dense

polycrystalline ceramic pellets) with a lsquomodelrsquo CMAS at 1500 degC are studied in some detail The

overall goal is to provide insights into the chemo-thermo-mechanical mechanisms of these


EBCs It is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-Y2Si2O7 show

distinctly different behavior compared to the Y-free group of EBC ceramics viz β-Yb2Si2O7 β-

Sc2Si2O7 and β-Lu2Si2O7

Briefly Y-containing EBC ceramics show extensive reaction-crystallization and no grain-

boundary penetration of the CMAS glass In contrast the Y-free EBC ceramics show little to no

reaction-crystallization and extensive grain-boundary penetration resulting in a dilatation gradient

and a new type of lsquoblisterrsquo cracking damage The former group of EBC ceramics are presented in

this chapter and the latter group is presented in the next chapter

19

YAlO3 (yttrium aluminate perovskite or YAP) is a line compound of orthorhombic crystal

structure [120] with no phase transformation from room temperature up to its congruent melting

point of 1913 degC [121] Its average CTE is 6-7 x 10-6 degC-1 [120122] Youngrsquos modulus is 316 GPa

[123] and density is 535 Mgm-3 [122] Although the YAlO3 CTE is on the high side compared

to the CTE of SiC (47 x 10-6 degC-1) [16] the major CMC material its most attractive feature for

EBC application is that it is Si-free YAlO3 has been included in a patent as a potential EBC

ceramic [115] but there has been no significant research reported in the open literature on this

ceramic in the context of EBCs

In the case of γ-Y2Si2O7-based EBCs there have been limited studies on their high-

temperature interaction with CMAS [2569] Y2Si2O7 has five polymorphs [37] but the γ-Y2Si2O7

monoclinic phase is the most desirable for EBC application It has a melting point of 1775 degC

[124] average CTE of 39 x 10-6 degC-1 [125] Youngrsquos modulus of 155 GPa [125] and a density of

396 Mgm-3 [125] While achieving the γ-Y2Si2O7 polymorph in the deposition of EBCs is a

challenge and its temperature capability is relatively low γ-Y2Si2O7 has an excellent CTE-match

with SiC and it is also relatively lightweight

22 Experimental Procedure

221 Processing

The YAlO3 powder was prepared in-house by combining stochiometric amounts of Al2O3

(Nanophase Technologies Corporation Romeoville IL) and Y2O3 (Nanocerox Ann Arbor MI)

LiCl was added to this mixture in a 21 ratio of LiClAl2O3+Y2O3 to reduce the temperature

required to form the YAlO3 powder [126] The mixture was then ball-milled using ZrO2 media in

ethanol for 48 h The mixed slurry was then dried at 90 degC while being stirred The dry powder

20

mixture was placed in a Pt crucible and calcined at 1400 degC in air for 4 h in a box furnace (CM

Furnaces Inc Bloomfield NJ) to complete the solid-state reaction between Al2O3 and Y2O3 The

reacted mixture was washed at least four times with hot deuterium-depleted water and filtered to

remove the LiCl from the mixture The YAlO3 powder was then dried and crushed

The γ-Y2Si2O7 powder was also prepared in-house by combining stochiometric amounts

of Y2O3 (Nanocerox Ann Arbor MI) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)

respectively [127] This mixture was then ball-milled and dried using the same procedure

described above The dried powder mixture was placed in a Pt crucible for calcination at 1600 degC

in air for 4 h in the box furnace The resulting γ-Y2Si2O7 powder was then ball-milled for an

additional 24 h dried and crushed

The powders were then loaded into graphite dies (20mm diameter) lined with graphfoil and

densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA) in

an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating

rate 1600 degC hold temperature 5 min hold time and 100 degCmin-1 cooling rate The surfaces of

the resulting dense pellets (sim2mm thickness) were ground to remove the graphfoil and the pellets

were heat-treated at 1500 degC in air for 1 h (10 degCmin-1 heating and cooling rates) in the box

furnace The top surfaces of the pellets were polished to a 1-μm finish using standard

ceramographic polishing techniques for CMAS-interaction testing Some pellets were cut using a

low-speed diamond saw and the cross-sections were polished to a 1-μm finish

222 CMAS interactions

The composition of the CMAS used is (mol) 515 SiO2 392 CaO 41 Al2O3 and 52

MgO which is from a previous study [128] and it is close to the composition of the AFRL-03

21

standard CMAS (desert sand) Powder of this CMAS glass composition was prepared using a

procedure described elsewhere [7086] CMAS interaction studies were performed by applying the

CMAS powder paste (in ethanol) uniformly over the center of the polished surfaces of the YAlO3

and the γ-Y2Si2O7 pellets at sim15 mg cm-2 loading The specimens were then placed on a Pt sheet

with the CMAS-coated surface facing up and heat-treated in the box furnace at 1500 degC in air for

different durations (10 degC min-1 heating and cooling rates) The CMAS-interacted pellets were

then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm finish

In separate experiments the CMAS powder and the YAlO3 powder or the γ-Y2Si2O7

powder were mixed in 11 ratio by weight and ball-milled for 24 h using the procedure described

in Section 221 The resulting dry powder-mixtures were placed in Pt crucibles heat-treated in the

box furnace for 1500 degC in air for 24 h and crushed into fine powders

223 Characterization

The as-prepared YAlO3 and γ-Y2Si2O7 powders were characterized using an X-ray

diffractometer (XRD D8 Advance Bruker AXS Karlsruhe Germany) to check for phase purity

The heat-treated mixtures of YAlO3-CMAS and γ-Y2Si2O7-CMAS powders were also

characterized using XRD The phases present in the reaction products were identified using the

PDF2 database

The densities of the as-SPSed pellets were measured using the Archimedes principle with

distilled water as the immersion medium The polished cross-sections of the as-SPSed pellets were

thermally-etched at 1500 degC for 1 min (10 degC min-1 heating and cooling rates)

The cross-sections of the as-SPSed and CMAS-interacted pellets were observed in a

scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany or Helios 600

FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy (EDS) systems

22

(Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS

elemental maps particularly Ca and Si were also collected and used to determine CMAS

penetration into the pellets Cross-sectional SEM micrographs (3ndash4 per material) were used to

measure the average grain sizes (linear-intercept method) of the as-SPSed pellets

Transmission electron microscopy (TEM) specimens from specific locations within the

polished cross-sections of the CMAS-interacted pellets were prepared using focused ion beam

(FIB Helios 600 FEI Hillsboro Oregon USA) and in situ lift-out These samples were then

examined using a TEM (2100 F JEOL Peabody MA) equipped with an EDS system (Inca

Oxford Instruments Oxfordshire UK) operated at 200 kV accelerating voltage Selected-area

electron diffraction patterns (SAEDPs) from various phases in the TEM micrographs were

recorded and indexed using standard procedures

23 Results

231 Polycrystalline Pellets

Figures 9A and 9B show a SEM micrograph and a XRD pattern of SPSed YAlO3 pellet

respectively The density of the pellet is 522 Mgmminus3 (sim97) and the average grain size is sim8

μm The indexed XRD pattern shows the presence of some Y3Al5O12 (yttrium aluminum garnet or

YAG) and Y4Al2O9 (yttrium aluminum monoclinic or YAM) in the pellet It is not unusual to have

YAG or YAM impurities in YAlO3 (YAP) ceramics due to slight shifts in the stoichiometry during

processing Also it is difficult to obtain phase pure YAlO3 powders using conventional ceramic-

powder processing

23



present)

Figures 10A and 10B are a SEM micrograph and a XRD pattern of a SPSed γ-Y2Si2O7

pellet respectively The density of the pellet is 394 Mgmminus3 (sim99) and the average grain size

is sim31 μm Some cracking is observed in these pellets The indexed XRD pattern shows phase-

pure γ-Y2Si2O7


XRD pattern showing phase-pure γ-Y2Si2O7

A B

B A

24

232 YAlO3-CMAS Interactions

Figures 11A and 11B are cross-sectional SEM micrographs showing interaction between

the YAlO3 ceramic and CMAS at 1500 degC for 1 min and 1 h respectively and the corresponding

EDS elemental compositions of the marked regions are presented in Table 3 YAlO3 appears to

have reacted with the CMAS within 1 min forming two reaction layers (sim30 μm total thickness)

The top layer (region 2) consists of vertically-aligned needle-shaped grains containing Y Ca Si

and O primarily and the composition roughly corresponds to Y8Ca2(SiO4)6O2 apatite with some

Al in solid solution (Y-Ca-Si apatite (ss)) Some CMAS glass is also observed in that layer

although it appears to contain excess Y and Al (region 1) The second layer (region 3) contains

lsquoblockyrsquo grains and they have a composition presented in Table 3 It is assumed to be a YAG (ss)

phase with Ca and Si in solid solution The base YAlO3 pellet (region 4) has a Y-rich

composition




boxes in (B) indicate regions from where the TEM specimens were extracted using the FIB

A B

Figure 12A

Figure 12B

25

The total thickness of the reaction zone increases up to sim40 μm after 1-h heat-treatment at

1500 degC (Figure 11B) and it appears to have three layers The top layer (region 5) still consists

of needle-shaped Y-Ca-Si apatite (ss) phase which is confirmed using SAEDP in the TEM (Figure

12A) The second layer (region 6) still contains the YAG (ss) phase whereas the third layer

(region 7) is Si-free and it also is assumed to be a YAG (ss) phase The base YAlO3 pellet

(regions 8 and 11) is still Y-rich composition while the minor lsquograyrsquo inclusions (regions 9 and

10) appear to be a Y-rich YAG phase (see XRD in Figure 9B)



ideal compositions of the three main phases and CMAS are also included

Region Y Al Ca Si Mg Phase

1 18 23 23 31 5 CMAS Glass

2 47 2 15 36 - Y-Ca-Si Apatite (ss)

3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)

- 79 376 495 50 Original CMAS Glass

Figures 12A and 12B are TEM micrographs from top and bottom regions as indicated in

Figure 11B and Table 4 includes the EDS elemental compositions of the marked regions The

indexed SAEDP (Figure 12A inset) confirms that the region 1 is Y-Ca-Si apatite (ss) phase While

26

region 2 has significant amounts of Ca and Si regions 3-7 have near-ideal YAl ratio of YAG

with some Ca in solid solution Thus the SEM and the TEM characterization results are consistent






respectively


TEM micrographs in Figure 12 of YAlO3 interaction with CMAS at 1500 degC for 1 h


1 46 - 12 42 - Y-Ca-Si Apatite (ss)

2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27

Upon further interaction of YAlO3 with CMAS glass for 24 h at 1500 degC the reaction-

layer thickness has doubled (sim80 μm) Figure 13A is a SEM micrograph of the entire YAlO3 pellet

showing no evidence of lsquoblisteringrsquo cracking that is typically observed in Y-free (β-Yb2Si2O7 β-

Sc2Si2O7 and β-Lu2Si2O7) EBC ceramics in Chapter 3 [117119] Figure 13B is a higher-

magnification SEM image of the reaction zone and Figures 13C and 13D are corresponding Ca

and Si elemental EDS maps respectively

28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29

The chemical composition of the different regions in the higher-magnification SEM images

in Figures 14A and 14B from the top and bottom (marked in Figure 13B) respectively are given

in Table 5 From these results the remnants of the three reaction layers can be seen with the top

Si-rich layer being mostly Y-Ca-Si apatite (ss) the middle Ca-lean layer being mostly YAG (ss)

and the bottom layer being a mixture of Y-Ca-Si apatite (ss) and YAG (ss) The boundary between

the bottom reaction layer and the base YAlO3 is still sharp It also appears that all the CMAS glass

has been consumed during its reaction with YAlO3 as no obvious CMAS pockets are found




elemental compositions were obtained using EDS and they are reported in Table 5


SEM micrographs in Figure 14 of YAlO3 interaction with CMAS at 1500 degC for 24 h



2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30

Figure 15 presents a XRD pattern of the YAlO3-CMAS powder mixture heat-treated at

1500 degC for 24 h The XRD results confirm the presence of the Y-Ca-Si apatite (ss) and YAG

phases along with some unreacted YAlO3 and YAM phases



(YAM) in addition to unreacted YAlO3

233 Y2Si2O7-CMAS Interactions

Figure 16 is a cross-sectional SEM micrograph showing interaction between γ-Y2Si2O7

EBC ceramic and CMAS at 1500 degC for 1 h and the EDS elemental compositions of the marked

regions are presented in Table 6 The γ-Y2Si2O7 appears to have reacted with CMAS glass to a

depth of sim400 μm from the top which is about an order-of-magnitude deeper than in the YAlO3

case under the same conditions The reaction zone has two layers The top layer contains only

needle-shaped Y-Ca-Si apatite (ss) and CMAS glass In contrast to the YAlO3 case a significant

amount of CMAS glass remains on top which is Y-enriched and Ca-depleted The second layer

(sim150 μm) comprises Y-Ca-Si apatite (ss) grains primarily with some CMAS glass pockets

31



were measured by EDS and they are reported in Table 6


SEM micrograph in Figure 16 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 1 h


1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

Figure 17A shows cross-section SEM micrograph of the entire γ-Y2Si2O7 pellet after

CMAS interaction at 1500 degC for 24 h Similar to the YAlO3 case no lsquoblisteringrsquo cracks are

observed The higher magnification SEM image (Figure 17B) shows that the total reaction layer

thickness is sim300 μm and the amount of CMAS glass remaining at the top has decreased compared

with the 1-h case The thickness of the bottom Y-Ca-Si apatite (ss) layer has increased to sim200

μm indicating the consumption of the CMAS glass and the growth of the Y-Ca-Si apatite (ss)

layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33

Figures 18A and 18B shows the top and the bottom area respectively of the reaction zone

at a higher magnification The compositions of the Y-Ca-Si apatite (ss) and the CMAS glass (Table

7) appear to be very similar to the ones in the 1-h case (Table 6)






SEM micrographs in Figure 18 of γ-Y2Si2O7 interaction with CMAS at 1500 degC for 24 h


1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass


Figure 19 presents a XRD pattern of the γ-Y2Si2O7-CMAS powder mixture heat-treated at

1500 degC for 24 h confirming the presence of the Y-Ca-Si apatite (ss) phase along with some

unreacted γ-Y2Si2O7

A B

34



24 Discussion

The results from this study show that the lsquomodelrsquo Y-bearing YAlO3 and γ-Y2Si2O7 EBC

ceramics react with the lsquomodelrsquo CMAS glass despite the fact that their OBs are quite similar

resulting in extensive reaction-crystallization but no lsquoblisterrsquo cracking The reaction-

crystallization propensity is attributed to the strong affinity between Y in the EBC ceramics and

the Ca in the CMAS highlighting the limitation of the use of the OBs-difference screening

criterion

In the case of the YAlO3 EBC ceramic it reacts with the CMAS glass very rapidly It

appears that the first reaction product is vertically-aligned needle-shaped Y-Ca-Si apatite (ss)

Similar Y-Ca-Si apatite (ss) formation has been observed in the cases of 2ZrO2∙Y2O3 [94129130]

and rare-earth zirconate [71128131ndash133] TBCs interacting with CMASs of wide range of

compositions This typically occurs by the dissolution of the ceramic in the CMAS glass

supersaturation and reaction-crystallization of needle-shaped grains of Y-Ca-Si apatite (ss) This

35

same mechanism is likely to be responsible in the case of YAlO3 dissolution of YAlO3 in the

CMAS glass and reaction-crystallization of Y-Ca-Si apatite (ss) from the supersaturated CMAS

glass melt The formation of the YAG (ss) layer containing Ca and Si in solid solution appears to

be related to inadequate access to the CMAS glass precluding further Y-Ca-Si apatite (ss)

formation but Y-depletion can still occur Solid solutions of YAG Y(3-x)CaxAl(5-x)SixO12 are also

known to exist where Ca2+ and Si4+ co-substitute for Y3+ and Al3+ in the octahedral and tetrahedral

sites respectively [134] Further down in the third layer the YAG (ss) phase is devoid of Si which

could be the result of no access to the CMAS glass In this context YAG (ss) is known to have

appreciable solubility for Ca where Ca2+ occupies Y3+ sites according to the following defect

reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)

Rapid reaction with the CMAS and the formation of a relatively thin protective reaction

layer could be advantageous in YAlO3 EBCs for CMAS resistance Also the silica activity of

YAlO3 is zero which is also a big advantage over Si-containing EBC ceramics from the standpoint

of high-temperature high-velocity water-vapor corrosion Finally the very high temperature-

capability and the potential low-cost of YAlO3 makes it an attractive EBC ceramic However the

moderate CTE mismatch of YAlO3 with SiC-based CMCs is a disadvantage but CTE-mismatch-

induced cracking at sharp interfaces can be mitigated by including a CTE-graded bond-coat

between the CMC and the YAlO3 EBC

γ-Y2Si2O7 EBC ceramic also reacts with the chosen CMAS but the nature of the reaction

is quite different from that observed in the case of YAlO3 The reaction zone is almost an order-

of-magnitude thicker in the case of γ-Y2Si2O7 compared to that in YAlO3 and there is significant

amount of CMAS remaining after 24 h heat-treatment (at 1500 degC) in the former This is primarily

36

because YAlO3 is Si-free resulting in more rapid consumption of the CMAS The mechanism of

reaction-crystallization of the needle-shaped Y-Ca-Si apatite (ss) in γ-Y2Si2O7 appears to be

similar to that in YAlO3 and also in Zr-containing ceramics However unlike YAlO3 where YAG

(ss) phases form underneath the Y-Ca-Si apatite (ss) layer no other phases form in the case of γ-

Y2Si2O7 This is consistent with what has been observed by others [2569]

While the CTE match with SiC is very good and it is relatively lightweight the formation

of the significantly thicker reaction layer in γ-Y2Si2O7 is a concern making this EBC ceramic less

effective against high-temperature CMAS attack Also the deposition of phase-pure γ-Y2Si2O7

EBCs will be a significant challenge because Y2Si2O7 can exist as four other undesirable

polymorphs Furthermore the temperature capability of γ-Y2Si2O7 is limited to sim1700 degC and its

silica activity is very high Considering all these drawbacks overall γ-Y2Si2O7 may not be an

attractive candidate ceramic for EBCs

25 Summary

Here we have systematically studied the high-temperature (1500 degC) interactions between

two promising dense polycrystalline EBC ceramics YAlO3 (YAP) and γ-Y2Si2O7 and a CMAS

glass Despite the small differences in the OBs of the two EBC ceramics and that of the CMAS

they both react with the CMAS In the case of the Si-free YAlO3 the reaction zone is small and it

comprises three regions of reaction-crystallization products (i) needle-like Y-Ca-Si apatite (ss)

grains (ii) blocky grains of YAG (ss) and (iii) a mixture of Y-Ca-Si apatite (ss) and YAG (ss)

blocky grains The YAG (ss) is found to contain Ca Al and Si in solid solution In contrast only

Y-Ca-Si apatite (ss) needle-like grains form in the case of Si-containing γ-Y2Si2O7 and the

reaction zone is an order-of magnitude thicker These CMAS interactions are analyzed in detail

37

and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb2Si2O7

β-Sc2Si2O7 and β-Lu2Si2O7) in Chapter 3 [117119] This is attributed to the presence of the Y in

the YAlO3 and γ-Y2Si2O7 EBC ceramics

38


MOLTEN CMAS

This chapter was modified from previously published articles along with unpublished data

LR Turcer AR Krause HF Garces L Zhang and NP Padture ldquoEnvironmental-barrier

coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS)

glass Part II β-Yb2Si2O7 and β-Sc2Si2O7rdquo Journal of the European Ceramic Society 38 3914-

3924 (2018) [117] and LR Turcer and NP Padture ldquoTowards multifunctional thermal

environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution ceramicsrdquo

Scripta Materialia 154 111-117 (2018) [119]

31 Introduction

In Chapter 2 it is found that the Y-containing group of EBC ceramics viz YAlO3 and γ-

Y2Si2O7 show distinctly different behavior compared to the Y-free group of EBC ceramics viz β-

Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 Briefly Y-containing EBC ceramics show extensive

reaction-crystallization and no grain-boundary penetration of the CMAS glass [116] In contrast

the Y-free EBC ceramics show little to no reaction-crystallization and extensive grain-boundary

penetration resulting in a dilatation gradient and a new type of lsquoblisterrsquo cracking damage

β-Yb2Si2O7 has a melting point of 1850 degC [136] average CTE of 40 x 10-6 degC-1 [137]

Youngrsquos modulus of 205 GPa [33] density of 613 Mgm-3 [34] High-temperature interactions

between Yb2Si2O7 (pellets or powders or coatings) and CMAS have been studied by others [2533ndash

3669] Stolzenburg et al [33] and Liu et al [25] have shown limited reaction between Yb2Si2O7

(pellets andor powders) and CMAS However The testing temperature used by Stolzenburg et al

[33] is limited to 1300 degC and the density of the β-Yb2Si2O7 pellet is not specified Interestingly

the same authors report extensive CMAS infiltration and reaction with porous air-plasma sprayed

(APS) Yb2Si2O7 EBC at 1300 degC [34] Liu et al [25] conducted their tests on Yb2Si2O7 pellets that

are sim25 porous at 1400 degC in water vapor environment complicating the interpretation of the

results Ahlborg et al [69] reported extensive reaction between Yb2Si2O7 pellets and CMAS at

39

1500 degC However the density of the pellets is not reported and their microstructures appear to

be heterogeneous Zhao et al [36] reported reaction between dense Yb2Si2O7 APS EBC and

CMAS at a lower temperature of 1300 degC However the APS Yb2Si2O7 EBC contains appreciable

quantities of Yb2SiO5 making these EBCs two-phase thus complicating the issue Finally

Poerschke et al [35] have studied the interaction between Yb2Si2O7 EBC deposited using electron-

beam directed-vapor deposition (EB-DVD) and CMAS at 1300 degC and 1500 degC However in their

experiments the EBC is buried under a Yb4Hf3O12 TBC or a bi-layer Yb4Hf3O12Yb2SiO5 TEBC

making these interactions indirect and strongly influenced by the TBC or the TEBC [35]

β-Sc2Si2O7 has a melting point of 1860 degC [138] average CTE of 54 x 10-6 deg C-1 [137]

Youngrsquos modulus of 200 GPa [139] and density of 340 Mgm-3 [138] There has been only one

report in the open literature on the high-temperature interaction between Sc2Si2O7 and CMAS Liu

et al [25] conducted their tests on a sim19 porous Sc2Si2O7 pellet at 1400 degC in water vapor

environment They showed penetration of the molten CMAS in the porous pellet and some

reaction resulting in the formation of Ca3Sc2Si3O12 However the highly porous nature of the pellet

precludes proper understanding of the high-temperature interactions of Sc2Si2O7 with CMAS

β-Lu2Si2O7 has a melting point of 2000 degC [140] average CTE of 38-39 x 10-6 degC-1

[137141] Youngrsquos modulus of 178 GPa [142] and density of 625 Mgm-3 [143] Liu et al [25]

is the only report in the open literature on the high-temperature interaction between Lu2Si2O7 and

CMAS They showed penetration of the molten CMAS in the porous pellet and a limited reaction

between Lu2Si2O7 pellets and CMAS However the tests were conducted on a sim25 porous

Lu2Si2O7 pellet at 1400 degC in water vapor environment which complicates the interpretation of

the results [25]

40

Thus the objective of this study is to use fully dense phase-pure β-Yb2Si2O7 β-Sc2Si2O7

and β-Lu2Si2O7 lsquomodelrsquo EBC ceramic pellets and to investigate their interaction with a lsquomodelrsquo

CMAS at 1500 degC in air The overall goal is to provide insights into the thermo-chemo-mechanical

mechanisms of these interactions and to use this understanding to guide the design and

development of future CMAS-resistant EBCs


321 Processing

The β-Yb2Si2O7 powders were obtained commercially from Oerlikon Metco (AE 11073

Oerlikon Metco Westbury NY)

The β-Sc2Si2O7 powder was prepared in-house by combining stochiometric amounts of

Sc2O3 (Reade Advanced Materials Riverside RI) and SiO2 (Atlantic Equipment Engineers

Bergenfield NJ) powders [144] The β-Lu2Si2O7 powder was prepared in-house by combining

stochiometric amounts of Lu2O3 (Sigma Aldrich St Louis MO) and SiO2 (Atlantic Equipment

Engineers Bergenfield NJ) powders The powder mixtures were then ball-milled using ZrO2 balls

media in ethanol for 48 h The mixed slurries were then dried while being stirred The dried

powder-mixtures were placed in Pt crucibles for calcination at 1600 degC for 4 h in air in a box

furnace (CM Furnaces Inc Bloomfield NJ) The resulting β-Sc2Si2O7 powder and β-Lu2Si2O7

powder were then ball-milled for an additional 24 h and dried

The powders were then densified into 20 mm diameter polycrystalline pellets using spark

plasma sintering (SPS) like the Y-containing EBC ceramics from the previous chapter More

details can be found in Section 221

41

In addition the β-Yb2Si2O7 powder was mixed with 1 vol CMAS powder and ball-milled

for 48 h The powder mixture was then dried and dry-pressed into pellets (25mm diameter)

followed by cold isostatic pressing (AIP Columbus OH) at 275 MPa The pellets were

pressureless sintered at 1500 degC in air for 4 h in the box furnace The thickness of the sintered

pellets was sim25 mm

The top surfaces of the pellets were polished to a 1-μm finish using standard ceramographic

polishing techniques for CMAS-interaction testing Some pellets were cut through the center using

a low-speed diamond saw and the cross-sections were polished to a 1-μm finish In some

instances the polished cross-sections were etched using dilute HF for 10 min

322 CMAS Interactions

CMAS interaction experiments were preformed like the CMAS interaction with Y-

containing EBC ceramics in Chapter 2 Briefly CMAS (515 SiO2 392 CaO 41 Al2O3 and 52

MgO in mol) [128] was applied uniformly over the center of the polished surfaces of pellets (β-

Yb2Si2O7 β-Sc2Si2O7 β-Lu2Si2O7 and β-Yb2Si2O7 + 1 vol CMAS) at 15 mgcm-2 loading The

specimens were then heat-treated in the box furnace at 1500 degC in air for different durations (10

degCmin-1 heating and cooling rates) and then cross-sectioned to observe the interaction zone

CMAS powder and Y-free EBC ceramic powders (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7) were

mixed in 11 ratio by weight ball-milled heat-treated for 24 h in air at 1500 degC and crushed into

fine powders Please see Section 222 for more details


The characterization for these experiments is similar to the Y-containing EBC ceramics

found in Chapter 2 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)

42

was conducted on the as-received β-Yb2Si2O7 powder the as-prepared β-Sc2Si2O7 and β-Lu2Si2O7

powders and the heat-treated mixtures Densities of the as-SPSed and pressureless-sintered pellets

were measured using the Archimedes principle (immersion medium = distilled water)

Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were

used to observe the cross-sections of the as-SPSed as-pressureless-sintered and CMAS-interacted

pellets Transmission electron microscopy (TEM) equipped with an EDS system was used to

observe specific locations within the cross-sections of the CMAS-interacted pellets These samples

were prepared using focused ion beam and in-situ lift-out

33 Results


Figures 20A and 20B show a SEM micrograph and a XRD pattern of SPSed β-Yb2Si2O7

pellet respectively The density of the pellet is 608 Mgm-3 (99) and the average grain size is

sim10 μm The indexed XRD pattern shows phase-pure β-Yb2Si2O7


indexed XRD pattern showing phase-pure β-Yb2Si2O7

A B

43

Figures 21A and 21B show a SEM micrograph and a XRD pattern of SPSed β-Sc2Si2O7


sim8 μm The indexed XRD pattern shows phase-pure β-Sc2Si2O7


XRD pattern showing phase-pure β-Sc2Si2O7

Figures 22A and 22B show a SEM micrograph and a XRD pattern of SPSed β-Lu2Si2O7


sim8 μm The indexed XRD pattern shows phase-pure β-Lu2Si2O7

B A

44


indexed XRD pattern showing phase-pure β-Lu2Si2O7

332 Yb2Si2O7-CMAs Interactions

Figure 23A is a cross-sectional SEM image of a β-Yb2Si2O7 pellet that has interacted with

CMAS at 1500 degC for 1 h A thick CMAS layer on top is observed and its interaction with the β-

Yb2Si2O7 pellet appears to be limited The latter is confirmed in Figures 23B and 23C which are

higher magnification SEM image and corresponding Ca elemental EDS map respectively of the

interaction zone The EDS elemental compositions of regions 1 to 4 are reported in Table 8 The

amount of Yb in the CMAS glass (region 1) is sim8 at which is similar to what has been observed

for Y in the case of YAlO3 and γ-Y2Si2O7 EBC ceramics [116] despite the somewhat higher

solubility of Y3+ in the CMAS glass Region 2 has a composition similar to that of Yb-Ca-Si

apatite solid solution (ss) phase which is confirmed using the indexed SAEDP (Figure 24A) The

distribution of Yb-Ca-Si apatite (ss) phase (Ca-containing grains) is clearly seen in Figure 23C

which does not appear to form a continuous layer Thus the amount of Yb-Ca-Si apatite (ss)

formed is significantly less than that in the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) in

Chapter 2 Region 3 appears to be reprecipitated Ca-containing β-Yb2Si2O7 while region 4 is

A B

45

base β-Yb2Si2O7 Also CMAS glass can be found in pockets in the base β-Yb2Si2O7 below the

Yb-Ca-Si apatite (ss) in Figure 24B which is typically not the case in Y-containing EBC ceramics

[116]






the TEM specimens were extracted using the FIB

A

B C

Figure 23B

Figure 24A

Figure 24B

46



ideal compositions of the two main phases and the CMAS are also included

Region Yb Al Ca Si Mg Phase

1 8 5 27 57 3 CMAS Glass

2 47 - 13 41 - Yb-Ca-Si Apatite (ss)

3 46 - 1 53 - β-Yb2Si2O7 (Re-precipitated)

4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions

500 - 125 375 - Yb8Ca2(SiO4)6O2 Apatite

500 - - 500 - β-Yb2Si2O7 (Base)





are marked Transmitted beam and zone axis are denoted by lsquoTrsquo and lsquoBrsquo respectively

Upon further interaction between β-Yb2Si2O7 and CMAS glass at 1500 degC for 24 h lsquoblisterrsquo

cracks form under the CMAS deposit (Figure 25A) but the occurrence of Yb-Ca-Si apatite (ss)

phase is rare (see Figures 25B and 25C and Table 9) The latter is confirmed by XRD results in

Figure 26 from β-Yb2Si2O7-CMAS powder mixture heat-treated at 1500 degC for 24 h Also no

CMAS glass is found on top which is the opposite of the γ-Y2Si2O7 case [116] Throughout the

pellet small Ca EDS signal is detected (Figure 25C) and CMAS glass pockets are found (Figure

A B

47

27) with the latter containing sim10 at Yb (Table 9) This indicates that there is reaction between

β-Yb2Si2O7 and the CMAS glass but there is little reprecipitation of β-Yb2Si2O7 or reaction-

crystallization of Yb-Ca-Si apatite (ss) The Yb-saturated CMAS glass appears to have penetrated

throughout the pellet most likely via the grain-boundary network as the pellet is fully dense The

higher-magnification SEM image of the lsquoblisterrsquo cracks in Figure 25D shows that the cracks are

wide and blunt reminiscent of typical high-temperature cracking observed in ceramics [145] This

indicates that the lsquoblisterrsquo cracks formed at a high temperature and not during cooling

48






was extracted using the FIB

A B

C

D

Figure 25B

Figure 25D

Figure 27

49



CMAS at 1500 degC for 24 h



2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass






obtained using EDS and it is reported in Table 9

50

Figures 28Andash28D show the evolution of the lsquoblisterrsquo cracking in β-Yb2Si2O7 pellets (sim2

mm thickness) after interaction with CMAS glass at 1500 degC At 1-h heat-treatment no significant

damage is visible in the optical micrograph collage of the whole pellet (Figure 28A) and same is

the case at 2 h (not shown here) At 3 h (Figure 28B) lsquoblisterrsquo cracks start to appear beneath the

interaction zone At 6 h (Figure 28C) the lsquoblisterrsquo cracks are fully formed and remain at 24 h

(Figure 28D) Similar lsquoblisterrsquo cracks are also observed in thinner pellets (sim1 mm thickness) in

Figure 28E





micrographs is epoxy from the sample mounting

Figures 29A and 29B are SEM micrographs of β-Yb2Si2O7 pellet (sim2 mm thickness) after

interaction with the CMAS glass at 1500 degC for 6 h from the top and the bottom regions of the

A

B

C

D

E

51

pellet respectively The HF-etching reveals gradient in the CMAS glass where there is large

amount of CMAS near the top of the pellet and hardly any CMAS glass near the bottom



333 Sc2Si2O7-CMAS Interactions

Figures 30A and 30B are cross-sectional SEM micrograph and corresponding Ca elemental

EDS map respectively of β-Sc2Si2O7 pellet that has interacted with CMAS glass at 1500 degC for 1

h Region 1 is CMAS glass with sim9 at Sc (Table 10) regions 2 and 3 are reprecipitated β-

Sc2Si2O7 grains containing a small amount of Ca and region 4 is base β-Sc2Si2O7 No Sc-Ca-Si

apatite (ss) could be detected This is in contrast with the β-Yb2Si2O7 case where some reaction-

crystallized Yb-Ca-Si apatite (ss) is found

A B

52



where elemental compositions were obtained using EDS and they are reported in Table 10


the SEM micrograph in Figure 30A of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 1 h

Region Sc Al Ca Si Mg Phase

1 9 6 31 50 4 CMAS Glass

2 51 - 1 48 - β-Sc2Si2O7 (Reprecipitated)


4 51 - - 49 - β-Sc2Si2O7 (Base)

After 24-h interaction between β-Sc2Si2O7 pellet and CMAS glass at 1500 degC there is no

CMAS glass remaining on top but lsquoblisterrsquo cracks are observed (Figure 31A) similar to those in

β-Yb2Si2O7 Once again no reaction-crystallized Sc-Ca-Si apatite (ss) is detected (Figures 31B

and 31C)

A B

53




where the TEM specimen was extracted using the FIB

A B

C

Figure 31B

Figure 31C

Figure 32A

54

TEMSAEDP (Figure 32A) and XRD (Figure 33) results confirm that β-Sc2Si2O7 is the

only crystalline phase and there are Sc-bearing CMAS glass pockets in the interior of the pellet

(Figures 32B and 32C) Similar to the β-Yb2Si2O7 case the Sc-saturated CMAS glass appears to

have penetrated throughout the pellet Once again this is most likely via the grain-boundary

network as the β-Sc2Si2O7 pellet is also fully dense







reported in Table 11

Figure 32B

A

A

B

C

55


the TEM micrograph in Figure 32B of β-Sc2Si2O7 interaction with CMAS at 1500 degC for 24 h


1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)


1500 degC for 24 h showing only the presence of unreacted β-Sc2Si2O7

334 Lu2Si2O7-CMAS Interactions

Figure 34A is a cross-sectional SEM micrograph of the entire CMAS-interacted zone in

the β-Lu2Si2O7 pellet at 1500 degC for 1 h A cross-sectional SEM micrograph of the pellet thickness

in the CMAS-interacted zone can be seen in Figure 34B Figures 34D and 34F are cross-sectional

SEM micrographs and Figures 34E and 34G are their corresponding Ca elemental EDS maps

respectively CMAS glass is not found on the surface of the β-Lu2Si2O7 pellet after 1 h at 1500 degC

Instead pockets of CMAS are found in-between grains and in triple junctions which can be seen

in regions 3 ndash 6 (Table 12) and lsquoblisterrsquo cracks are observed near the surface of the pellet No

56

Lu-Ca-Si apatite (ss) could be detected This is similar to the β-Sc2Si2O7 case and in contrast with

the β-Yb2Si2O7 case where some reaction-crystallized Yb-Ca-Si apatite (ss) is found








A

B

D

C

E

F G

Figure 34C Figure 34B

Figure 34D

Figure 34F

57


the SEM micrograph in Figure 34A of β-Lu2Si2O7 interaction with CMAS at 1500 degC for 1 h

Region Lu Al Ca Si Mg Phase

1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7

After 24 h at 1500 degC the lsquoblisterrsquo cracks are more prevalent which can be seen in Figure

35A These lsquoblisterrsquo cracks can be seen throughout the thickness of the pellet A noticeable change

in porosity is seen from the top to the bottom of the β-Lu2Si2O7 pellet This change in porosity can

also be seen in Figure 36 from the CMAS-interacted region (left) to the edge of the pellet (right)

Figures 36B and 36C are cross-sectional images taken from regions in the CMAS-interacted zone

(close to the bottom of the pellet) and away from the CMAS-interacted zone (close to the edge of

the pellet) respectively

Like in the β-Sc2Si2O7 Lu-Ca-Si apatite (ss) was not found in the β-Lu2Si2O7 pellets XRD

(Figure 36) confirms that β-Lu2Si2O7 is the only crystalline phase Similar to both β-Yb2Si2O7 and

β-Sc2Si2O7 the CMAS glass appears to have penetrated through the pellet Once again this is most

likely via the grain-boundary network as the β-Lu2Si2O7 pellet is also fully dense

58



region from where (B) was collected (C) EDS elemental Ca map corresponding to (B)

A

B

C

Figure 35B

59




zone close to the edge of the pellet


at 1500 degC for 24 h showing only the presence of unreacted β-Lu2Si2O7

A

B C

60

34 Discussion

In stark contrast with the Y-containing EBC ceramics (YAlO3 and γ-Y2Si2O7) [116] the

reaction-recrystallization of apatite (ss) is minimal in β-Yb2Si2O7 and non-existent in β-Sc2Si2O7

and β-Lu2Si2O7 This is consistent with the fact that Y3+ (0900 Aring) with its larger ionic radius than

those of Sc3+ (0745 Aring) Lu3+ (0861 Aring) and Yb3+ (0868 Aring) has stronger propensity for Ca and

provides a higher driving force for the reaction-crystallization of apatite (ss) [128146147] Instead

of reaction-crystallization the CMAS glass appears to penetrate the grain boundaries of the dense

β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 EBC ceramic pellets Assuming the glass is in chemical

equilibrium with the crystal the driving force for penetration of molten glass into grain boundaries

in ceramics is reduction in the total energy of the system due to the formation of two glassceramic

interfaces from one ceramicceramic interface typically a high-angle grain boundary [148ndash150]

120574119866119861 gt 2120574119868 (Equation 6)

where γGB is the grain-boundary energy and γI is the ceramicglass interface energy The lsquostuffingrsquo

of the grain boundaries by CMAS glass results in the dilatation of the ceramic However unlike

porous ceramics (eg TBCs) where penetration of molten CMAS glass is very rapid (within

minutes at 1500 degC) its grain boundary penetration in dense ceramics is a very slow process

Therefore the top region has more CMAS than the bottom region as confirmed in Figure 29 This

results in a dilatation gradient where the top region wants to expand compared to the bottom

unaffected region as depicted schematically in Figure 38A But the constraint provided by the

unpenetrated (undilated) base material creates effective compression in the top dilated layer This

compression is likely to build up as the top dilated layer thickens albeit some relaxation due to

creep When the top dilated layer is sufficiently thick with increasing heat-treatment duration (eg

3 h at 1500 degC for β-Yb2Si2O7 (Figure 28)) the built-up compressive strain in that layer appears

61

to cause the lsquoblisterrsquo cracking perhaps by a mechanism akin to buckling of compressed films

(Figure 38B) [151] The wide and blunt nature of the lsquoblisterrsquo cracks confirms that the cracking

occurred at high temperature as hypothesized and not during cooling to room temperature



top dilated layer

It appears that the genesis of this new type of lsquoblisterrsquo cracking damage mode in EBC

ceramics subjected to CMAS attack is the slow buildup of the dilatation gradient and possibly

inadequate creep relaxation of the built-up compressive strain While full understanding of this

phenomenon is lacking at this time in order to address this issue and mitigate the lsquoblisterrsquo cracking

damage a new approach is explored mdash add a small amount of CMAS glass to the EBC ceramic

powders before sintering This CMAS glass is expected to segregate at grain boundaries in the

sintered EBC ceramics and its lsquosoftrsquo nature at high temperatures will accomplish two goals (i)

facilitate relatively rapid penetration of the deposited CMAS glass along grain boundaries thereby

reducing the severity of the dilatation gradient and (ii) facilitate rapid creep relaxation of the

compression To that end 1 vol CMAS glass powder was mixed in with the β-Yb2Si2O7 powder

before sintering as a case study Figures 39A and 39B are the SEM micrograph and corresponding

A

B

62

Ca elemental EDS map respectively of the β-Yb2Si2O71 vol CMAS pellet (polished and etched

cross-section) showing a near-full density (588 Mgmminus3 or sim96) equiaxed microstructure

(average grain size sim20 μm) Somewhat uniform distribution of CMAS glass can also be seen in

Figure 39B


Yb2Si2O71 vol CMAS pellet and (B) corresponding EDS elemental Ca map

Figure 40A is an optical-micrograph collage of the whole pellet after its interaction with

CMAS glass deposit on top at 1500 degC for 24 h where no evidence of lsquoblisterrsquo cracks can be found

Figure 40B is a SEM micrograph of the region marked in Figure 40A once again showing no

lsquoblisterrsquo cracks Figures 40C and 40D are a higher magnification SEM image and its corresponding

Ca elemental EDS map showing some Yb-Ca-Si apatite (ss) formation and minor cracks (sharp

narrow) during cooling due to CTE mismatch at the surface

A B

63





marked by the dashed box in (B) and (D) corresponding EDS elemental Ca map

A

B C

D

Figure 40B

Figure 40C

64

These results clearly demonstrate the success of this approach in mitigating the lsquoblisterrsquo

cracking damage mode in β-Yb2Si2O7 EBC ceramics and it is likely to work in β-Sc2Si2O7 β-

Lu2Si2O7 and other EBC ceramics as well Most importantly the amount of CMAS glass additive

needed is very small (1 vol) which is unlikely to affect other properties of EBC ceramic

significantly Thus for EBC ceramics where reaction-crystallization upon interaction with CMAS

glass does not occur the mitigation of the lsquoblisterrsquo cracking damage using this approach is very

attractive

In the case of β-Yb2Si2O7 its good CTE match with SiC and high-temperature capability

are advantages However its high silica activity is a disadvantage Also APS deposition of phase-

pure β-Yb2Si2O7 can be a challenge where the substrate needs to be held at sim1000 degC in a furnace

during APS deposition [43] In the case of β-Sc2Si2O7 it is lightweight in addition to having good

CTE match with SiC and high temperature capability β-Lu2Si2O7 also has a good CTE match and

high temperature capabilities But the high silica activity and high cost are disadvantages for both

β-Sc2Si2O7 and β-Lu2Si2O7 and the challenges associated with the APS deposition of phase-pure

β-Sc2Si2O7 and β-Lu2Si2O7 are not known

Finally while the new damage mode of lsquoblisterrsquo cracking is seen in EBC ceramic pellets

in this study it is likely to persist in actual EBCs on CMCs This is because the CMC substrate

with its very high stiffness is likely to provide similar if not greater constraint as the unpenetrated

(undilated) bottom part of the ceramic pellet Thus the lsquoblisterrsquo cracking damage mode is likely to

be important in actual EBCs on CMCs Furthermore the approach demonstrated here for the

mitigation of lsquoblisterrsquo cracking in pellets should also work in actual EBCs on CMCs but that

remains to be demonstrated

65

35 Summary

Here we have systematically studied the high-temperature (1500 degC) interactions of three

promising dense polycrystalline EBC ceramics β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7 with a

CMAS glass Unlike Y-containing YAlO3 and γ-Y2Si2O7 in Chapter 2 [116] little or no reaction

is found between the Y-free EBC ceramics and the CMAS



Y2Si2O7 (B) Yb2Si2O7 (C) Sc2Si2O7 and (D) Lu2Si2O7

A B

C D

66

In the case of β-Yb2Si2O7 a small amount of reaction-crystallization product Yb-Ca-Si

apatite (ss) is detected whereas none is detected in the cases of β-Sc2Si2O7 and β-Lu2Si2O7

Instead the CMAS glass is found to penetrate the grain boundaries of β-Yb2Si2O7 β-Sc2Si2O7 and

β-Lu2Si2O7 EBC ceramics and they all suffer from a new type of lsquoblisterrsquo cracking damage

comprising large and wide cracks This is attributed to the through-thickness dilatation-gradient

caused by the slow penetration of the CMAS glass into the grain boundaries Based on this

understanding a lsquoblisteringrsquo-damage-mitigation approach is devised and successfully

demonstrated where 1 vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering

The resulting EBC ceramic does not show the lsquoblisterrsquo cracking damage as the presence of the

CMAS-glass phase at the grain boundaries appears to promote rapid CMAS-glass penetration

thereby avoiding the dilatation-gradient

67



CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS

This chapter was modified from a submitted (February 20 2020) article LR Turcer and

NP Padture ldquoRare-earth pyrosilicate solid-solution environmental-barrier coating ceramics for

resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glassrdquo Journal of

Materials Research submitted for focus issue sand-phobic thermalenvironmental barrier

coatings for gas turbine engines (2020)

41 Introduction

In Chapter 3 it was shown that while Yb2Si2O7 EBC ceramic has minimal reaction with a

CMAS at 1500 ˚C large lsquoblisterrsquo cracks form as a result of the dilatation gradient set up due to the

progressive penetration of CMAS glass into the Yb2Si2O7 ceramic grain boundaries [117] In

contrast Y2Si2O7 is found to react with the CMAS to form a Y-Ca-Si apatite (ss) preventing the

CMAS from penetrating the grain boundaries and forming lsquoblisterrsquo cracks (Chapter 2) [116] This

raises the interesting possibility of tempering these extreme CMAS-interaction behaviors by

forming Yb(2 x)YxSi2O7 solid-solution EBC ceramics Furthermore the thermal conductivities of

substitutional solid-solutions with large atomic-number contrast (ZYb=70 ZY=39) are expected to

be low for potential thermal-environmental barrier coating (TEBC) applications [119] which will

be discussed further in Chapter 5

In this context although there have been several studies focused on the interactions

between RE-pyrosilicates and CMAS [23ndash2733ndash3669146152] there is little known about

CMAS interactions with pyrosilicate solid-solutions Figure 42A shows the polymorphism of

several RE2Si2O7 [37] It is seen that Yb2Si2O7 does not undergo polymorphic transformation and

remains as β-phase from room temperature up to its melting point In contrast Y2Si2O7 shows

several polymorphic transformations in that temperature range In this context it has been shown

68

that the β-phase can be stabilized in Yb(2-x)YxSi2O7 solid-solutions where x lt 11 (Figure 42B)

[38153]




chosen in this chapter Adapted from Ref [38]

Here we have studied the interactions at 1500 degC of two solid-solution lsquomodelrsquo EBC

ceramics (dense polycrystalline ceramic pellets) of compositions Yb18Y02Si2O7 (x = 02) and

Yb1Y1Si2O7 (x= 1) with three lsquomodelrsquo CMAS compositions with different CaSi ratios (i) Naval

Air Systems Command (NAVAIR) CMAS (CaSi = 076) [116117128] (ii) National Aeronautics

and Space Administration (NASA) CMAS (CaSi = 044) [61] and (iii) Icelandic volcanic ash

(IVA) CMAS (CaSi = 010) [71] The chemical compositions of these CMASs are reported in

Table 13 Interactions of these CMASs with pure RE-pyrosilicates (Y2Si2O7 (x = 2) and Yb2Si2O7

(x = 0)) are also studied for comparison This is with the overall goal of providing insights into the

chemo-thermo-mechanical mechanisms of these interactions and to use this understanding to

guide the design and development of future CMAS-resistant low thermal-conductivity TEBCs

A B

69


each

Phase CaO MgO AlO15 SiO2 CaSi

NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044

Icelandic Volcanic Ash [71] 79 50 79 792 010

42 Experimental Procedures

421 Powders

Experimental procedures for making γ-Y2Si2O7 powder have already been reported and

can be found in Section 221 The β-Yb2Si2O7 powders were obtained commercially from

Oerlikon Metco (AE 11073 Oerlikon Metco Westbury NY) β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7

solid-solution powders were prepared in-house by combining stoichiometric amounts of β-

Yb2Si2O7 and γ-Y2Si2O7 powders The mixture was then ball-milled and dried using the same

procedure described in Section 221 The dried powders were placed in Pt crucibles for calcination

at 1600 ˚C in air for 24 h in the box furnace The resulting powders were then crushed ball-milled

for an additional 24 h and dried

These ceramic powders followed the same procedure as stated for YAlO3 Y2Si2O7

Yb2Si2O7 Sc2Si2O7 and Lu2Si2O7 which can be found in Section 221 for more detail Briefly

pellets (~2 mm thick 20 mm in diameter) were made using spark plasma sintering (SPS 75 MPa

applied pressure 50 degCmin-1 heating rate 1500 degC hold temperature 5 min hold time and 100

degCmin-1 cooling rate) The pellets were ground heat-treated (1500 degC 1 h) and polished for

CMAS-interaction testing

70

422 CMAS Interaction

Three different simulated CMASs were used in this study NAVAIR CMAS (CaSi = 076)

NASA CMAS (CaSi = 044) and IVA CMAS (CaSi = 010) The chemical compositions of these

CMASs are reported in Table 13 and they have been chosen to study the effect of CMAS CaSi

ratio on the interaction of the CMAS with RE2Si2O7 (RE = Yb Y YbY) NAVIAR CMAS is

from Chapters 2 and 3 and a previous study [116117128] and it is close to the composition of

the AFRL-03 standard CMAS (desert sand) The NASA CMAS [61] and the IVA CMAS [71]

compositions are based on literature where the CaSi ratio is changed while maintaining the same

amounts of MgO and AlO15

Powders of the CMAS glasses of these compositions were prepared using a procedure

described elsewhere [7086] CMAS interaction studies were performed by applying the CMAS

powder paste (in ethanol) uniformly over the center of the polished surfaces of the Yb2Si2O7

Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets at sim15 mgcm-2 loading The specimens were

then placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box

furnace at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted

pellets were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-

μm finish


The characterization for these experiments is similar to the EBC ceramics found in

Chapters 2 and 3 Please refer to Section 223 for more detail Briefly X-ray diffraction (XRD)

was conducted on the as-prepared β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 powders and the heat-

71

treated pellets Densities of the as-SPSed pellets were measured using the Archimedes principle

(immersion medium = distilled water)

Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy

(EDS) was used to observe the cross-sections of the as-SPSed and CMAS-interacted pellets

Transmission electron microscopy (TEM) equipped with an EDS system was used to observe the

β-Yb1Y1Si2O7 as-SPSed sample The sample was prepared using focused ion beam and in-situ lift-

out

43 Results

431 Powder and Polycrystalline Pellets

Figures 43A and 43B are SEM micrographs of as-processed Yb18Y02Si2O7 and

Yb1Y1Si2O7 powders respectively Figures 43C and 43D are cross-sectional SEM micrographs of

Yb18Y02Si2O7 and Yb1Y1Si2O7 thermally-etched SPSed pellets respectively The density of the

Yb18Y02Si2O7 pellet is found to be 593 Mgm-3 (~99 dense) and the average grain size is ~14

μm The density of the Yb1Y1Si2O7 pellet is found to be 503 Mgm-3 (~99 dense) and the

average grain size is ~15 μm Figure 43E presents indexed XRD patterns of the Yb18Y02Si2O7 and

Yb1Y1Si2O7 pellets along with that of the Yb2Si2O7 pellet The progressive peak-shift with

increasing x from 0 to 1 as evident in the higher-resolution XRD pattern in Figure 43F indicates

single-phase (β) solid solutions

72




resolution XRD patterns

73

Figure 44A is a bright-field TEM micrograph of the as-SPSed Yb1Y1Si2O7 pellet with

Figure 44B showing a higher magnification image from the area marked in Figure 44A The EDS

composition (at cation basis) corresponding to the points marked (encircled numbers) in Figure

44B are presented in Table 14 which appear to be uniform Also there is no visible contrast within

the grains Figure 44C is another high-magnification bright-field TEM image showing no phase

contrast within the grains and a grain boundary Figure 44D presents EDS line scans (Si Yb Y)

along the line marked L-R The YYb ratios along the entire line are within the EDS detection

limit indicating compositional homogeneity ie no evidence of nanoscale phase separation Thus

the XRD data in Figures 43E and 43F coupled with the TEM and EDS data in Figure 44 and Table

14 unambiguously confirm that the as-SPSed Yb1Y1Si2O7 pellet is a RE-pyrosilicate ceramic solid-

solution Although Yb1Y1Si2O7 was the focus of this TEM analysis Yb18Y02Si2O7 is expected to

form a complete solid-solution without phase separation as well

74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50

432 NAVAIR CMAS Interactions

Figures 45A 45B 45C and 45D are cross-sectional SEM micrographs of Yb2Si2O7

Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with the

NAVAIR CMAS (CaSi = 076) at 1500 ˚C for 24 h Figure 45A is from Chapter 3 [117] and

Figure 45D is from Chapter 2 [116] As mentioned earlier Y2Si2O7 has extensive reaction with

NAVAIR CMAS resulting in the formation of a needle-like Y-Ca-Si apatite reaction product In

contrast Yb2Si2O7 does not form Yb-Ca-Si-apatite readily and instead large lsquoblisterrsquo cracks

(horizontal) are observed in the pellet Figures 45B and 45C clearly show the tempering of these

extreme behaviors in the Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solutions respectively In the

Yb18Y02Si2O7 pellet no lsquoblisterrsquo cracks are seen and the higher magnification SEM image in

Figure 45E shows some formation of Yb-Y-Ca-Si apatite (region 1 in Table 15) See also the

corresponding EDS elemental Ca map in Figure 45F Thus with the addition of 10 at Y (x = 02)

to Yb2Si2O7 the lsquoblisterrsquo cracks are eliminated in exchange for a slightly higher propensity for

reaction with the CMAS However the small amount of Yb-Y-Ca-Si apatite does not appear to

arrest the penetration of the NAVAIR CMAS into the grain boundaries CMAS pockets can be

found (regions 3 and 6 in Table 15) Figure 45G is a higher magnification SEM image of the

Yb1Y1Si2O7 pellet and the corresponding EDS Ca elemental map is presented in Figure 45H With

76

the higher amount of Y3+ in Yb1Y1Si2O7 it appears to react with NAVAIR CMAS in a manner

similar to that of the Y2Si2O7 pellet (Figure 45D) There are two reaction layers a CMAS-rich

zone on the top of the sample and an Yb-Y-Ca-Si apatite zone at the interface The Yb-Y-Ca-Si

apatite layer is 80-100 μm thick which is approximately half the thickness of the Y-Ca-Si apatite

layer found in the Y2Si2O7 pellet (Figure 45D) Once again no lsquoblisterrsquo cracks are observed in

Figure 45C

77







[116] respectively

Figure 45E Figure 45G

78




are also included

Region Yb Y Ca Mg Al Si Phase

1 amp 2 39 5 12 - - 44 Yb-Y-Ca-Si Apatite

3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions

45 5 125 - - 375 Yb72Y08Ca2(SiO4)6O2 Apatite


45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7

433 NASA CMAS Interactions

Figures 46Andash46D are cross-sectional SEM micrographs of Yb2Si2O7 Yb18Y02Si2O7

Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with NASA CMAS (CaSi =

044) at 1500 ˚C for 24 h Unlike the NAVAIR CMAS case the Yb2Si2O7 pellet does not show

lsquoblisterrsquo cracks in Figure 46A The higher magnification SEM image in Figure 46E the EDS Ca

elemental map (Figure 46I) and the EDS compositions in Table 16 of the regions marked in Figure

46E all confirm that there is no Yb-Ca-Si apatite present Similarly lsquoblisterrsquo cracks and apatite are

absent in Yb18Y02Si2O7 (Figures 46B 46F and 46J and Table 16) and Yb1Y1Si2O7 (Figures 46C

46G and 46K and Table 16) pellets that have interacted with the NASA CMAS Pockets of NASA

CMAS can be seen in triple junctions in the Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 pellets Y-Ca-

Si apatite formation is found in the Y2Si2O7 pellets that has interacted with the NASA CMAS

79

(regions 13 and 14 in Figure 46H and Table 16) but the apatite layer is much thinner (~50 μm

thickness) and NASA CMAS is also found in pockets between Y2Si2O7 grains (region 15 in

Figure 46H and Table 16) The porosity in the Y2Si2O7 pellet also appears to be affected after

NASA-CMAS interaction where in Figure 46D larger pores can be seen near the top of the sample

as compared to the middle of the sample (toward the bottom of the micrograph)







obtained (see Table 16)

Figure 46E Figure 46F

Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass

13 amp 14 - 42 14 - - 44 Y-Ca-Si Apatite

15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7

Includes signal from surrounding material

434 Icelandic Volcanic Ash CMAS Interactions


Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7 pellets respectively that have interacted with IVA

CMAS (CaSi = 010) at 1500 ˚C for 24 h The corresponding higher magnification SEM images

and EDS Ca elemental maps are presented in Figures 47E-47H and Figures 47I-47L respectively

This low CaSi-ratio CMAS shows the most unusual behavior where crystallization of pure SiO2

(α-cristobalite phase) grains is observed within the CMAS Neither lsquoblisterrsquo cracks nor apatite

formation is detected in any of these pellets Only slight penetration of the IVA CMAS is observed

in the Y2Si2O7 pellet (Figures 47H and 47L) In Yb2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 pellets

reprecipitated phases can be seen in the CMAS pool at the top of the sample Their chemical

compositions are reported in Table 17 (regions 3 7 and 10)

81







Table 17)


Figure 47G Figure 47H

82




Ash CMAS at 1500 ˚C for 24 h


1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass

3 36 - 2 - - 62 Re-precipitated Yb2Si2O7

4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2

7 32 4 2 - - 62 Re-precipitated Yb18Y02Si2O7

8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion

The results from this study show systematically that the CaSi ratio in the CMAS can

influence profoundly its interaction with Yb(2-x)YxSi2O7 EBC ceramics which also depends

critically on the x value First consider the propensity for the formation of the apatite reaction

product Y-Ca-Si apatite is significantly more stable compared to Yb-Ca-Si apatite as the ionic

radius of Y3+ is closer to that of Ca2+ than is Yb3+ to Ca2+ This is the driving force for apatite

formation [128146147] Thus the combination of CMAS with the highest Ca content (CaSi =

076 NAVAIR) and EBC ceramic with the highest Y content (x = 2 Y2Si2O7) shows the greatest

propensity for apatite formation Apatite formation is a lsquodouble edged swordrsquo On the one hand

formation of apatite consumes the CMAS and arrests its further penetration into the EBC (pores

andor grain boundaries) On the other hand extensive formation of apatite is detrimental as this

reaction-product layer does not have the desirable thermal (CTE) and mechanical properties of the

83

EBC itself As expected a reduction in the Y3+ content (x value) in the Yb(2-x)YxSi2O7 EBC

ceramic for the same high Ca-content CMAS (NAVAIR) reduces the propensity for apatite

formation Next consider the lsquoblisterrsquo cracks formation This occurs when Y3+ is completely

eliminated (x = 0) in Yb2Si2O7 where the lack of apatite formation allows the CMAS glass to

penetrate into Yb2Si2O7 grain boundaries This sets up a dilatation gradient which is the driving

force for lsquoblisterrsquo cracking Thus the benefit of solid-solution EBCs is clearly demonstrated in this

study where the CMAS-interaction behavior is tuned to prevent lsquoblisterrsquo crack formation and to

reduce apatite formation

As the CaSi ratio decreases in the NASA CMAS (CaSi = 044) the overall propensity for

apatite formation decreases This is expected due to insufficient Ca2+ availability in the NASA

CMAS But surprisingly lsquoblisterrsquo cracking is also suppressed in Yb2Si2O7 despite the grain-

boundary penetration of the NASA CMAS The reason for this is not clear at this time but it could

be related to the relatively facile grain-boundary penetration of NASA CMAS which may

preclude the formation of a dilatation gradient

With further decrease in the CaSi ratio to 010 in IVA CMAS the propensity for apatite

formation decreases further The amount of molten CMAS that can react or interact with the pellets

decreases due to the crystallization of pure SiO2 cristobalite However this increases the CaSi

ratio in the remaining CMAS complicating the issue Nonetheless the CaSi ratio in the remaining

CMAS is still less than 044 that is in NASA CMAS (Table 16) resulting in virtually no apatite

formation and the suppression of lsquoblisterrsquo cracks

This first systematic report on CMAS interactions with Yb(2-x)YxSi2O7 EBC ceramics

clearly shows the benefit of solid-solutions This allows tuning of the CMAS interaction by

84

reducing the amount of apatite formation and suppressing lsquoblisterrsquo cracking while maintaining

polymorphic β-phase stability and the desirable CTE match with SiC-based CMCs

45 Summary

Here a systematic study of the high-temperature (1500 degC) interactions between promising

dense polycrystalline EBC ceramic pellets Yb2Si2O7 Yb18Y02Si2O7 Yb1Y1Si2O7 and Y2Si2O7

and three CMAS glasses NAVAIR (CaSi = 076) NASA (CaSi = 044) Icelandic Volcanic Ash

(CaSi = 010) was performed Yb(2-x)YxSi2O7 solid solutions are confirmed to be pure β-phase

NAVAIR CMAS with its highest CaSi ratio shows a tempering effect between the extensive

reaction-crystallization (apatite formation) in Y2Si2O7 and the lsquoblisterrsquo crack formation in

Yb2Si2O7 EBC ceramics The Yb18Y02Si2O7 and Yb1Y1Si2O7 solid-solution EBC ceramics do not

show any lsquoblisterrsquo cracks There is some apatite formation but it is not as extensive as in the case

of Y2Si2O7 EBC ceramics The NASA CMAS when reacted with the EBC ceramics does not show

lsquoblisterrsquo cracks although CMAS still penetrates the grain boundaries In the Yb2Si2O7

Yb18Y02Si2O7 and Yb1Y1Si2O7 EBC ceramics no reaction products are observed In the case of

Y2Si2O7 EBC ceramic there is an apatite reaction zone but it is much smaller compared to the

NAVAIR CMAS (CaSi = 076) case Penetration of the NASA CMAS into grain boundaries and

pores are also observed in the Y2Si2O7 EBC ceramics The IVA CMAS with its lowest CaSi ratio

does not show apatite formation in any of the EBC ceramics studied There is some crystallization

of pure SiO2 (α-cristobalite) in the CMAS melt No lsquoblisterrsquo cracks are observed in any of the EBC

ceramics This study highlights the interplay between the CMAS and the EBC ceramic

compositions in determining the nature of the high-temperature interaction and suggests a way to

tune that interaction in rare-earth pyrosilicate solid-solutions

85

CHAPTER 5 THERMAL CONDUCTIVITY

This chapter was modified from a previously published article along with unpublished data

that may be used in future publications LR Turcer and NP Padture ldquoTowards multifunctional

thermal environmental barrier coatings (TEBCs) based on rare-earth pyrosilicate solid-solution

ceramicsrdquo Scripta Materialia 154 111-117 (2018)

51 Introduction

EBC-coated CMC components need to be attached to the lower-temperature metallic

hardware within the engine which invariably results in temperature gradients It is therefore

imperative that EBCs have enhanced thermal-insulation properties There is also an increasing

demand for thermal protection of CMCs for even higher temperature applications [41335154]

Furthermore thin-shelled hollow CMCs are being developed using the integral ceramic textile

structure (ICTS) approach which can be actively cooled [4155156] In all of these cases an

additional thermally-insulating TBC top-coat capable of withstanding higher temperatures (gt1700

degC) is needed ndash the concept of TEBC (Figures 48A and 48B) [413146154157]



concept

The TBC top-coat is typically made of low thermal-conductivity refractory oxides such as

a RE-zirconate or RE-hafanate However the CTEs of Si-free TBC oxides (~10times10minus6 degC) are

typically significantly higher than that of SiC (~45times10minus6 degC) While the cracks and pores in TBC

A B

C

86

top-coats can provide strain-tolerance exposure of the TBC top-coat to temperatures approaching

1700 degC can result in their sintering This leads to a reduction in the strain-tolerance and increases

the thermal conductivity of the TBC top-coat The introduction of an intermediate layer or

gradation between the TBC top-coat and the underlying EBC can mitigate the CTE-mismatch

problems to some extent However the options of available high-temperature materials for this

additional layer or gradation that satisfy the various onerous requirements is vanishingly small

intermediate CTE high-temperature capability phase stability chemical compatibility with both

TBC and EBC robust mechanical properties etc Thus at operating temperatures approaching

1700 degC deleterious reactions between the different layers and homogenization of any gradations

are inevitable over time Also any additional interfaces can become sources of failure during in-

service thermal cyclingexcursions

In order to avoid these shortcomings of the current TEBCs it is highly desirable to replace

the EBC the intermediate layergradation and the TBC top-coat with a single layer of one material

that can perform both the thermal- and environmental-barrier functions (Figure 48C) ndash the TEBC

concept Thus the four most important properties among several other requirements this single

material must possess are (i) good CTE match with SiC (ii) high-temperature phase stability (iii)

inherently low thermal conductivity in its dense state and (iv) resistance to CMAS attack This

chapter proposes that solid-solutions of some RE-pyrosilicates (or RE-disilicates ndash RE2Si2O7) may

satisfy these key requirements for TEBC applications

511 Coefficient of Thermal Expansion

As previously stated individual RE-pyrosilicate ceramics are showing promise for EBC

application as they have good CTE match with SiC Figure 49A shows the measured average CTEs

87

of several RE2Si2O7 polymorphs [137158] The β polymorph of RE2Si2O7 (RE = Sc Lu Yb Er

Y) and γ polymorph of RE2Si2O7 (RE = Y Ho) have average CTEs that are close to that of SiC

[137] Both β (space groups C2m C2 Cm) and γ (space group P21a) polymorphs have the

monoclinic crystal structure and therefore their CTEs are anisotropic [137158] (Note that the

polymorphs β γ δ and α correspond to C D E and B respectively in the original notation by

Felsche [37])



the various RE2Si2O7 pyrosilicate polymorphs redrawn with data from Ref [37]

512 Phase Stability

While CTEs of the above RE-pyrosilicate polymorphs are acceptable for EBC application

some of them undergo polymorphic phase transformation in the temperature range 25ndash1700 degC

Figure 49B presents the phase-stability diagram for the different RE-pyrosilicates (excluding RE

= Sc and Y) showing that except for Yb2Si2O7 (MP 1850 degC [136]) and Lu2Si2O7 (MP 2000 degC

[140]) all RE-pyrosilicates undergo phase transformation(s) [37] While Er2Si2O7 and Ho2Si2O7

have a good CTE match with SiC they may not be suitable for EBC application as both undergo

phase transformations Y2Si2O7 (MP 1775 degC [124]) may also seem unsuitable for EBC application

88

as Y3+ has an ionic radius very close to that of Ho3+ and it also undergoes phase transformation

δrarrγrarrβrarrα during cooling [159] On the other hand Sc2Si2O7 with its very small Sc3+ ionic

radius (0745 Aring coordination number 6) has only one polymorph β up to its melting point (1860

degC [138]) [144] This narrows the list of RE pyrosilicate ceramics suitable for EBCs to β-Yb2Si2O7

β-Sc2Si2O7 and β-Lu2Si2O7 (Note that some of the polymorphic transformations in RE-

pyrosilicates can be sluggish and therefore the high temperature polymorphs can be kinetically

stabilized at lower temperatures Also the volume change associated with some of the

polymorphic transformations can be small making them relatively benign for high-temperature

structural applications but the CTEs of the product phases may be undesirable (Figure 49A))

513 Solid solutions

Phase equilibria in Y2Si2O7-Yb2Si2O7 [38160] Y2Si2O7-Lu2Si2O7 [160161] and Y2Si2O7-

Sc2Si2O7 [144] have been studied and are all shown to form complete solid-solutions While

Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 all exist only as the β phase their respective solid solutions with

Y2Si2O7 exist as β γ or δ phase depending on the Y content and the temperature the trend follows

βrarrγrarrδ with increasing Y-content and temperature [38] For example the β phase is stable up to

1700 degC for x lt 11 for both YxYb(2-x)Si2O7 and YxLu(2-x)Si2O7 and x lt 17 for YxSc(2-x)Si2O7 Since

these solid-solutions are isomorphous without any low-melting eutectics they are expected to have

higher MPs compared to pure Y2Si2O7 which has the lowest MP among the four RE-pyrosilicates

considered here [38] Thus Y2Si2O7 when alloyed with higher-melting Yb2Si2O7 Lu2Si2O7 or

Sc2Si2O7 becomes a viable ceramic for EBC application The Sc2Si2O7-Lu2Si2O7 system is shown

to form complete β-phase solid-solution [162] While phase equilibria studies in the Sc2Si2O7-

Yb2Si2O7 and the Lu2Si2O7-Yb2Si2O7 systems have not been reported in the open literature it is

likely that they also form complete solid-solutions considering that these RE-pyrosilicates are

89

isostructural and that the ionic radius of Yb3+ is only slightly larger than that of Lu3+ (Figure 49B)

Thus in addition to individual β-phase RE-pyrosilicates Yb2Si2O7 Lu2Si2O7 and Sc2Si2O7 the

list of potential candidates for TEBC application includes the following β-phase RE-pyrosilicate

solid-solutions (i) YxYb(2-x)Si2O7 (x lt 11) (ii) YxLu(2-x)Si2O7 (x lt 11) (iii) YxSc(2-x)Si2O7 (x lt

17) (iv) YbxSc(2-x)Si2O7 (v) LuxSc(2-x)Si2O7 and (vi) LuxYb(2-x)Si2O7 While the CTEs of these

solid-solutions are likely to follow rule-of-mixtures behavior their thermal conductivities may be

depressed significantly relative to the rule-of-mixtures behavior and is discussed in the next

section

52 Calculated Thermal Conductivity of Binary Solid-Solutions


In order to calculate the thermal conductivity of solid-solutions (RE119909I RE(2minus119909)

II Si2O7)

experimentally collected data on the pure RE2Si2O7 ceramics were needed including thermal

conductivity and Youngrsquos modulus

Dense polycrystalline ceramic pellets (~2 mm thickness) of γ-Y2Si2O7 β-Yb2Si2O7 and

β-Sc2Si2O7 from previous studies were used to measure their thermal diffusivity They were sent

to NETZSCH Instruments North America LLC (Burlington MA) for thermal diffusivity (κ)

measurements They machined the pellets to fit their testing apparatus and followed the ASTM

E1461-13 ldquoStandard Test Method for Thermal Diffusivity by the Flash Methodrdquo Using the flash

diffusivity method on a NETZSCH LFA 467 HT HyperFlashreg instrument the thermal diffusivities

at 27 200 400 600 800 and 1000 degC were measured Using the Neumann-Kopp rule for oxides

[163] the specific heat capacities for the RE2Si2O7 (RE = Y Yb and Sc) were calculated by the

specific heat capacities (CP) of the present constituent oxides Yb2O3 Y2O3 Sc2O3 and SiO2 [164]

90

The thermal conductivity (k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is

the measured room-temperature density

The Youngrsquos modulus of Sc2Si2O7 was obtained by nanoindentation on random grains

using the TI950 Triboindenter (Hysitron Minneapolis MN) The Berkovich diamond tip was used

to estimate the E values with a maximum load of 25 mN and a rate of 27778 microNs-1 The load-

displacement curves were then used to determine the E using the Oliver-Pharr analysis [165] Nine

indentations were made and the average E of Sc2Si2O7 was found to be 202 GPa with a minimum

of 153 GPa and a maximum of 323 GPa This large scatter is attributed to the anisotropic E of

monoclinic β-Sc2Si2O7

522 Pure RE2Si2O7 (RE = Yb Y Lu Sc) Thermal Conductivity

Among the four β-RE-pyrosilicates considered here the high temperature thermal

conductivities of Y2Si2O7 [142] Yb2Si2O7 [123142] and Lu2Si2O7 [142] have been measured

experimentally However the pellets used were not completely dense and instead thermal

conductivity data was extrapolated Dense polycrystalline Yb2Si2O7 and Y2Si2O7 pellets similar

to those used in Chapters 2 and 3 were measured experimentally by NETZSCH These results are

plotted in Figure 50 along with the Lu2Si2O7 data from literature The thermal conductivities of

the Y2Si2O7 and Lu2Si2O7 RE-pyrosilicates are low and they are in the range of 15ndash2 Wmiddotmminus1middotKminus1

(at 1000 degC) To the best of our knowledge the thermal conductivity of Sc2Si2O7 has not been

reported in the open literature In order to address this paucity the thermal conductivities of a fully

dense phase-pure Sc2Si2O7 ceramic pellet in the temperature range 27ndash1000 degC were measured

These are reported in Figure 50 It is seen that Sc2Si2O7 has a significantly higher thermal

conductivity 32 Wmiddotm-1middotK-1 (at 1000 degC) compared to other RE-pyrosilicates

91


as a function of temperature The data for Lu2Si2O7 is from Ref [142]

523 Thermal Conductivity Calculations for Binary Solid-Solutions

None of the thermal conductivities of the RE-pyrosilicate solid-solutions have been

reported in literature In this context there is a tantalizing possibility of obtaining even lower

thermal conductivities in dense RE-pyrosilicate solid-solutions where the substitutional-solute

point defects can be used as effective phonon scatterers especially where the atomic number (ZRE)

contrast between the host and the solute RE-ions is large To that end analytical calculations have

been performed to estimate the thermal conductivities of RE-pyrosilicate solid-solutions in six

systems YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and

LuxYb(2-x)Si2O7 with ZSc = 21 ZY = 39 ZYb = 70 and ZLu = 71

92

The thermal conductivity of a solid-solution in relation with its pure host material as a

function of temperature is given by [166]

119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)

Here ωo is the phonon frequency at which the mean free paths due to point-defect

scattering and intrinsic scattering are equal and ωM is the phonon frequency corresponding to the

maximum of the acoustic branch of the phonon spectrum The latter is given by ωDm-13 where m

is the number atoms per molecular unit and ωD is the Debye frequency given by (6π2v3a)13 Here

a is the atomic volume (a3 = MWmNA where MW is the molecular weight and NA is Avagadros

number) and v is the transverse phonon velocity (v = (μρ)12 where ρ is the density and μ is the

shear modulus) Also γ2 is the Gruumlneisen anharmoncity parameter kB is the Boltzmann constant

c is the concentration of the solute differing in mass from the host atom of mass M by ΔM (for a

simple substitutional solid-solution) and ψ is an adjustable parameter included to obtain an

empirical fit between the theory and experiment at room temperature (298 K) and it is set to unity

in this case The function f(T) takes into account the lsquominimum thermal conductivityrsquo and it is

given empirically by [167]

119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)

Using the available values for all the parameters (listed in Table 18) [34125138142143]

the thermal conductivities kss of the six RE-pyrosilicate solid-solutions are plotted in Figure 51

Note that E of Sc2Si2O7 coating is mentioned to be 200 GPa in the literature [25] Here it was

confirmed that the average E is 202 GPa using nanoindentation of different individual grains in a

93

dense polycrystalline Sc2Si2O7 ceramic pellet (see Section 521 for experimental details)

However the E appears to be highly anisotropic ranging from 153 to 323 GPa for individual

grains The Poissons ratio is assumed to be 031 The experimental data points from Figure 50 are

included on the y-axes in Figure 51

Table 18 Properties and parameters for pure β-RE-pyrosilicates

β-Sc2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 β-Lu2Si2O7

ρ (Mgmiddotm-3) 340 393dagger 613Dagger 625sect

v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182

kMin (Wmiddotm-1middotK-1) 159 109 090 095 This work paraFitted value Ref [138] daggerRef [125] DaggerRef [34] sectRef [143] All other values are

from Ref [142]

94





The dashed lines represent 1 Wmiddotm-1middotK-1

95

As expected the largest Z-contrast solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-

x)Si2O7 and LuxSc(2-x)Si2O7 show the largest decrease in thermal conductivities due to alloying

Whereas the solid-solutions with the smallest Z-contrast YxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 show

the smallest decrease LuxYb(2-x)Si2O7 shows a rule-of-mixtures behavior since Yb and Lu are next

to each other in the periodic table and both have high Z All but the last two of the dense solid-

solutions of RE-pyrosilicates can have thermal conductivities below 1 Wmiddotm-1middotK-1 at 1000 degC This

is unprecedented even for TBC ceramics [168] making dense RE-pyrosilicate solid-solutions good

candidates for the new single-material TEBCs discussed earlier So far only binary solid-solutions

have been considered but phonon scattering in ternary solid-solutions with high Z-contrast REs

eg Sc(2-x-y)YxLuySi2O7 could prove to be even more effective

In this context the lsquominimum thermal conductivityrsquo (kMin) where the phonon mean free

path approaches interatomic spacing [169] may limit how low the thermal conductivity of RE-

pyrosilicate solid-solutions can be depressed For pure RE-pyrosilicates the lsquominimum thermal

conductivityrsquo (kMin) is estimated using the following relation [170]

119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)

where E is the Youngs modulus (minimum value if anisotropic) and the corresponding properties

(see Table 18) The properties in Equation 10 for isomorphous solid-solutions are not known but

are expected to follow rule-of-mixture behavior In Figure 51 where the x values display the lowest

thermal conductivity the rule-of-mixture properties of the solid-solutions are estimated They are

listed in Table 19 Substituting these property values into Equation 10 the kMin for the six solid-

solutions are calculated and are also reported in Table 19 It should be noted that Equation 10 is

derived based on approximations and provides a rough estimate for the lsquominimum thermal

conductivityrsquo Thus it remains to be seen if high-temperature thermal conductivities below 1 Wmiddotm-

96

1middotK-1 can in fact be achieved experimentally in dense RE-pyrosilicate solid-solution (binary or

ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin

(Wmiddotm-1middotK-1)

YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099

53 Experimental Yb(2-x)YxSi2O7 Solid-Solutions Thermal Conductivity


Dense polycrystalline ceramic pellets (~2 mm thickness) of β-Yb18Y02Si2O7 and β-

Yb1Y1Si2O7 from the previous study in Chapter 4cedil were used to measure their thermal diffusivity

They were sent to NETZSCH Instruments North America LLC (Burlington MA) for thermal

diffusivity (κ) measurements like the pure RE2Si2O7 ceramics For more details on this process

please refer to Section 521 Using the flash diffusivity method on a NETZSCH LFA 467 HT

HyperFlashreg instrument the thermal diffusivities at 27 200 400 600 800 and 1000 degC were

measured following ASTM E1461-13 Using the Neumann-Kopp rule for oxides [163] specific

heat capacities for the RE2Si2O7 (RE = Yb18Y02 and Yb1Y1) were calculated by the specific heat

capacities (CP) of the constituent oxides Yb2O3 Y2O3 and SiO2 [164] The thermal conductivity

(k) at each temperature was then calculated using 119896 = 120581120588119862119875 where ρ is the measured room-

temperature density

97

Other experimental data including density Youngrsquos modulus etc were obtained by using

rule-of-mixture calculations

532 Comparison of Experimental and Calculated Thermal Conductivity

Figure 52 shows the thermal conductivity measurements for Yb2Si2O7 Y2Si2O7 Yb18Y-

02Si2O7 and Yb1Y1Si2O7 At room temperature (27 degC) the thermal conductivity of Yb1Y1Si2O7 is

the lowest For the rest of the thermal conductivity measurements the solid-solutions

Yb18Y02Si2O7 and Yb1Y1Si2O7 fall in the range of the thermal conductivity values of the pure

components Yb2Si2O7 and Y2Si2O7



represents 1 Wmiddotm-1middotK-1

98

To more easily compare this data the experimental data points are plotted against the

calculated values from Section 523 which can be seen in Figure 53 The experimental data does

not have as significant a decrease in thermal conductivity as expected from the analytical

calculations From room temperature to 600 degC the data shows a decrease in thermal conductivity

lower than the rule-of-mixtures prediction This comparison can also be seen in Table 20 From

600 to 1000 degC the solid-solution thermal conductivities seem to follow a rule-of-mixtures

estimate



which can also be found in Figure 52 (circles) The dashed line represents 1 Wmiddotm-1middotK-1

99



Temperature

(degC)

Thermal Conductivities (Wmiddotm-1middotK-1)

Yb18Y02Si2O7 Yb1Y1Si2O7

Experimental Rule-of-Mixture Experimental Rule-of-Mixture

27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209

Similarly Tian et al [171] have measured the thermal conductivities of RE2SiO5 solid-

solutions hot-pressed ceramics (YxYb1-x)2SiO5 as a function of x (0 to 1) and temperature (27 to

1000 degC) for possible TEBCs They did not observe the expected lsquodiprsquo in the thermal

conductivities which could be attributed to the ldquominimum conductivityrdquo limit [171] However

they observed lower than expected thermal conductivity in a Yb-rich RE2SiO5 composition (x =

017) [171] They attributed this to the presence of oxygen vacancies created by some reduction of

Yb3+ to Yb2+ in the ceramic fabricated using hot-pressing [171] which invariably has a reducing

atmosphere While such oxygen vacancies are unlikely to exist in equilibrium ceramics in an

oxidizing environment of a gas-turbine engine equilibrium oxygen vacancies can be formed by

alloying them with group IIA aliovalent substitutional cations such as Mg2+ (ZMg = 12) Ca2+ (ZCa

= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)

It is known that point defects such as oxygen vacancies are potent phonon scatterers in

RE2O3-ZrO2 solid-solutions and compounds [5167168172] Thus for example alloying a RE-

pyrosilicate such as Yb2Si2O7 with a group IIA oxide such as MgO will result in high Z-contrast

cation substitution and oxygen vacancies 2119872119892119874 ⟷ 2119872119892119884119887prime + 2119874119874 + 119881119874

∙∙ This effect could be

further enhanced in ternary or even quaternary solid-solutions of RE-pyrosilicates and group IIA

oxides notwithstanding the lsquominimum thermal conductivityrsquo limit Unfortunately phase equilibria

100

studies in these systems have not been reported in the open literature and therefore the relative

solid-solubilities are not known Also there is the danger of forming low-melting eutectics andor

glasses in such multicomponent silicate systems which may limit their utility in high-temperature

TEBC applications

Another possible way to decrease the thermal conductivity in RE-pyrosilicates would be

to use equiatomic solid-solution mixtures like high-entropy ceramics This will be discussed

further in the following section

54 Thermal Conductivity of a 5-Component Equiatomic Solid-Solution

541 Introduction to High-Entropy Ceramics

High-entropy alloys were first studied in 2004 [173] These were made by mixing

equimolar amounts of metallic elements which creates a disordered solid-solution This increases

the entropy of the system which causes a decrease in the energy of the system Since then many

studies have focused on high-entropy ceramic materials to enhance certain properties High-

entropy oxides [174ndash176] borides [177] carbides [178ndash180] nitrides [181] sulfides [182] and

silicides [183184] have all been studied They have demonstrated phase stability and have been

shown to have adjustable and enhanced properties [185]

In 2019 high-entropy ceramics of RE2Si2O7 [186] and RE2SiO5 [187188] were first

studied Chen et al [187] synthesized a homogenous (Yb025Y025Lu025Er025)2SiO5 ceramic which

was confirmed by EDS mapping on a SEM and high temperature XRD Ridley et al [188] studied

the thermal conductivity and coefficient of thermal expansion for (Sc02Y02Dy02Er02Yb02)2SiO5

compared to pure RE2SiO5 ceramics Again only EDS mapping on a SEM and XRD confirmed

solid-solution high-entropy ceramics To the best of my knowledge the only high-entropy

101

RE2Si2O7 found in literature is β-(Y02Y02Lu02Sc02Gd02)2Si2O7 [186] Dong et al [186] confirms

a phase pure homogenous solid-solution through XRD TEM and SAEDP However the lsquohigh-

entropyrsquo nature of this system has not been confirmed

For the focus of this project the thermal conductivity of a 5-compontent equiatomic solid-

solution or β-(Y02Y02Lu02Sc02Gd02)2Si2O7 was studied Here it will not be referred to as lsquohigh-

entropyrsquo due to insufficient evidence However it has been shown to form a phase pure solid-

solution and due to the difference in Z-contrast (ZSc = 21 ZY = 39 ZGd = 64 ZYb = 70 and ZLu =

71) and the randomly distributed RE cations in a β-RE2Si2O7 structure it is believed that the

thermal conductivity will decrease The overall goal is to provide insights into the thermal

conductivity of the 5-component equiatomic β-(Y02Y02Lu02Sc02Gd02)2Si2O7 and to use this

understanding to guide the design and development of future low thermal-conductivity TEBCs


The β-(Y02Y02Lu02Sc02Gd02)2Si2O7 powder was prepared in-house by combining

stochiometric amounts of Y2O3 (Nanocerox Ann Arbor MI) Yb2O3 (Sigma Aldrich St Louis

MO) Lu2O3 (Sigma Aldrich St Louis MO) Sc2O3 (Reade Advanced Materials Riverside RI)

Gd2O3 (Alfa AESAR Ward Hill MA) and SiO2 (Atlantic Equipment Engineers Bergenfield NJ)

This mixture was then ball-milled and dried while stirring The dried powder mixture was placed

in a Pt crucible for calcination at 1600 degC in air for 4 h in the box furnace The resulting β-(Y02Y-

02Lu02Sc02Gd02)2Si2O7 powder was then ball-milled for an additional 24 h dried and crushed

The powders were then loaded into graphite dies (20 mm diameter) lined with graphfoil

and densified in a spark plasma sintering (SPS) unit (Thermal Technologies LLC Santa Rosa CA)

in an argon atmosphere The SPS conditions were 75 MPa applied pressure 50 degCmin-1 heating

102


the resulting dense pellets (sim2 mm thickness) were ground to remove the graphfoil and the pellets



ceramographic polishing techniques Some pellets were cut using a low-speed diamond saw and

the cross-sections were polished to a 1-μm finish

The as-prepared powder was characterized using an X-ray diffractometer (XRD D8

Advance Bruker AXS Karlsruhe Germany) to check for phase purity The phase present was

identified using the PDF2 database The densities of the as-SPSed pellets were measured using the

Archimedes principle with distilled water as the immersion medium

The cross-sections of the as-SPSed pellet was observed in a SEM (LEO 1530VP Carl

Zeiss Munich Germany or Helios 600 FEI Hillsboro Oregon USA) equipped with EDS (Inca

Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS elemental

maps were also collected and used to determine homogeneity in the pellets

A transmission electron microscopy (TEM) specimen from a location within the polished

cross-section of the as-SPSed pellet was prepared using focused ion beam (FIB Helios 600 FEI

Hillsboro Oregon USA) and in situ lift-out The sample was then examined using a TEM (2100

F JEOL Peabody MA) equipped with an EDS system (Inca Oxford Instruments Oxfordshire

UK) operated at 200 kV accelerating voltage Selected-area electron diffraction patterns

(SAEDPs) from various phases in the TEM micrographs were recorded and indexed using standard

procedures

103

543 Solid Solution Confirmation

Although the material was confirmed to be solid-solution by Dong et al [186] they made

samples using a sol-gel process Here the samples were made by mixing oxide constituents and

calcinating the powders Therefore due to the difference in materials processing a confirmation

of the solid-solubility of β-(Y02Y02Lu02Sc02Gd02)2Si2O7 is needed

Figure 54 shows an XRD pattern of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet compared

to Yb2Si2O7 and the solid-solution mixtures Yb18Y02Si2O7 and Yb1Y1Si2O7 (from Chapter 4 and

Section 53 in this chapter) The indexed XRD pattern shows a β-phase pure material The density

of the β-(Y02Y02Lu02Sc02Gd02)2Si2O7 pellet is 508 Mgm-3 (~98 dense compared to the

theoretical density obtained by reitveld analysis)


compared to β-Yb2Si2O7 β-Yb18Y02Si2O7 and β-Yb1Y1Si2O7 pellets

Figure 55 shows a SEM micrograph of the as-SPSed β-(Y02Y02Lu02Sc02Gd02)2Si2O7

pellet and its corresponding elemental EDS maps Y Yb Lu Sc Gd and Si The elemental EDS

104

maps show a homogenous dispersion of the 5 RE components and Si EDS elemental compositions

were also collected in different grains across this sample and were Y7-Yb9-Lu9-Sc10-Gd9-Si56 (at

cation basis) which is similar to the ideal composition of Y10-Yb10-Lu10-Sc10-Gd10-Si50 (at

cation basis)


the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si

Figure 56A shows a TEM sample collected from the as-SPSed β-(Y02Y02Lu-

02Sc02Gd02)2Si2O7 pellet An indexed SAEDP confirms β-phase Figures 56B and 56C are two

higher magnification TEM micrographs of regions marked in Figure 56A Elemental EDS maps

for Y Yb Lu Sc Gd and Si are also shown Within the grain and along grain boundaries the EDS

maps are showing a homogenous material EDS elemental compositions were collected (circled

numbers) and can be found in Table 21

105

Figure 56 (A) Bright-field TEM micrograph and indexed SAEDP of the as-SPSed β-(Y02Y02Lu-

02Sc02Gd02)2Si2O7 pellet β-RE2Si2O7 grains are found Transmitted beam and zone axis are

denoted by lsquoTrsquo and lsquoBrsquo respectively (B C) Two higher magnification regions showing grain

boundaries and the corresponding EDS elemental maps Y Yb Lu Sc Gd and Si The circled

regions are where EDS elemental compositions were obtained and can be found in Table 21

Figure 56B

Figure 56C

106



EBC ceramic pellet

Region Yb Y Lu Sc Gd Si

1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47

TEMSAEDP (Figure 56 and Table 21) and XRD (Figure 54) results confirm that β-

(Y02Yb02Lu02Sc02Gd02)2Si2O7 is the only crystalline phase and that there does not appear to be

nano-scale phase separation in this material ie the material is confirmed to be a solid-solution of

β-(Y02Yb02Lu02Sc02Gd02)2Si2O7

544 Experimental Thermal Conductivity Results

Thermal conductivity β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was measured by NETZSCH and

can be seen below in Figure 57 Room temperature thermal conductivity of the β-

(Y02Yb02Lu02Sc02Gd02)2Si2O7 is 215 Wmiddotm-1middotK-1 which is much lower than the thermal

conductivities of Yb2Si2O7 Y2Si2O7 Yb18Y02Si2O7 and Yb1Y1Si2O7 However as temperature is

increased the thermal conductivity starts to align with that of the Y2Si2O7 sample (~151 Wmiddotm-

1middotK-1 at 800 and 1000 degC)

107



temperature The dashed line represents 1 Wmiddotm-1middotK-1

Interestingly this shows a similar relationship to the Yb(2-x)YxSi2O7 solid-solutions The 5-

component equiatomic RE2Si2O7 shows much lower thermal conductivities up to 600 degC The

solid-solutions saw a greater decrease than the rule-of-mixtures up to 600 degC From 600 to 1000

degC β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 follows the thermal conductivity of Y2Si2O7 In the same

temperature range the thermal conductivity of the Yb(2-x)YxSi2O7 solid-solutions did not show a

decrease in thermal conductivity compared to the rule-of-mixtures calculations At the higher

temperatures (gt 600 degC) the lack of the expected decrease in thermal conductivity could be

attributed to the ldquominimum conductivityrdquo limit [171]

55 Summary

Analytical calculations of the thermal conductivities for six systems YxYb(2-x)Si2O7

YxLu(2-x)Si2O7 YxSc(2-x)Si2O7 YbxSc(2-x)Si2O7 LuxSc(2-x)Si2O7 and LuxYb(2-x)Si2O7 were

108

performed Substitutional-solute point defects are an effective way to scatter phonons and decrease

thermal conductivity especially when the Z-contrast is high As expected the largest Z-contrast

solid-solutions YxYb(2-x)Si2O7 YxLu(2-x)Si2O7 YbxSc(2-x)Si2O7 and LuxSc(2-x)Si2O7 show the

largest decrease in thermal conductivities due to alloying

Solid-solutions of Yb(2-x)YxSi2O7 were studied in more detail and experimental thermal

conductivity data was obtained for Yb18Y02Si2O7 and Yb1Y1Si2O7 The experimental data does

not have as significant a decrease in thermal conductivity as expected by the analytical

calculations

A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was also studied XRD and

TEMSAEDP were used to confirm powder processing by mixing oxide constituents results in a

single phase homogeneous solid-solution β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has a much lower

room temperature thermal conductivity than the previous RE2Si2O7 (pure and Yb-Y pyrosilicate

solid-solutions) However as the temperature increases the thermal conductivity plateaus at ~151

Wmiddotm-1middotK-1 At higher temperatures (gt 600 degC) the lack of the expected decrease in thermal

conductivity could be attributed to the ldquominimum conductivityrdquo limit [171]

109



BY MOLTEN CALCIA-MAGNESIA-ALUMINOSILICATE (CMAS) GLASS

This chapter is unpublished data that may be used in a future publication

61 Introduction

In Chapters 2 and 3 how potential RE2Si2O7 (Y Yb Lu Sc) EBC ceramics interact with

a lsquomodelrsquo CMAS (NAVAIR CaSi = 076) was demonstrated In Chapter 4 Yb2Si2O7 Y2Si2O7

and their solid-solution (Yb18Y02Si2O7 and Yb1Y1Si2O7) EBC ceramics were also analyzed with

CMAS They were tested with 3 different CMAS compositions (with different CaSi ratios) It was

shown that in some cases solid-solutions can temper the failure mechanisms of the pure

components like in the NAVAIR CMAS while also lowering the thermal conductivity of the EBC

(Chapter 5) It has been shown that dense polycrystalline pellets can be used as lsquomodelrsquo

experiments to determine the reaction between EBC materials and CMAS glass However the

microstructure of coatings is different to that of polycrystalline pellets Therefore the next step

was to determine how air plasma sprayed (APS) EBCs would interact with CMAS

Unfortunately EBC deposition is still a significant challenge [3940] Conventional air

plasma spray (APS) is preferred due to its efficiency and relative low cost However the EBCs

typically deposit as an amorphous coating [41] To crystallize the coating during spraying many

researchers have performed APS inside a box furnace where the substrate is heated to temperatures

above 1000 degC [1733364243] but this is difficult in a manufacturing setting Garcia et al [41]

has studied the microstructural evolution when a post-deposition heat treatment is performed on

APS Yb2Si2O7 EBC coatings with different spray conditions Crystallization has a significant

volume change which can lead to porous coatings Also undesirable phases may form during

110

crystallization However it was determined that a more amorphous coating included less porosity

initially and fewer SiO2 inclusions

In this context there are only a few studies on Yb2Si2O7 EBC coatings and their interactions

with CMAS [333536] Stolzenburg et al [33] and Zhao et al [36] both used APS coatings

Stolzenburg et al [33] obtained and studied coatings produced by Rolls Royce however the APS

processing parameters were not disclosed Zhao et al [36] sprayed coatings into a furnace at 1200

degC to produce a crystalline coating Poerschke et al [35] used electron-beam-directed vapor

deposition (EB-DVD) to produce coatings Poerschke et al [35] applied a TBC on top of the Yb-

silicate EBC which makes the interactions indirect and strongly influenced by the TBC

Zhao et al [36] and Stolzenburg et al [33] used the same CMAS composition (a high CaSi

ratio (= 073)) but found differing results Zhao et al [36] showed Yb-Ca-Si apatite (ss) formation

in APS coatings when interacted with CMAS whereas Stolzenburg et al [33] showed little

reaction between the Yb2Si2O7 EBC and the CMAS This could be due to Yb2SiO5 areas found in

the Yb2Si2O7 coatings used by Zhao et al [36]

There is little known about the interaction between CMAS and solid-solution ie

Yb1Y1Si2O7 APS coatings

Here the interactions at 1500 degC of two APS EBCs of compositions Yb2Si2O7 and

Yb1Y1Si2O7 with a lsquomodelrsquo CMAS Naval Air Systems Command (NAVAIR) CMAS (CaSi =

076) have been studied [116117128] The objective is to provide insights into the chemo-thermo-

mechanical mechanisms of these interactions and to use this understanding to guide the design

and development of future CMAS-resistant low thermal-conductivity TEBCs

111


621 Air Plasma Sprayed Coatings


Oerlikon Metco Westbury NY) The β-Yb1Y1Si2O7 powders were also obtained from Oerlikon

Metco in collaboration with Dr Gopal Dwivedi as an experimental RampD powder

The coatings were sprayed by our colleagues at Stony Brook University Professor Sanjay

Sampath and Dr Eugenio Garcia The coatings Yb2Si2O7 and Yb1Y1Si2O7 were air plasma

sprayed using a F4MB-XL plasma gun (Oerlikon Metco Westbury NY) controlled by a 9MC

console (Oerlikon-Metco Westbury NY) The spray parameters used for both powders were as-

plasma forming gas Ar with a flow rate of 475 standard liters per minute (slpm) a secondary

gas H2 with a flow rate of 9 slpm and a current of 550 A These conditions reported a voltage of

712 V or a power of 392 kW The stand-of distance was maintained at 150 mm The raster speed

was 500 mms-1 A mass rate of 12 gmin-1 was used for both powders

622 Heat Treatments

Some as-sprayed β-Yb2Si2O7 and β-Yb1Y1Si2O7 coatings were analyzed as arrived which

will be described below in Section 624 Some of the as-sprayed coatings were placed on Pt sheets

for a heat treatment at 1300 degC for 4 h in air in a box furnace (CM Furnaces Inc Bloomfield NJ)



MgO which is from a previous study [128] and in Chapters 2-4 and it is close to the composition

of the AFRL-03 standard CMAS (desert sand) Powder of this CMAS glass composition was

112

prepared using a procedure described elsewhere [7086] CMAS interaction studies were

performed by applying the CMAS powder paste (in ethanol) uniformly over the center of the heat-

treated Yb2Si2O7 and Yb1Y1Si2O7 APS coatings at sim15 mgcm-2 loading The specimens were then

placed on a Pt sheet with the CMAS-coated surface facing up and heat-treated in the box furnace

at 1500 degC in air for 24 h (10 degCmin-1 heating and cooling rates) The CMAS-interacted coatings

were then cut using a low-speed diamond saw and the cross-sections were polished to a 1-μm

finish


The as-sprayed and heat-treated APS coatings were characterized using an X-ray


The phases present were identified using the PDF2 database In-situ high-temperature XRD of the

as-sprayed Yb1Y1Si2O7 APS coating at 25 800 900 1000 1100 1200 1300 and 1350 degC were

conducted to determine the temperature needed for the coatings to crystallize A ramping rate of

10 degCmin-1 was used and the temperatures were held for 10 minutes before the XRD scan was

performed

The densities of the as-sprayed and heat-treated coatings were measured using the


Cross-sections of the as-sprayed heat-treated and CMAS-interacted APS coatings were

observed in a scanning electron microscope (SEM LEO 1530VP Carl Zeiss Munich Germany

or Helios 600 FEI Hillsboro Oregon USA) equipped with energy-dispersive spectroscopy

(EDS Inca Oxford Instruments Oxfordshire UK) operated at 20 kV accelerating voltage EDS

113


penetration into the pellets

63 Results

631 As-sprayed and Heat-Treated Coatings

As-received as-sprayed Yb2Si2O7 APS coatings were cross-sectioned and SEM

micrographs can be found in Figures 58A and 58B The Yb2Si2O7 coating is ~1 mm thick and

some porosity is observed There are lighter and darker gray regions in this microstructure

indicating a change in silica concentration Lighter regions have lower amounts of silica which

was confirmed using EDS Figure 58C shows the indexed XRD patterns for the Yb2Si2O7 APS

coating XRD was collected on both the top and bottom of the coating Slight differences can be

seen between the top to bottom of the coating but both confirm that the coating is mostly

amorphous with small amounts of un-melted particles




showing a mostly amorphous coating

114

Figures 59A and 59B show SEM micrographs of the as-received as-sprayed Yb1Y1Si2O7

APS coating Like the Yb2Si2O7 coating porosity is observed and there are lighter (less silica) and

darker (more silica) gray regions in this microstructure The Yb1Y1Si2O7 coating is ~15 mm thick

Figure 59C shows the indexed XRD pattern for the Yb1Y1Si2O7 APS coating Again XRD patterns

were collected on both the top and bottom of the coating The bottom of the coating is almost

purely amorphous The top of the coating shows more peaks indicating it contains more un-melted

Yb1Y1Si2O7 particles Both show a mostly amorphous coating





To determine the heat treatment needed to crystallize the coatings in-situ high-temperature

XRD on the Yb1Y1Si2O7 APS coating was conducted and can be found in Figure 60 Between 25

and 900 degC the coating remains amorphous At 1000 degC crystalline peaks begin to emerge The

coating at 1100 and 1200 degC seems to be forming Yb1Y1SiO5 over β-Yb1Y1Si2O7 At 1300 degC the

coating is crystalline and contains more β-Yb1Y1Si2O7 than Yb1Y1SiO5 At 1350 degC the XRD

remains the same as the 1300 degC XRD pattern Therefore 1300 degC was selected as the heat

treatment temperature for the APS coatings

115




the square markers and dashed line index the Yb1Y1SiO5 phase

Heat treatments at 1300 degC for 4 hours were performed on both coatings Figures 61A and

61B show SEM micrographs of the heat-treated crystalline Yb2Si2O7 APS coating The density of

all the coatings can be found in Table 22 The density of the Yb2Si2O7 coating after heat treatment

is 612 Mgm-3 When compared to the theoretical density of Yb2Si2O7 the relative density is 99

However as seen in the micrographs and the XRD (Figure 61C) there is also Yb2SiO5 present

which has a higher density of 692 Mgm-3 [189] This would increase the coatings relative density

compared to pure Yb2Si2O7

116





showing both Yb2Si2O7 and Yb2SiO5 are present


treated (HT 1300 degC 4 h) Yb2Si2O7 and Yb1Y1Si2O7 APS coatings

Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity

Yb2Si2O7 As-sprayed 639 615 104 4

Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5

Yb1Y1Si2O7 As-sprayed 492 5045 98 4

Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3

Figures 62A and 62B show SEM micrographs of the heat-treated (1300 degC 4 h) crystalline

Yb1Y1Si2O7 APS coating Porosity is observed along with Yb1Y1Si2O7 and Yb1Y1SiO5 This is

also confirmed by XRD in Figure 62C Based on the peak height ratio of the XRD patterns the

Yb1Y1Si2O7 APS coating contains less RE2SiO5 than the Yb2Si2O7 APS coating which is also

confirmed in the SEM micrographs The density of the heat-treated (1300degC 4 h) Yb1Y1Si2O7

APS coating is 481 Mgm-3 which is ~95 dense relative to pure Yb1Y1Si2O7 (calculated by rule-

of-mixtures from Yb2Si2O7 and Y2Si2O7) As stated above the relative density could be skewed

due the presence of Yb1Y1SiO5 The theoretical density of Yb1Y1SiO5 calculated by rule-of-

117

mixtures of Yb2SiO5 and Y2SiO5 (444 Mgm-3 [190]) is 568 Mgm-3 which is higher than that of

the pure Yb1Y1Si2O7





sides showing Yb1Y1Si2O7 and Yb1Y1SiO5 are present


All CMAS interactions were performed on the crystalline or heat-treated (1300 degC 4 h)

APS coatings

Figure 63A is a cross-sectional SEM micrograph of a Yb2Si2O7 APS coating that has

interacted with CMAS at 1500 degC for 24 h Figure 63B is a higher magnification image of the

region indicated in Figure 63A and its corresponding Si Ca and Yb elemental EDS maps No

CMAS glass is observed on the top of the coating The dashed line indicates the approximate

CMAS penetration

118




corresponding Si Ca and Yb elemental EDS maps

Figures 64A 64B and 64D are higher magnification cross-sectional SEM images of a

Yb2Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 64C and 64E

are Ca elemental EDS maps corresponding to Figures 64B and 64D respectively The EDS

elemental compositions of regions 1 to 7 are reported in Table 23 The top of the coating has a

thin Yb-Ca-Si apatite (ss) layer (region 1) Further into the coating more Yb-Ca-Si apatite (ss)

can be found (region 2) In the region containing the Yb-Ca-Si apatite phase (ss) Yb2Si2O7 is

also present However there is no Yb2SiO5 present in that region (~40 μm in depth) Even further

into the coating Yb2Si2O7 (regions 4 and 6) and Yb2SiO5 (regions 3 5 and 7) can be found

119










Region Yb Ca Si Phase

1 45 12 43 Yb-Ca-Si Apatite (ss)


3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions

500 125 375 Yb8Ca2(SiO4)6O2 Apatite

500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120

Figure 65A is a cross-sectional SEM micrograph of a Yb1Y1Si2O7 APS coating that has




CMAS penetration




its corresponding Si Ca Y and Yb elemental EDS maps


Yb1Y1Si2O7 APS coating that has interacted with CMAS at 1500 degC for 24 h Figures 66C and

66E are Ca elemental EDS maps corresponding to Figures 66B and 66D respectively The EDS


layer of Yb-Y-Ca-Si apatite (ss) (region 1) Further into the coating more Yb-Y-Ca-Si apatite

(ss) can be found (region 3 and Figure 66C) In the region containing the Yb-Y-Ca-Si apatite

phase (ss) Yb1Y1Si2O7 is also present (regions 2 and 4) However there is no Yb1Y1SiO5

present in that region (~150 μm in depth) This is clearly observed in the Si elemental EDS map

121

in Figure 65 Even further into the coating (Figure 66D) Yb2Si2O7 (regions 5 and 7) and

Yb2SiO5 (regions 6 and 8) can be found







122




Region Yb Y Ca Si Phase

1 21 21 12 46 Yb-Y-Ca-Si Apatite (ss)

2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions

250 250 125 375 Yb4Y4Ca2(SiO4)6O2 Apatite

250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion

Both APS coatings Yb2Si2O7 and Yb1Y1Si2O7 showed apatite (ss) formation In Chapter

3 it was demonstrated that Yb2Si2O7 when in contact with the same CMAS (NAVAIR CaSi ratio

= 076) can form Yb-Ca-Si apatite (ss) However it did not form as readily as the Yb1Y1Si2O7

pellet seen in Chapter 4 There is higher propensity to form apatite (ss) in Y3+ containing materials

than in the Yb3+ due to the ionic radii size This can also be seen in the APS coatings More apatite

formation is found in the Yb1Y1Si2O7 APS coating

Another explanation for the formation of apatite (ss) can be the RE2SiO5 phase found in

the APS coatings It has an enhanced effect on the formation of apatite (ss) [3672] Zhao et al

[36] compared Yb2Si2O7 and Yb2SiO5 APS coatings and their interactions with CMAS (CaSi ratio

= 073) Yb2SiO5 was shown to react more readily with CMAS to form Yb-Ca-Si apatite (ss) [36]

Jang et al [72] also observed Yb-Ca-Si apatite (ss) forms as a continuous layer on dense sintered

polycrystalline Yb2SiO5 pellets

123

In both the Yb2Si2O7 and Yb1Y1Si2O7 APS coatings a nearly continuous layer of apatite

(ss) is found on the surface of the coating No pockets of CMAS glass were found Below the

surface there are grains of apatite (ss) which can be seen in Figures 64 and 66 for Yb2Si2O7 and

Yb1Y1Si2O7 respectively The formation of apatite (ss) could be due to the RE2SiO5 (RE = Yb

YbY) present The depth of CMAS penetration in the Yb2Si2O7 APS coating based on the

elemental Ca map is ~40 μm which is relatively small compared to that of the Yb1Y1Si2O7 (~150

μm) This could be due to the placement of the cross-section (slightly off center of the CMAS

interaction zone) or the amount of Yb2SiO5 in the Yb2Si2O7 coating The more RE2SiO5 (RE = Yb

YbY) in the coating the faster the CMAS is consumed This is due to the reaction between the

RE2SiO5 (RE = Yb YbY) and the CMAS melt CaO and SiO2 are needed to form apatite (ss) The

example reaction for the pure Yb system is shown

4Yb2SiO5 + 2CaO (melt) + 2SiO2(melt) rarr Ca2Yb8(SiO4)6O2 (Equation 11)

Yb2Si2O7 contains the required amount of SiO2 to form apatite (ss) so only CaO is removed from

the melt

4Yb2Si2O7 + 2CaO (melt) rarr Ca2Yb8(SiO4)6O2 + 2SiO2(melt) (Equation 12)

In fact excess SiO2 from the Yb2Si2O7 is added into the melt

In the pellets of pure Yb2Si2O7 and Yb1Y1Si2O7 the CMAS remained either in grain

boundaries or on the surface of the pellet respectively However in the APS coatings RE2SiO5

(RE = Yb YbY) is present and another reaction with the CMAS can occur

Yb2SiO5 + 2SiO2(melt) rarr Yb2Si2O7 (Equation 13)

This is observed in both coatings but it is more apparent in the Yb1Y1Si2O7 APS coating in the Si

elemental EDS map in Figure 65 The top region shows only apatite (ss) and Yb1Y1Si2O7 which

have approximately the same Si concentration this is the CMAS interaction zone Below that in

124

the bottom region there are areas of lower Si concentration or Yb1Y1SiO5 Due to these reactions

the CMAS is almost completely consumed by the formation of apatite (ss) and RE2Si2O7 (RE =

Yb YbY) in these APS coatings

The lsquoblisteringrsquo damage mechanism was not observed in the either APS coating This could

be due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the

RE2SiO5 (RE = Yb YbY) and the melt or the porosity seen in these coatings which stops the

formation of a dilatation gradient

65 Future Work

There is ongoing work for the APS coatings and CMAS interaction studies Currently a

post-doctoral fellow Dr Hadas Sternlicht is focusing on the crystallization of these coatings She

is also working on confirming solid-solutions of the Yb1Y1Si2O7 coating using TEM

The quantitative amounts of RE2Si2O7 and RE2SiO5 in the APS coatings will also be

determined through high-resolution XRD and rietveld analysis

CMAS interaction studies (1500 degC 24 h) of these APS coatings with the CMASs used in

Chapter 4 (NASA CMAS and Icelandic Volcanic Ash (IVA) CMAS) should be done to complete

a systematic study However it is believed that the other CMASs with lower CaSi ratios (NASA

= 044 and IVA = 010) would mostly show RE2Si2O7 formation and limited or no apatite (ss)

formation

66 Summary

Here amorphous as-sprayed APS coatings of Yb2Si2O7 and Yb1Y1Si2O7 were studied A

heat treatment of 4 h at 1300 degC was performed to obtain crystalline coatings The crystalline

125

coatings were found to contain both β-RE2Si2O7 and RE2SiO5 (RE = Yb YbY) Based on XRD

and cross-sectional SEM micrographs the Yb2Si2O7 APS coating has a higher RE2SiO5 to β-

RE2Si2O7 ratio than the Yb1Y1Si2O7 APS coatings

The high-temperature (1500 degC 24 h) interactions of the two promising APS EBCs

Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS glass (NAVAIR CaSi ratio = 076) were studied

CMAS glass was consumed by the formation of apatite (ss) and RE2Si2O7 (RE = Yb YbY) due to

the presence of RE2SiO5 (RE = Yb YbY) in the APS coatings and CaO and SiO2 in the CMAS

melt Therefore no remaining CMAS glass was observed in either coatings

The lsquoblisteringrsquo damage mechanism was not observed in the APS coatings This could be

due to the depletion of CMAS through the formation of RE2Si2O7 (RE = Yb YbY) from the



126

CHAPTER 7 CONCLUSIONS AND FUTURE WORK

71 Summary and Conclusions

Ceramic-matrix-composites (CMCs) typically comprising of a SiC-based matrix and

fibers are showing great promise in the enginersquos hot-section due to their inherently high

temperature capabilities [46ndash8] However the oxygen and steam present in the high-velocity hot-

gas stream in the engine causes the SiC-based CMCs to undergo active oxidation and recession

[411ndash13] Thus SiC-based CMCs need to be protected by ceramic environmental barrier coatings

(EBCs) [49131617] EBCs must also have low SiO2 activity among other requirements

[131617]

Gas-turbine engines can ingest silicates collectively referred to as calcia-magnesia-

aluminosilicate (CMAS) [3459146] CMAS can be in the form of airborne sand runway debris

or volcanic ash in aircraft engines and ambient dust andor fly ash in power-generation engines

Since the surface temperatures of EBCs are expected to be well above the melting point of most

CMAS the ingested CMAS will melt adhere to the EBC surface and attack the EBC The CMAS

attack of EBCs is expected to be severe due to the high operating temperatures and the fact that

all the relevant processes (diffusion reaction viscosity etc) are thermally-activated [4146]

Since EBCs need to be dense it is preferred that they have low reactivity with the CMAS

to retain the EBCrsquos integrity Optical-basicity (OB or Λ) is introduced as a screening criterion for

choosing CMAS-resistant EBC ceramics In this context a small OB difference between CMAS

and potential EBC ceramics is desired [78] Therefore rare-earth pyrosilicates (RE = rare earth

RE2Si2O7) such as γ-Y2Si2O7 and β-Yb2Si2O7 have been identified as promising CMAS-resistant

EBC ceramics [78] It should be emphasized that the OB-difference analysis provides a rough

screening criterion based purely on chemical considerations The actual reactivity will depend on

127

many other factors including the nature of the cations in the EBC ceramics the CMAS

composition and the relative stability of the reaction products

In Chapter 2 the high-temperature (1500 ˚C) interactions of two promising dense

polycrystalline EBC ceramics YAlO3 (YAP) and -Y2Si2O7 with a CMAS (NAVAIR CaSi ratio

= 076) glass have been explored as part of a model study Despite the fact that the optical basicities

of both the Y-containing EBC ceramics and the CMAS are similar reactions with the CMAS

occur In the case of the Si-free YAlO3 the reaction zone is small and it comprises three regions

of reaction-crystallization products including Y-Ca-Si apatite solid-solution (ss) and Y3Al5O12

(YAG (ss)) In contrast only Y-Ca-Si apatite (ss) forms in the case of Si-containing -Y2Si2O7

and the reaction zone is an order-of-magnitude thicker This is attributed to the presence of the Y

in the YAlO3 and γ-Y2Si2O7 EBC ceramics These CMAS interactions are found to be strikingly

different than those observed in Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7 and β-Lu2Si2O7)

in Chapter 3

Little or no reaction is found between the Y-free EBC ceramics (β-Yb2Si2O7 β-Sc2Si2O7

and β-Lu2Si2O7) and the CMAS in Chapter 3 In the case of β-Yb2Si2O7 a small amount of

reaction-crystallization product Yb-Ca-Si apatite (ss) forms whereas none is detected in the cases

of β-Sc2Si2O7 and β-Lu2Si2O7 The CMAS glass penetrates the grain boundaries of the Y-free EBC

ceramics and they suffer from a new damage mechanism lsquoblisterrsquo cracking This is attributed to

the through-thickness dilatation-gradient caused by the slow grain-boundary-penetration of the

CMAS glass The success of a lsquoblisteringrsquo-damage-mitigation approach is demonstrated where 1

vol CMAS glass is mixed into the β-Yb2Si2O7 powder prior to sintering The CMAS-glassy

phase at the grain boundaries promotes rapid CMAS glass penetration thereby eliminating the

dilatation-gradient

128

Based on the interactions with CMAS in Chapters 2 and 3 an interesting possibility of

tempering these extreme CMAS-interaction behaviors by forming binary solid-solution EBC

ceramics was proposed and studied in Chapter 4 High-temperature (1500 degC) interactions of

environmental-barrier coating (EBC) ceramics in the rare-earth pyrosilicates system Yb(2-

x)YxSi2O7 (x=0 02 1 or 2) with three different CMAS glass compositions are explored Only the

CaSi ratio is varied in the CMAS 076 (NAVAIR) 044 (NASA) or 010 (Icelandic Volcanic

Ash) Interaction between the highest-CaSi CMAS and the EBC ceramic with the lowest x (= 0

Yb2Si2O7) promotes no reaction and formation of lsquoblisterrsquo cracks In contrast the highest x (= 2

Y2Si2O7) promotes formation of an apatite (ss) reaction product but no lsquoblisterrsquo cracks

Observationally it is found that a decrease in the CMAS CaSi ratio (076 to 010) and a decrease

in Y-content or x (2 to 0) decreases the propensity for the reaction-crystallization (apatite

formation) and lsquoblisterrsquo cracks These observations are rationalized based on the ionic radii size

Y3+ is closer to that of Ca2+ than is Yb3+ which is the driving force for apatite (ss) formation This

suggests a way to tune the CMAS interactions in rare-earth pyrosilicate solid-solutions

Chapter 5 introduces a new concept based on the formation of solid-solutions thermal

environmental barrier coatings (TEBCs) or a coating that has the ability to act as both an EBC

and a TBC The thermal conductivities of six binary solid-solutions were analytically calculated

The thermal conductivities of Yb(2-x)YxSi2O7 (x = 02 and 1) were obtained experimentally and

compared to calculated data A 5-component equiatomic β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 was

also studied Between room temperature and 600 degC a large decrease in thermal conductivity

compared to the other materials studied in this chapter was observed However at higher

temperatures the thermal conductivity plateaued The lack of the expected decrease in thermal

129

conductivity of the Yb(2-x)YxSi2O7 (x = 02 and 1) solid-solutions and β-

(Y02Yb02Lu02Sc02Gd02)2Si2O7 could be attributed to the ldquominimum conductivityrdquo limit

Based on interactions with CMAS in the previous chapters (2ndash4) two potential EBC

ceramics Yb2Si2O7 and Yb1Y1Si2O7 were chosen to be deposited as coatings using air plasma

spray (APS) In Chapter 6 the high-temperature (1500 ˚C) interactions of two promising APS

coatings Yb2Si2O7 and Yb1Y1Si2O7 with a CMAS (NAVAIR CaSi ratio = 076) glass have been

explored as part of a model study Before CMAS testing could occur the APS coatings needed to

be heat-treated (1300 degC 4 h) to obtain a crystalline structure The coatings contained RE2SiO5 as

well as the desired β-RE2Si2O7 The high-temperature (1500 degC 24 h) CMAS interactions found

the presence of apatite (ss) near the surface of the coatings while no CMAS glass was observed

Instead the CMAS glass has interacted with the APS coatings to not only form apatite (ss) but

also RE2Si2O7 (RE = Yb YbY) This is due to the presence of RE2SiO5 (RE = Yb YbY) in the

APS coatings and SiO2 in the CMAS melt The lsquoblisteringrsquo damage mechanism found in the pellets

was not observed in the APS coatings which could be due to the depletion of CMAS or the

porosity in the coatings

72 Future Work

Although we have gained insight into potential coatings used as EBCs on hot-section

components in gas-turbine engines there is more that needs to be researched In the context of

dense polycrystalline pellets the interaction with NASA CMAS (CaSi ratio = 044) should be

studied in more detail The results obtained show no lsquoblisteringrsquo cracks and full penetration of

CMAS into grain boundaries which is not the case for the NAVAIR CMAS The reason behind

this is not known and should be investigated further

130

Another area of focus will be water vapor corrosion studies on the dense polycrystalline

solid-solution pellets Yb18Y02Si2O7 and Yb1Y1Si2O7 and their pure components Yb2Si2O7 and

Y2Si2O7 Most of this testing has already been conducted by our colleagues at the University of

Virginia Professor Elizabeth Opila Dr Rebekah Webster and Mr Mackenzie Ridley These data

are still in the process of being analyzed to determine the recession of the pellet and the reaction

products The impingement site can be seen in Figures 67Andash67D Cross-sectional SEM

micrographs of the impingement zone can be seen in Figures 67Endash67H Their corresponding Si

elemental EDS maps can be seen in Figures 67Indash67L respectively




corresponding Si elemental EDS maps to (E-H) respectively

The equiatomic solid-solution RE2Si2O7 mixtures should be a major subject of interest

moving forward So far β-(Y02Yb02Lu02Sc02Gd02)2Si2O7 has been studied confirmed to be a

homogeneous solid-solution and showed a decrease in thermal conductivity compared to pure

131

RE2Si2O7 ceramics However the CMAS resistance and water-vapor corrosion has not yet been

studied

Another investigation exploring other potential 4 or 5 equiatomic RE2Si2O7 using

combinations of known RE2Si2O7 (RE = Y Yb Sc Lu Gd Nb Ho etc) should be conducted

As mentioned in Chapter 6 there is ongoing work on the crystallization porosity and solid-

solution homogeneity of the APS Yb2Si2O7 and Yb1Y1Si2O7 coatings Quantitative analysis should

also be explored through high-resolution XRD and Rietveld analysis Finally CMAS interaction

studies (1500 degC 24 h) of these APS coatings with the other two CMASs used in Chapter 4 will

be done to complete this systematic study

These tests have been conducted but the data have not been analyzed yet due to a labmicroscopy

facility shutdown

132

REFERENCES

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Applications Science 296 (2002) 280ndash284 httpsdoiorg101126science1068609

[2] R Darolia Thermal barrier coatings technology critical review progress update remaining

challenges and prospects International Materials Reviews 58 (2013) 315ndash348

httpsdoiorg1011791743280413Y0000000019

[3] DR Clarke M Oechsner NP Padture Thermal-barrier coatings for more efficient gas-

turbine engines MRS Bull 37 (2012) 891ndash898 httpsdoiorg101557mrs2012232

[4] NP Padture Advanced structural ceramics in aerospace propulsion Nature Mater 15 (2016)

804ndash809 httpsdoiorg101038nmat4687

[5] W Pan SR Phillpot C Wan A Chernatynskiy Z Qu Low thermal conductivity oxides

MRS Bull 37 (2012) 917ndash922 httpsdoiorg101557mrs2012234

[6] JH Perepezko The Hotter the Engine the Better Science 326 (2009) 1068ndash1069

httpsdoiorg101126science1179327

[7] NP Bansal J Lamon Ceramic Matrix Composites Materials Modelling and Technology

John Wiley amp Sons Hoboken NJ USA 2014

[8] FW Zok Ceramic-matrix composites enable revolutionary gains in turbine engine

efficiency American Ceramic Society Bulletin 95 (nd) 7

[9] E Bakan DE Mack G Mauer R Vaszligen J Lamon NP Padture High-temperature

materials for power generation in gas turbines in O Guillon (Ed) Advanced Ceramics for

Energy Conversion and Storage Elsevier 2020

[10] NP Bansal Handbook of Ceramic Composites Kluwer Academic Publishers New York

2005

[11] EJ Opila JL Smialek RC Robinson DS Fox NS Jacobson SiC Recession Caused by

SiO 2 Scale Volatility under Combustion Conditions II Thermodynamics and Gaseous-

Diffusion Model Journal of the American Ceramic Society 82 (1999) 1826ndash1834

httpsdoiorg101111j1151-29161999tb02005x

[12] PJ Meschter EJ Opila NS Jacobson Water VaporndashMediated Volatilization of High-

Temperature Materials Annu Rev Mater Res 43 (2013) 559ndash588

httpsdoiorg101146annurev-matsci-071312-121636

[13] D Zhu Advanced environmental barrier coatings in T Ohji M Singh (Eds) Engineered

Ceramics Current Status and Future Prospects John Wiley amp Sons Hoboken NJ USA

2016

133

[14] NS Jacobson Corrosion of Silicon-Based Ceramics in Combustion Environments J

American Ceramic Society 76 (1993) 3ndash28 httpsdoiorg101111j1151-

29161993tb03684x

[15] EJ Opila RE Hann Paralinear Oxidation of CVD SiC in Water Vapor Journal of the


29161997tb02810x

[16] KN Lee Current status of environmental barrier coatings for Si-Based ceramics Surface

and Coatings Technology 133ndash134 (2000) 1ndash7 httpsdoiorg101016S0257-

8972(00)00889-6

[17] KN Lee DS Fox NP Bansal Rare earth silicate environmental barrier coatings for

SiCSiC composites and Si3N4 ceramics Journal of the European Ceramic Society 25

(2005) 1705ndash1715 httpsdoiorg101016jjeurceramsoc200412013

[18] KN Lee DS Fox JI Eldridge D Zhu RC Robinson NP Bansal RA Miller Upper

Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS Journal

of the American Ceramic Society 86 (2003) 1299ndash1306 httpsdoiorg101111j1151-

29162003tb03466x

[19] S Ueno DD Jayaseelan T Ohji Development of Oxide-Based EBC for Silicon Nitride

International Journal of Applied Ceramic Technology 1 (2004) 362ndash373


[20] WD Summers DL Poerschke AA Taylor AR Ericks CG Levi FW Zok Reactions

of molten silicate deposits with yttrium monosilicate J Am Ceram Soc 103 (2020) 2919ndash

2932 httpsdoiorg101111jace16972




[22] CG Parker EJ Opila Stability of the Y 2 O 3 ndashSiO 2 system in high‐temperature high‐

velocity water vapor J Am Ceram Soc 103 (2020) 2715ndash2726

httpsdoiorg101111jace16915

[23] G Costa BJ Harder VL Wiesner D Zhu N Bansal KN Lee NS Jacobson D Kapush

SV Ushakov A Navrotsky Thermodynamics of reaction between gas-turbine ceramic

coatings and ingested CMAS corrodents Journal of the American Ceramic Society 102

(2019) 2948ndash2964 httpsdoiorg101111jace16113

[24] VL Wiesner BJ Harder NP Bansal High-temperature interactions of desert sand CMAS

glass with yttrium disilicate environmental barrier coating material Ceramics International

44 (2018) 22738ndash22743 httpsdoiorg101016jceramint201809058

134

[25] J Liu L Zhang Q Liu L Cheng Y Wang Calciumndashmagnesiumndashaluminosilicate corrosion

behaviors of rare-earth disilicates at 1400degC Journal of the European Ceramic Society 33


[26] JL Stokes BJ Harder VL Wiesner DE Wolfe High-Temperature thermochemical

interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating

materials Journal of the European Ceramic Society 39 (2019) 5059ndash5067

httpsdoiorg101016jjeurceramsoc201906051

[27] WD Summers DL Poerschke D Park JH Shaw FW Zok CG Levi Roles of

composition and temperature in silicate deposit-induced recession of yttrium disilicate Acta

Materialia 160 (2018) 34ndash46 httpsdoiorg101016jactamat201808043

[28] J Xiao Q Liu J Li H Guo H Xu Microstructure and high-temperature oxidation behavior

of plasma-sprayed SiYb2SiO5 environmental barrier coatings Chinese Journal of

Aeronautics 32 (2019) 1994ndash1999 httpsdoiorg101016jcja201809004

[29] BT Richards S Sehr F de Franqueville MR Begley HNG Wadley Fracture

mechanisms of ytterbium monosilicate environmental barrier coatings during cyclic thermal

exposure Acta Materialia 103 (2016) 448ndash460

httpsdoiorg101016jactamat201510019

[30] X Zhong Y Niu H Li T Zhu X Song Y Zeng X Zheng C Ding J Sun Comparative

study on high-temperature performance and thermal shock behavior of plasma-sprayed

Yb2SiO5 and Yb2Si2O7 coatings Surface and Coatings Technology 349 (2018) 636ndash646

httpsdoiorg101016jsurfcoat201806056

[31] M-H Lu H-M Xiang Z-H Feng X-Y Wang Y-C Zhou Mechanical and Thermal

Properties of Yb 2 SiO 5  A Promising Material for TEBCs Applications J Am Ceram Soc

99 (2016) 1404ndash1411 httpsdoiorg101111jace14085

[32] T Zhu Y Niu X Zhong J Zhao Y Zeng X Zheng C Ding Influence of phase

composition on microstructure and thermal properties of ytterbium silicate coatings deposited

by atmospheric plasma spray Journal of the European Ceramic Society 38 (2018) 3974ndash

3985 httpsdoiorg101016jjeurceramsoc201804047

[33] F Stolzenburg P Kenesei J Almer KN Lee MT Johnson KT Faber The influence of

calciumndashmagnesiumndashaluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer

environmental barrier coatings Acta Materialia 105 (2016) 189ndash198


[34] F Stolzenburg MT Johnson KN Lee NS Jacobson KT Faber The interaction of

calciumndashmagnesiumndashaluminosilicate with ytterbium silicate environmental barrier materials

Surface and Coatings Technology 284 (2015) 44ndash50


135

[35] DL Poerschke DD Hass S Eustis GGE Seward JS Van Sluytman CG Levi Stability

and CMAS Resistance of Ytterbium-SilicateHafnate EBCsTBC for SiC Composites J Am

Ceram Soc 98 (2015) 278ndash286 httpsdoiorg101111jace13262

[36] H Zhao BT Richards CG Levi HNG Wadley Molten silicate reactions with plasma

sprayed ytterbium silicate coatings Surface and Coatings Technology 288 (2016) 151ndash162


[37] J Felsche The crystal chemistry of the rare-earth silicates in Rare Earths Springer Berlin

Heidelberg Berlin Heidelberg 1973 pp 99ndash197 httpsdoiorg1010073-540-06125-8_3

[38] AJ Fernaacutendez-Carrioacuten MD Alba A Escudero AI Becerro Solid solubility of Yb2Si2O7

in β- γ- and δ-Y2Si2O7 Journal of Solid State Chemistry 184 (2011) 1882ndash1889

httpsdoiorg101016jjssc201105034

[39] E Bakan D Marcano D Zhou YJ Sohn G Mauer R Vaszligen Yb2Si2O7 Environmental

Barrier Coatings Deposited by Various Thermal Spray Techniques A Preliminary

Comparative Study J Therm Spray Tech 26 (2017) 1011ndash1024

httpsdoiorg101007s11666-017-0574-1

[40] E Bakan G Mauer YJ Sohn D Koch R Vaszligen Application of High-Velocity Oxygen-

Fuel (HVOF) Spraying to the Fabrication of Yb-Silicate Environmental Barrier Coatings

Coatings 7 (2017) 55 httpsdoiorg103390coatings7040055

[41] E Garcia H Lee S Sampath Phase and microstructure evolution in plasma sprayed

Yb2Si2O7 coatings Journal of the European Ceramic Society 39 (2019) 1477ndash1486


[42] BT Richards KA Young F de Francqueville S Sehr MR Begley HNG Wadley

Response of ytterbium disilicatendashsilicon environmental barrier coatings to thermal cycling in

water vapor Acta Materialia 106 (2016) 1ndash14


[43] BT Richards HNG Wadley Plasma spray deposition of tri-layer environmental barrier

coatings Journal of the European Ceramic Society 34 (2014) 3069ndash3083


[44] S Ramasamy SN Tewari KN Lee RT Bhatt DS Fox Slurry based multilayer

environmental barrier coatings for silicon carbide and silicon nitride ceramics mdash I

Processing Surface and Coatings Technology 205 (2010) 258ndash265


[45] Y Lu Y Wang Formation and growth of silica layer beneath environmental barrier coatings

under water-vapor environment Journal of Alloys and Compounds 739 (2018) 817ndash826

httpsdoiorg101016jjallcom201712297

[46] MP Appleby D Zhu GN Morscher Mechanical properties and real-time damage

evaluations of environmental barrier coated SiCSiC CMCs subjected to tensile loading under

136

thermal gradients Surface and Coatings Technology 284 (2015) 318ndash326


[47] T Yokoi N Yamaguchi M Tanaka D Yokoe T Kato S Kitaoka M Takata Preparation

of a dense ytterbium disilicate layer via dual electron beam physical vapor deposition at high

temperature Materials Letters 193 (2017) 176ndash178

httpsdoiorg101016jmatlet201701085

[48] SN Basu T Kulkarni HZ Wang VK Sarin Functionally graded chemical vapor

deposited mullite environmental barrier coatings for Si-based ceramics Journal of the

European Ceramic Society 28 (2008) 437ndash445


[49] P Mechnich Y2SiO5 coatings fabricated by RF magnetron sputtering Surface and Coatings

Technology 237 (2013) 88ndash94 httpsdoiorg101016jsurfcoat201308015

[50] DD Jayaseelan S Ueno T Ohji S Kanzaki Solndashgel synthesis and coating of

nanocrystalline Lu2Si2O7 on Si3N4 substrate Materials Chemistry and Physics 84 (2004)

192ndash195 httpsdoiorg101016jmatchemphys200311028

[51] KN Lee Yb 2 Si 2 O 7 Environmental barrier coatings with reduced bond coat oxidation

rates via chemical modifications for long life J Am Ceram Soc 102 (2019) 1507ndash1521


[52] NS Jacobson Silica Activity Measurements in the Y 2 O 3 -SiO 2 System and Applications

to Modeling of Coating Volatility J Am Ceram Soc 97 (2014) 1959ndash1965


[53] GCC Costa NS Jacobson Mass spectrometric measurements of the silica activity in the

Yb2O3ndashSiO2 system and implications to assess the degradation of silicate-based coatings in

combustion environments Journal of the European Ceramic Society 35 (2015) 4259ndash4267


[54] XF Zhang KS Zhou M Liu CM Deng CG Deng SP Niu SM Xu Oxidation and

thermal shock resistant properties of Al-modified environmental barrier coating on SiCfSiC

composites Ceramics International 43 (2017) 13075ndash13082

httpsdoiorg101016jceramint201706167

[55] MA Carpenter EKH Salje A Graeme-Barber Spontaneous strain as a determinant of

thermodynamic properties for phase transitions in minerals European Journal of Mineralogy

(1998) 621ndash691 httpsdoiorg101127ejm1040621

[56] W Pabst E Gregorovaacute ELASTIC PROPERTIES OF SILICA POLYMORPHS ndash A

REVIEW (2013) 18

[57] KN Lee JI Eldridge RC Robinson Residual Stresses and Their Effects on the Durability

of Environmental Barrier Coatings for SiC Ceramics Journal of the American Ceramic

Society 88 (2005) 3483ndash3488 httpsdoiorg101111j1551-2916200500640x

137

[58] Gregory Corman Krishan Luthra Jill Jonkowski Joseph Mavec Paul Bakke Debbie

Haught Merrill Smith Melt Infiltrated Ceramic Matrix Composites for Shrouds and

Combustor Liners of Advanced Industrial Gas Turbines 2011

httpsdoiorg1021721004879

[59] CG Levi JW Hutchinson M-H Vidal-Seacutetif CA Johnson Environmental degradation of

thermal-barrier coatings by molten deposits MRS Bull 37 (2012) 932ndash941

httpsdoiorg101557mrs2012230

[60] J Kim MG Dunn AJ Baran DP Wade EL Tremba Deposition of Volcanic Materials

in the Hot Sections of Two Gas Turbine Engines J Eng Gas Turbines Power 115 (1993)

641ndash651 httpsdoiorg10111512906754

[61] JL Smialek FA Archer RG Garlick Turbine airfoil degradation in the persian gulf war

JOM 46 (1994) 39ndash41 httpsdoiorg101007BF03222663

[62] MP Borom CA Johnson LA Peluso Role of environment deposits and operating surface

temperature in spallation of air plasma sprayed thermal barrier coatings Surface and Coatings

Technology 86ndash87 (1996) 116ndash126 httpsdoiorg101016S0257-8972(96)02994-5

[63] FH Stott DJ de Wet R Taylor Degradation of Thermal-Barrier Coatings at Very High

Temperatures MRS Bull 19 (1994) 46ndash49 httpsdoiorg101557S0883769400048223

[64] S Kraumlmer S Faulhaber M Chambers DR Clarke CG Levi JW Hutchinson AG

Evans Mechanisms of cracking and delamination within thick thermal barrier systems in

aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration Materials

Science and Engineering A 490 (2008) 26ndash35 httpsdoiorg101016jmsea200801006

[65] S Kraumlmer J Yang CG Levi CA Johnson Thermochemical Interaction of Thermal

Barrier Coatings with Molten CaOndashMgOndashAl2O3ndashSiO2 (CMAS) Deposits Journal of the


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[66] RG Wellman G Whitman JR Nicholls CMAS corrosion of EB PVD TBCs Identifying

the minimum level to initiate damage (2010)

httpdxdoiorg101016jijrmhm200907005

[67] P Mechnich W Braue U Schulz High-Temperature Corrosion of EB-PVD Yttria Partially

Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay

Journal of the American Ceramic Society 94 (2011) 925ndash931

httpsdoiorg101111j1551-2916201004166x

[68] J Webb B Casaday B Barker JP Bons AD Gledhill NP Padture Coal Ash Deposition

on Nozzle Guide VanesmdashPart I Experimental Characteristics of Four Coal Ash Types J

Turbomach 135 (2013) httpsdoiorg10111514006571

138

[69] NL Ahlborg D Zhu Calciumndashmagnesium aluminosilicate (CMAS) reactions and

degradation mechanisms of advanced environmental barrier coatings Surface and Coatings


[70] JM Drexler K Shinoda AL Ortiz D Li AL Vasiliev AD Gledhill S Sampath NP

Padture Air-plasma-sprayed thermal barrier coatings that are resistant to high-temperature

attack by glassy deposits Acta Materialia 58 (2010) 6835ndash6844


[71] JM Drexler AD Gledhill K Shinoda AL Vasiliev KM Reddy S Sampath NP

Padture Jet Engine Coatings for Resisting Volcanic Ash Damage Adv Mater 23 (2011)

2419ndash2424 httpsdoiorg101002adma201004783

[72] B-K Jang F-J Feng K Suzuta H Tanaka Y Matsushita K-S Lee S Ueno Corrosion

behavior of volcanic ash and calcium magnesium aluminosilicate on Yb2SiO5 environmental

barrier coatings J Ceram Soc Japan 125 (2017) 326ndash332

httpsdoiorg102109jcersj216211

[73] M Shinozaki KA Roberts B van de Goor TW Clyne Deposition of Ingested Volcanic

Ash on Surfaces in the Turbine of a Small Jet Engine Deposition of Volcanic Ash Inside a

Jet Engine Adv Eng Mater (2013) na-na httpsdoiorg101002adem201200357

[74] AD Gledhill KM Reddy JM Drexler K Shinoda S Sampath NP Padture Mitigation

of damage from molten fly ash to air-plasma-sprayed thermal barrier coatings Materials

Science and Engineering A 528 (2011) 7214ndash7221

httpsdoiorg101016jmsea201106041

[75] JP Bons J Crosby JE Wammack BI Bentley TH Fletcher High-Pressure Turbine

Deposition in Land-Based Gas Turbines From Various Synfuels J Eng Gas Turbines Power

129 (2007) 135ndash143 httpsdoiorg10111512181181

[76] JM Crosby S Lewis JP Bons W Ai TH Fletcher Effects of Temperature and Particle

Size on Deposition in Land Based Turbines Journal of Engineering for Gas Turbines and

Power 130 (2008) 051503 httpsdoiorg10111512903901

[77] R Van Noorden Two plants to put ldquoclean coalrdquo to test Nature 509 (2014) 20

httpsdoiorg101038509020a

[78] AR Krause BS Senturk HF Garces G Dwivedi AL Ortiz S Sampath NP Padture

2ZrO 2 middotY 2 O 3 Thermal Barrier Coatings Resistant to Degradation by Molten CMAS Part

I Optical Basicity Considerations and Processing J Am Ceram Soc 97 (2014) 3943ndash3949


[79] WE Ford Danarsquos Textbook of Mineralogy John Wiley amp Sons New York 1954

[80] PTI Material Safety Data Sheet Arizona Test Dust (nd)

139

[81] HE Taylor FE Lichte Chemical composition of Mount St Helens volcanic ash

Geophysical Research Letters 7 (1980) 949ndash952

httpsdoiorg101029GL007i011p00949

[82] WH Chesner User guidelines for waste and by-product materials in pavement construction

US Dept of Transportation Federal Highway Administration Research and Development

Turner-Fairbank Highway Research Center  McLean VA  1998

[83] MP Bacos JM Dorvaux S Landais O Lavigne R Meacutevrel M Poulain C Rio MH

Vidal-Seacutetif 10 Years-Activities at ONERA on Advanced Thermal Barrier Coatings (2011)

1ndash14

[84] W Braue P Mechnich Recession of an EB-PVD YSZ Coated Turbine Blade by CaSO4 and

Fe Ti-Rich CMAS-Type Deposits Journal of the American Ceramic Society 94 (2011)

4483ndash4489 httpsdoiorg101111j1551-2916201104747x

[85] T Steinke D Sebold DE Mack R Vaszligen D Stoumlver A novel test approach for plasma-

sprayed coatings tested simultaneously under CMAS and thermal gradient cycling

conditions Surface and Coatings Technology 205 (2010) 2287ndash2295


[86] A Aygun AL Vasiliev NP Padture X Ma Novel thermal barrier coatings that are

resistant to high-temperature attack by glassy deposits Acta Materialia 55 (2007) 6734ndash

6745 httpsdoiorg101016jactamat200708028

[87] J Wu H Guo Y Gao S Gong Microstructure and thermo-physical properties of yttria

stabilized zirconia coatings with CMAS deposits Journal of the European Ceramic Society

31 (2011) 1881ndash1888 httpsdoiorg101016jjeurceramsoc201104006

[88] AK Rai RS Bhattacharya DE Wolfe TJ Eden CMAS-Resistant Thermal Barrier

Coatings (TBC) International Journal of Applied Ceramic Technology 7 (2010) 662ndash674

httpsdoiorg101111j1744-7402200902373x

[89] VL Wiesner NP Bansal Mechanical and thermal properties of calciumndashmagnesium

aluminosilicate (CMAS) glass Journal of the European Ceramic Society 35 (2015) 2907ndash


[90] WC Hasz MP Borom CA Johnson Protected thermal barrier coating composites with

multiple coatings (1999)

[91] BA Nagaraj JI Williams JF Ackerman Thermal barrier coating resistant to deposits and

coating method therefor (2003)

[92] GE Witz Multilayer thermal barrier coating (2012)

[93] P Mohan B Yao T Patterson YH Sohn Electrophoretically deposited alumina as

protective overlay for thermal barrier coatings against CMAS degradation Surface and

Coatings Technology 204 (2009) 797ndash801 httpsdoiorg101016jsurfcoat200909055

140

[94] AR Krause HF Garces BS Senturk NP Padture 2ZrO2middotY2O3 Thermal Barrier

Coatings Resistant to Degradation by Molten CMAS Part II Interactions with Sand and Fly

Ash Journal of the American Ceramic Society 97 (2014) 3950ndash3957


[95] JA Duffy MD Ingram An interpretation of glass chemistry in terms of the optical basicity

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[96] JA Duffy AcidndashBase Reactions of Transition Metal Oxides in the Solid State Journal of

the American Ceramic Society 80 (1997) 1416ndash1420 httpsdoiorg101111j1151-

29161997tb02999x

[97] T Nanba Y Miura S Sakida Consideration on the correlation between basicity of oxide

glasses and O1s chemical shift in XPS J Ceram Soc Jpn 113 (2005) 44ndash50


[98] JA Duffy Optical Basicity of Titanium(IV) Oxide and Zirconium(IV) Oxide Journal of the


29161989tb06022x

[99] JA Duffy A common optical basicity scale for oxide and fluoride glasses Journal of Non-

Crystalline Solids 109 (1989) 35ndash39 httpsdoiorg1010160022-3093(89)90438-9

[100] JA Duffy Optical basicity analysis of glasses containing trivalent scandium yttrium

gallium and indium (2005)

httpswwwingentaconnectcomcontentsgtpcg20050000004600000005art00003

(accessed February 25 2020)

[101] V Dimitrov S Sakka Electronic oxide polarizability and optical basicity of simple oxides

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[102] V Dimitrov T Komatsu AN INTERPRETATION OF OPTICAL PROPERTIES OF

OXIDES AND OXIDE GLASSES IN TERMS OF THE ELECTRONIC ION

POLARIZABILITY AND AVERAGE SINGLE BOND STRENGTH (REVIEW) Journal

of the University of Chemical Technoloy and Metallurgy 45 (2010) 219ndash250

[103] JA Duffy Acid-Base Reactions of Transition Metal Oxides in the Solid State Journal of


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[104] JA Duffy Relationship between Cationic Charge Coordination Number and

Polarizability in Oxidic Materials J Phys Chem B 108 (2004) 14137ndash14141

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[105] JA Duffy Polarisability and polarising power of rare earth ions in glass an optical

basicity assessment (2005)

141



[106] X Zhao X Wang H Lin Z Wang Electronic polarizability and optical basicity of

lanthanide oxides Physica B Condensed Matter 392 (2007) 132ndash136

httpsdoiorg101016jphysb200611015

[107] LS Dent-Glasser JA Duffy Analysis and prediction of acidndashbase reactions between

oxides and oxysalts using the optical basicity concept J Chem Soc Dalton Trans (1987)

2323ndash2328 httpsdoiorg101039DT9870002323




[109] D Ghosh VA Krishnamurthy SR Sankaranarayanan Application of optical basicity to

viscosity of high alumina blast furnace slags J Min Metall B Metall 46 (2010) 41ndash49

httpsdoiorg102298JMMB1001041G

[110] P Moriceau B Taouk E Bordes P Courtine Correlations between the optical basicity

of catalysts and their selectivity in oxidation of alcohols ammoxidation and combustion of

hydrocarbons Catalysis Today 61 (2000) 197ndash201 httpsdoiorg101016S0920-

5861(00)00380-1

[111] RL Jones CE Williams Hot corrosion studies of zirconia ceramics Surface and

Coatings Technology 32 (1987) 349ndash358 httpsdoiorg1010160257-8972(87)90119-8

[112] M Fu R Darolia M Gorman BA Nagaraj Thermal Barrier Coating Systems Including

a Rare Earth Aluminate Layer for Improved Resistance to CMAS Infiltration and Coated

Articles (2011)

[113] KM Grant S Kraumlmer GGE Seward CG Levi Calcium-Magnesium Alumino-Silicate

Interaction with Yttrium Monosilicate Environmental Barrier Coatings YMS Interaction

with YMS EBCs Journal of the American Ceramic Society 93 (2010) 3504ndash3511

httpsdoiorg101111j1551-2916201003916x

[114] CM Toohey Novel Environmental Barrier Coatings for Resistance Against Degradation

by Molten Glassy Deposit in the Presence of Water Vapor (2011)

[115] BT Hazel I Spitsberg ThermalEnvironmental Barrier Coating System for Silicon-

Containing Materials US Patent No 7862901 2011

[116] LR Turcer AR Krause HF Garces L Zhang NP Padture Environmental-barrier

coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate

(CMAS) glass Part I YAlO3 and γ-Y2Si2O7 Journal of the European Ceramic Society 38


142



(CMAS) glass Part II β-Yb2Si2O7 and β-Sc2Si2O7 Journal of the European Ceramic

Society 38 (2018) 3914ndash3924 httpsdoiorg101016jjeurceramsoc201803010

[118] LR Turcer NP Padture Rare-Earth Pyrosilicate Solid-Solution Environmental-Barrier

Coating Ceramics for Resistance Against Attack by Molten Calcia-Magnesia-

Aluminosilicate (CMAS) Journal of Materials Research Sumbitted (2020)

[119] LR Turcer NP Padture Towards multifunctional thermal environmental barrier coatings

(TEBCs) based on rare-earth pyrosilicate solid-solution ceramics Scripta Materialia 154

(2018) 111ndash117 httpsdoiorg101016jscriptamat201805032

[120] O Chaix-Pluchery B Chenevier JJ Robles Anisotropy of thermal expansion in YAlO3

and NdGaO3 Applied Physics Letters 86 (2005) 251911


[121] O Fabrichnaya H Seifert R Weiland T Ludwig F Aldinger A Navrotsky Phase

Equilibria and Thermodynamics in the Y2O3-Al2O3-SiO2 System Zeitschrift Fuumlr

Metallkunde v92 1083-1097 (2001) 92 (2001)

[122] RL Aggarwal DJ Ripin JR Ochoa TY Fan Measurement of thermo-optic properties

of Y3Al5O12 Lu3Al5O12 YAIO3 LiYF4 LiLuF4 BaY2F8 KGd(WO4)2 and

KY(WO4)2 laser crystals in the 80ndash300K temperature range Journal of Applied Physics 98

(2005) 103514 httpsdoiorg10106312128696

[123] Y-C Zhou C Zhao F Wang Y-J Sun L-Y Zheng X-H Wang Theoretical Prediction

and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β-

Yb2Si2O7 Journal of the American Ceramic Society 96 (2013) 3891ndash3900


[124] Z Sun Y Zhou J Wang M Li -Y 2 Si 2 O 7 a Machinable Silicate Ceramic Mechanical

Properties and Machinability J American Ceramic Society 90 (2007) 2535ndash2541

httpsdoiorg101111j1551-2916200701803x

[125] Z Sun L Wu M Li Y Zhou Tribological properties of γ-Y2Si2O7 ceramic against AISI

52100 steel and Si3N4 ceramic counterparts Wear 266 (2009) 960ndash967

httpsdoiorg101016jwear200812018

[126] J-S Lee Molten salt synthesis of YAlO3 powders Mater Sci-Pol 31 (2013) 240ndash245

httpsdoiorg102478s13536-012-0091-3

[127] Z Sun Y Zhou M Li Low-temperature synthesis and sintering of γ-Y 2 Si 2 O 7 J Mater

Res 21 (2006) 1443ndash1450 httpsdoiorg101557jmr20060173

[128] JM Drexler AL Ortiz NP Padture Composition effects of thermal barrier coating

ceramics on their interaction with molten CandashMgndashAlndashsilicate (CMAS) glass Acta


143

[129] AR Krause X Li NP Padture Interaction between ceramic powder and molten calcia-

magnesia-alumino-silicate (CMAS) glass and its implication on CMAS-resistant thermal

barrier coatings Scripta Materialia 112 (2016) 118ndash122

httpsdoiorg101016jscriptamat201509027

[130] AR Krause HF Garces CE Herrmann NP Padture Resistance of 2ZrO2middotY2O3 top

coat in thermalenvironmental barrier coatings to calcia-magnesia-aluminosilicate attack at

1500degC Journal of the American Ceramic Society 100 (2017) 3175ndash3187


[131] S Kraumlmer J Yang CG Levi Infiltration-Inhibiting Reaction of Gadolinium Zirconate

Thermal Barrier Coatings with CMAS Melts Journal of the American Ceramic Society 91

(2008) 576ndash583 httpsdoiorg101111j1551-2916200702175x

[132] JM Drexler C-H Chen AD Gledhill K Shinoda S Sampath NP Padture Plasma

sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten

CandashMgndashAlndashsilicate glass Surface and Coatings Technology 206 (2012) 3911ndash3916


[133] DL Poerschke TL Barth CG Levi Equilibrium relationships between thermal barrier

oxides and silicate melts Acta Materialia 120 (2016) 302ndash314


[134] S Tanabe c materials for optical amplifiers in Advances in Photoic Materials and

Devices Ceram Trans The American Ceramics Society Westerville OH 2005 pp 1ndash16

[135] A Richter M Goumlbbels Phase Equilibria and Crystal Chemistry in the System CaO-

Al2O3-Y2O3 J Phase Equilib Diffus 31 (2010) 157ndash163 httpsdoiorg101007s11669-

010-9672-1

[136] NA Toropov IA Bondar FY Galakhov High-temperature solid solutions of silicates

of the rare-earth elements Trans Intl Ceram Cong 8 (1962) 85ndash103

[137] AJ Fernaacutendez‐Carrioacuten M Allix AI Becerro Thermal Expansion of Rare-Earth

Pyrosilicates Journal of the American Ceramic Society 96 (2013) 2298ndash2305


[138] Y Suzuki PED Morgan K Niihara Improvement in Mechanical Properties of Powder-

Processed MoSi 2 by the Addition of Sc 2 O 3 and Y 2 O 3 J American Ceramic Society 81

(1998) 3141ndash3149 httpsdoiorg101111j1151-29161998tb02749x

[139] J Liu L Zhang Q Liu L Cheng Y Wang Structure design and fabrication of

environmental barrier coatings for crack resistance Journal of the European Ceramic Society


[140] CWE van Eijk in CR Ronda LE Shea AM Srivastava (Eds) Physics and

Chemistry of Luminescent Materials The Electrochemical Society Pennington NJ 2000

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[141] Eacute Darthout F Gitzhofer Thermal Cycling and High-Temperature Corrosion Tests of Rare

Earth Silicate Environmental Barrier Coatings J Therm Spray Tech 26 (2017) 1823ndash1837

httpsdoiorg101007s11666-017-0635-5

[142] Z Tian L Zheng Z Li J Li J Wang Exploration of the low thermal conductivities of

γ-Y2Si2O7 β-Y2Si2O7 β-Yb2Si2O7 and β-Lu2Si2O7 as novel environmental barrier

coating candidates Journal of the European Ceramic Society 36 (2016) 2813ndash2823


[143] HS Tripathi VK Sarin Synthesis and densification of lutetium pyrosilicate from lutetia

and silica Materials Research Bulletin 42 (2007) 197ndash202

httpsdoiorg101016jmaterresbull200606013

[144] A Escudero MD Alba AnaI Becerro Polymorphism in the Sc2Si2O7ndashY2Si2O7

system Journal of Solid State Chemistry 180 (2007) 1436ndash1445


[145] S Suresh Fatigue of Materials Cambridge Core (1998)

httpsdoiorg101017CBO9780511806575

[146] DL Poerschke RW Jackson CG Levi Silicate Deposit Degradation of Engineered

Coatings in Gas Turbines Progress Toward Models and Materials Solutions Annu Rev

Mater Res 47 (2017) 297ndash330 httpsdoiorg101146annurev-matsci-010917-105000

[147] A Quintas D Caurant O Majeacuterus T Charpentier Effect of changing the rare earth cation

type on the structure and crystallization behavior of an aluminoborosilicate glass (nd) 5

[148] TM Shaw PR Duncombe Forces between Aluminum Oxide Grains in a Silicate Melt

and Their Effect on Grain Boundary Wetting Journal of the American Ceramic Society 74


[149] J Jitcharoen NP Padture AE Giannakopoulos S Suresh Hertzian-Crack Suppression

in Ceramics with Elastic-Modulus-Graded Surfaces Journal of the American Ceramic

Society 81 (1998) 2301ndash2308 httpsdoiorg101111j1151-29161998tb02625x

[150] DC Pender NP Padture AE Giannakopoulos S Suresh Gradients in elastic modulus

for improved contact-damage resistance Part I The silicon nitridendashoxynitride glass system

Acta Materialia 49 (2001) 3255ndash3262 httpsdoiorg101016S1359-6454(01)00200-2

[151] JW Hutchinson Z Suo Mixed Mode Cracking in Layered Materials in JW

Hutchinson TY Wu (Eds) Advances in Applied Mechanics Elsevier 1991 pp 63ndash191

httpsdoiorg101016S0065-2156(08)70164-9

[152] Z Tian X Ren Y Lei L Zheng W Geng J Zhang J Wang Corrosion of RE2Si2O7

(RE=Y Yb and Lu) environmental barrier coating materials by molten calcium-magnesium-

alumino-silicate glass at high temperatures Journal of the European Ceramic Society 39


145

[153] N Maier G Rixecker KG Nickel Formation and stability of Gd Y Yb and Lu disilicates

and their solid solutions Journal of Solid State Chemistry 179 (2006) 1630ndash1635


[154] I Spitsberg J Steibel Thermal and Environmental Barrier Coatings for SiCSiC CMCs in

Aircraft Engine Applications International Journal of Applied Ceramic Technology 1


[155] DB Marshall BN Cox Integral Textile Ceramic Structures Annual Review of Materials

Research 38 (2008) 425ndash443 httpsdoiorg101146annurevmatsci38060407130214

[156] DB Marshall BN Cox Textile Composite Materials Ceramic Matrix Composites in

Encylopedia of Aerospace Engineering John Wiley amp Sons Hoboken NJ USA 2010

[157] J Xu VK Sarin S Dixit SN Basu Stability of interfaces in hybrid EBCTBC coatings

for Si-based ceramics in corrosive environments International Journal of Refractory Metals

and Hard Materials 49 (2015) 339ndash349 httpsdoiorg101016jijrmhm201408013

[158] MD Dolan B Harlan JS White M Hall ST Misture SC Bancheri B Bewlay

Structures and anisotropic thermal expansion of the α β γ and δ polymorphs of Y2Si2O7

Powder Diffraction 23 (2008) 20ndash25 httpsdoiorg10115412825308

[159] AI Becerro A Escudero Revision of the crystallographic data of polymorphic Y2Si2O7

and Y2SiO5 compounds Phase Transitions 77 (2004) 1093ndash1102

httpsdoiorg10108001411590412331282814

[160] N Maier KG Nickel G Rixecker High temperature water vapour corrosion of rare earth

disilicates (YYbLu)2Si2O7 in the presence of Al(OH)3 impurities Journal of the European

Ceramic Society 27 (2007) 2705ndash2713 httpsdoiorg101016jjeurceramsoc200609013

[161] AI Becerro A Escudero Polymorphism in the Lu2minusxYxSi2O7 system at high

temperatures Journal of the European Ceramic Society 26 (2006) 2293ndash2299


[162] H Ohashi MD Alba AI Becerro P Chain A Escudero Structural study of the

Lu2Si2O7ndashSc2Si2O7 system Journal of Physics and Chemistry of Solids 68 (2007) 464ndash

469 httpsdoiorg101016jjpcs200612025

[163] J Leitner P Voňka D Sedmidubskyacute P Svoboda Application of NeumannndashKopp rule

for the estimation of heat capacity of mixed oxides Thermochimica Acta 497 (2010) 7ndash13

httpsdoiorg101016jtca200908002

[164] O Kubaschewski CB Alcock PJ Spenser Materials Thermochemistry 6th ed

Pergamon Oxford UK 1993

[165] WC Oliver GM Pharr An improved technique for determining hardness and elastic

modulus using load and displacement sensing indentation experiments Journal of Materials

Research 7 (1992) 1564ndash1583 httpsdoiorg101557JMR19921564

146

[166] PG Klemens -- in RP Tye (Ed) Thermal Conductivity Academic Press London UK

1969

[167] J Wu NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi Thermal

conductivity of ceramics in the ZrO2-GdO15system Journal of Materials Research 17

(2002) 3193ndash3200 httpsdoiorg101557JMR20020462

[168] M Zhao W Pan C Wan Z Qu Z Li J Yang Defect engineering in development of

low thermal conductivity materials A review Journal of the European Ceramic Society 37


[169] JM Ziman Electrons and Photons Oxford University Press Oxford UK 1960

[170] DR Clarke Materials selection guidelines for low thermal conductivity thermal barrier

coatings Surface and Coatings Technology 163ndash164 (2003) 67ndash74

httpsdoiorg101016S0257-8972(02)00593-5

[171] Z Tian C Lin L Zheng L Sun J Li J Wang Defect-mediated multiple-enhancement

of phonon scattering and decrement of thermal conductivity in (YxYb1-x)2SiO5 solid

solution Acta Materialia 144 (2018) 292ndash304


[172] J Wu X Wei NP Padture PG Klemens M Gell E Garciacutea P Miranzo MI Osendi

Low-Thermal-Conductivity Rare-Earth Zirconates for Potential Thermal-Barrier-Coating

Applications Journal of the American Ceramic Society 85 (2002) 3031ndash3035


[173] J-W Yeh S-K Chen S-J Lin J-Y Gan T-S Chin T-T Shun C-H Tsau S-Y

Chang Nanostructured High-Entropy Alloys with Multiple Principal Elements Novel Alloy

Design Concepts and Outcomes Advanced Engineering Materials 6 (2004) 299ndash303

httpsdoiorg101002adem200300567

[174] CM Rost E Sachet T Borman A Moballegh EC Dickey D Hou JL Jones S

Curtarolo J-P Maria Entropy-stabilized oxides Nature Communications 6 (2015) 1ndash8

httpsdoiorg101038ncomms9485

[175] W Hong F Chen Q Shen Y-H Han WG Fahrenholtz L Zhang Microstructural

evolution and mechanical properties of (MgCoNiCuZn)O high-entropy ceramics Journal

of the American Ceramic Society 102 (2019) 2228ndash2237


[176] R Djenadic A Sarkar O Clemens C Loho M Botros VSK Chakravadhanula C

Kuumlbel SS Bhattacharya AS Gandhi H Hahn Multicomponent equiatomic rare earth

oxides Materials Research Letters 5 (2017) 102ndash109

httpsdoiorg1010802166383120161220433

[177] J Gild Y Zhang T Harrington S Jiang T Hu MC Quinn WM Mellor N Zhou K

Vecchio J Luo High-Entropy Metal Diborides A New Class of High-Entropy Materials

147

and a New Type of Ultrahigh Temperature Ceramics Scientific Reports 6 (2016) 1ndash10

httpsdoiorg101038srep37946

[178] P Sarker T Harrington C Toher C Oses M Samiee J-P Maria DW Brenner KS

Vecchio S Curtarolo High-entropy high-hardness metal carbides discovered by entropy

descriptors Nature Communications 9 (2018) 1ndash10 httpsdoiorg101038s41467-018-

07160-7

[179] E Castle T Csanaacutedi S Grasso J Dusza M Reece Processing and Properties of High-

Entropy Ultra-High Temperature Carbides Sci Rep 8 (2018) 8609

httpsdoiorg101038s41598-018-26827-1

[180] X Yan L Constantin Y Lu J-F Silvain M Nastasi B Cui

(Hf02Zr02Ta02Nb02Ti02)C high-entropy ceramics with low thermal conductivity



[181] T Jin X Sang RR Unocic RT Kinch X Liu J Hu H Liu S Dai Mechanochemical-

Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy Advanced

Materials 30 (2018) 1707512 httpsdoiorg101002adma201707512

[182] R-Z Zhang F Gucci H Zhu K Chen MJ Reece Data-Driven Design of Ecofriendly

Thermoelectric High-Entropy Sulfides Inorg Chem 57 (2018) 13027ndash13033

httpsdoiorg101021acsinorgchem8b02379

[183] Y Qin J-X Liu F Li X Wei H Wu G-J Zhang A high entropy silicide by reactive

spark plasma sintering J Adv Ceram 8 (2019) 148ndash152 httpsdoiorg101007s40145-019-

0319-3

[184] J Gild J Braun K Kaufmann E Marin T Harrington P Hopkins K Vecchio J Luo

A high-entropy silicide (Mo02Nb02Ta02Ti02W02)Si2 Journal of Materiomics 5 (2019)

337ndash343 httpsdoiorg101016jjmat201903002

[185] C Oses C Toher S Curtarolo High-entropy ceramics Nat Rev Mater (2020)

httpsdoiorg101038s41578-019-0170-8

[186] Y Dong K Ren Y Lu Q Wang J Liu Y Wang High-entropy environmental barrier

coating for the ceramic matrix composites Journal of the European Ceramic Society 39


[187] H Chen H Xiang F-Z Dai J Liu Y Zhou High entropy

(Yb025Y025Lu025Er025)2SiO5 with strong anisotropy in thermal expansion Journal of

Materials Science amp Technology 36 (2020) 134ndash139

httpsdoiorg101016jjmst201907022

[188] M Ridley J Gaskins PE Hopkins E Opila Tailoring Thermal Properties of Ebcs in

High Entropy Rare Earth Monosilicates Social Science Research Network Rochester NY

2020 httpspapersssrncomabstract=3525134 (accessed March 8 2020)

148

[189] F-J Feng B-K Jang JY Park KS Lee Effect of Yb2SiO5 addition on the physical

and mechanical properties of sintered mullite ceramic as an environmental barrier coating

material Ceramics International 42 (2016) 15203ndash15208


[190] AH Haritha RR Rao Sol-Gel synthesis and phase evolution studies of yttrium silicates

Ceramics International 45 (2019) 24957ndash24964


ii

copy Copyright 2020 by Laura R Turcer

iii



Date ________________________ _______________________________________



Date ________________________ _______________________________________


Date ________________________ _______________________________________



Date ________________________ _______________________________________


School

iv

CURRICULUM VITAE


Brown University


Brown University





v

PUBLICATIONS
















vi

DEDICATION


vii

ACKNOWLEDGEMENTS






















viii










defense












ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi




TABLE OF FIGURES xv






123 EBC Failure 7




14 Approach 13


142 Objectives 16



21 Introduction 18


221 Processing 19



23 Results 22


x



24 Discussion 34

25 Summary 36


MOLTEN CMAS 38

31 Introduction 38


321 Processing 40



33 Results 42





34 Discussion 60

35 Summary 65




41 Introduction 67


421 Powders 69



43 Results 71





44 Discussion 82

45 Summary 84

xi


51 Introduction 85
















55 Summary 107




61 Introduction 109






63 Results 113



64 Discussion 122

65 Future Work 124

66 Summary 124

xii



72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES




























each 69



is also included 75

xiv








˚C for 24 h 80








96














xv

TABLE OF FIGURES





























100 h and (B) 200 h [36] 11



present) 23



xvi










respectively 26




were collected 28














were collected 32







34

xvii

























49










xviii



51

































xix


























L-R in (C) 74












xx










Table 17) 81



concept 85























xxi




21 105


































xxii

















1














2







temperature [46]




3
















119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)




1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4

















A B

5


















A B

6
















in Figure 4B








7




application [50]

123 EBC Failure


















8













A

B

C

D

E

F

9

















10























11












100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12












in CMAS was reached










13

14 Approach






















14



CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]










Al2O3 [104]







15















(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004


16








142 Objectives















17








18







21 Introduction


















19

















221 Processing






20























21

















PDF2 database







22












23 Results








powder processing

23



present)




pure γ-Y2Si2O7



A B

B A

24












composition





A B

Figure 12A

Figure 12B

25












1 18 23 23 31 5 CMAS Glass


3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)





26








respectively





2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27







28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29
















2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30
















31







1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)







layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33











1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass





A B

34



24 Discussion






criterion







35










reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)













36













25 Summary










37




38


MOLTEN CMAS








31 Introduction

















39






















the results [25]

40







321 Processing















41























42









33 Results







A B

43









B A

44

















A B

45



[116]







A

B C

Figure 23B

Figure 24A

Figure 24B

46





1 8 5 27 57 3 CMAS Glass



4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions


500 - - 500 - β-Yb2Si2O7 (Base)












A B

47








48







A B

C

D

Figure 25B

Figure 25D

Figure 27

49






2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass







50







Figure 28E








A

B

C

D

E

51












A B

52







1 9 6 31 50 4 CMAS Glass



4 51 - - 49 - β-Sc2Si2O7 (Base)




and 31C)

A B

53





A B

C

Figure 31B

Figure 31C

Figure 32A

54













Figure 32B

A

A

B

C

55




1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)











56










A

B

D

C

E

F G


Figure 34D

Figure 34F

57




1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7












58




A

B

C

Figure 35B

59







A

B C

60

34 Discussion











120574119866119861 gt 2120574119868 (Equation 6)












61






top dilated layer












A

B

62




Figure 39B









A B

63






A

B C

D

Figure 40B

Figure 40C

64







attractive
















65

35 Summary








A B

C D

66












67









41 Introduction

















68


[38153]















A B

69


each


NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044



421 Powders















70

















μm finish





71







out

43 Results











72





73













74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50


















76






Figure 45C

77







[116] respectively


78




are also included



3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions



45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7












79














Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass


15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7













81







Table 17)



82






1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass


4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2


8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion












83























84



45 Summary




















85






51 Introduction











concept




A B

C

86























87






Felsche [37])




512 Phase Stability








88










513 Solid solutions














89








section




II Si2O7)












90























91












92



119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)













119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)





93








v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182


from Ref [142]

94






95















119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)









96


ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin


YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099













temperature density

97












98







estimate




99



Temperature

(degC)




27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209











= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)








100




TEBC applications



















101























102





















procedures

103















104




cation basis)









105






Figure 56B

Figure 56C

106



EBC ceramic pellet


1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47












107












55 Summary



108








calculations








109





61 Introduction



















110






















111














622 Heat Treatments








112







finish








performed







113



63 Results














114



















115












116








Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity


Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5


Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3









117


the pure Yb1Y1Si2O7








APS coatings





CMAS penetration

118













119













3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions


500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120





CMAS penetration













121









122






2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions


250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion













123














the melt










124








65 Future Work










formation

66 Summary



125













126









[131617]















127













in Chapter 3










dilatation-gradient

128























129
















72 Future Work







130
















131


studied









facility shutdown

132

REFERENCES





httpsdoiorg1011791743280413Y0000000019

















2005










2016

133



29161993tb03684x



29161997tb02810x



8972(00)00889-6







29162003tb03466x




















134





































135















httpsdoiorg101007s11666-017-0574-1























136


































REVIEW (2013) 18




137

























2916200601209x







httpsdoiorg101111j1551-2916201004166x




138




































139









1ndash14
















httpsdoiorg101111j1744-7402200902373x












140







httpsdoiorg1010160022-3093(76)90027-2



29161997tb02999x






29161989tb06022x















29161997tb02999x






141


















5861(00)00380-1





Articles (2011)




httpsdoiorg101111j1551-2916201003916x









142
















Metallkunde v92 1083-1097 (2001) 92 (2001)




(2005) 103514 httpsdoiorg10106312128696







httpsdoiorg101111j1551-2916200701803x





httpsdoiorg102478s13536-012-0091-3






143























010-9672-1














144



httpsdoiorg101007s11666-017-0635-5





























httpsdoiorg101016S0065-2156(08)70164-9





145



















httpsdoiorg10108001411590412331282814


















146


1969










httpsdoiorg101016S0257-8972(02)00593-5























httpsdoiorg1010802166383120161220433



147






07160-7



httpsdoiorg101038s41598-018-26827-1













0319-3





httpsdoiorg101038s41578-019-0170-8











148








iii



Date ________________________ _______________________________________



Date ________________________ _______________________________________


Date ________________________ _______________________________________



Date ________________________ _______________________________________


School

iv

CURRICULUM VITAE


Brown University


Brown University





v

PUBLICATIONS
















vi

DEDICATION


vii

ACKNOWLEDGEMENTS






















viii










defense












ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi




TABLE OF FIGURES xv






123 EBC Failure 7




14 Approach 13


142 Objectives 16



21 Introduction 18


221 Processing 19



23 Results 22


x



24 Discussion 34

25 Summary 36


MOLTEN CMAS 38

31 Introduction 38


321 Processing 40



33 Results 42





34 Discussion 60

35 Summary 65




41 Introduction 67


421 Powders 69



43 Results 71





44 Discussion 82

45 Summary 84

xi


51 Introduction 85
















55 Summary 107




61 Introduction 109






63 Results 113



64 Discussion 122

65 Future Work 124

66 Summary 124

xii



72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES




























each 69



is also included 75

xiv








˚C for 24 h 80








96














xv

TABLE OF FIGURES





























100 h and (B) 200 h [36] 11



present) 23



xvi










respectively 26




were collected 28














were collected 32







34

xvii

























49










xviii



51

































xix


























L-R in (C) 74












xx










Table 17) 81



concept 85























xxi




21 105


































xxii

















1














2







temperature [46]




3
















119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)




1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4

















A B

5


















A B

6
















in Figure 4B








7




application [50]

123 EBC Failure


















8













A

B

C

D

E

F

9

















10























11












100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12












in CMAS was reached










13

14 Approach






















14



CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]










Al2O3 [104]







15















(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004


16








142 Objectives















17








18







21 Introduction


















19

















221 Processing






20























21

















PDF2 database







22












23 Results








powder processing

23



present)




pure γ-Y2Si2O7



A B

B A

24












composition





A B

Figure 12A

Figure 12B

25












1 18 23 23 31 5 CMAS Glass


3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)





26








respectively





2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27







28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29
















2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30
















31







1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)







layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33











1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass





A B

34



24 Discussion






criterion







35










reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)













36













25 Summary










37




38


MOLTEN CMAS








31 Introduction

















39






















the results [25]

40







321 Processing















41























42









33 Results







A B

43









B A

44

















A B

45



[116]







A

B C

Figure 23B

Figure 24A

Figure 24B

46





1 8 5 27 57 3 CMAS Glass



4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions


500 - - 500 - β-Yb2Si2O7 (Base)












A B

47








48







A B

C

D

Figure 25B

Figure 25D

Figure 27

49






2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass







50







Figure 28E








A

B

C

D

E

51












A B

52







1 9 6 31 50 4 CMAS Glass



4 51 - - 49 - β-Sc2Si2O7 (Base)




and 31C)

A B

53





A B

C

Figure 31B

Figure 31C

Figure 32A

54













Figure 32B

A

A

B

C

55




1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)











56










A

B

D

C

E

F G


Figure 34D

Figure 34F

57




1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7












58




A

B

C

Figure 35B

59







A

B C

60

34 Discussion











120574119866119861 gt 2120574119868 (Equation 6)












61






top dilated layer












A

B

62




Figure 39B









A B

63






A

B C

D

Figure 40B

Figure 40C

64







attractive
















65

35 Summary








A B

C D

66












67









41 Introduction

















68


[38153]















A B

69


each


NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044



421 Powders















70

















μm finish





71







out

43 Results











72





73













74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50


















76






Figure 45C

77







[116] respectively


78




are also included



3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions



45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7












79














Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass


15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7













81







Table 17)



82






1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass


4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2


8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion












83























84



45 Summary




















85






51 Introduction











concept




A B

C

86























87






Felsche [37])




512 Phase Stability








88










513 Solid solutions














89








section




II Si2O7)












90























91












92



119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)













119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)





93








v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182


from Ref [142]

94






95















119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)









96


ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin


YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099













temperature density

97












98







estimate




99



Temperature

(degC)




27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209











= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)








100




TEBC applications



















101























102





















procedures

103















104




cation basis)









105






Figure 56B

Figure 56C

106



EBC ceramic pellet


1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47












107












55 Summary



108








calculations








109





61 Introduction



















110






















111














622 Heat Treatments








112







finish








performed







113



63 Results














114



















115












116








Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity


Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5


Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3









117


the pure Yb1Y1Si2O7








APS coatings





CMAS penetration

118













119













3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions


500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120





CMAS penetration













121









122






2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions


250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion













123














the melt










124








65 Future Work










formation

66 Summary



125













126









[131617]















127













in Chapter 3










dilatation-gradient

128























129
















72 Future Work







130
















131


studied









facility shutdown

132

REFERENCES





httpsdoiorg1011791743280413Y0000000019

















2005










2016

133



29161993tb03684x



29161997tb02810x



8972(00)00889-6







29162003tb03466x




















134





































135















httpsdoiorg101007s11666-017-0574-1























136


































REVIEW (2013) 18




137

























2916200601209x







httpsdoiorg101111j1551-2916201004166x




138




































139









1ndash14
















httpsdoiorg101111j1744-7402200902373x












140







httpsdoiorg1010160022-3093(76)90027-2



29161997tb02999x






29161989tb06022x















29161997tb02999x






141


















5861(00)00380-1





Articles (2011)




httpsdoiorg101111j1551-2916201003916x









142
















Metallkunde v92 1083-1097 (2001) 92 (2001)




(2005) 103514 httpsdoiorg10106312128696







httpsdoiorg101111j1551-2916200701803x





httpsdoiorg102478s13536-012-0091-3






143























010-9672-1














144



httpsdoiorg101007s11666-017-0635-5





























httpsdoiorg101016S0065-2156(08)70164-9





145



















httpsdoiorg10108001411590412331282814


















146


1969










httpsdoiorg101016S0257-8972(02)00593-5























httpsdoiorg1010802166383120161220433



147






07160-7



httpsdoiorg101038s41598-018-26827-1













0319-3





httpsdoiorg101038s41578-019-0170-8











148








iv

CURRICULUM VITAE


Brown University


Brown University





v

PUBLICATIONS
















vi

DEDICATION


vii

ACKNOWLEDGEMENTS






















viii










defense












ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi




TABLE OF FIGURES xv






123 EBC Failure 7




14 Approach 13


142 Objectives 16



21 Introduction 18


221 Processing 19



23 Results 22


x



24 Discussion 34

25 Summary 36


MOLTEN CMAS 38

31 Introduction 38


321 Processing 40



33 Results 42





34 Discussion 60

35 Summary 65




41 Introduction 67


421 Powders 69



43 Results 71





44 Discussion 82

45 Summary 84

xi


51 Introduction 85
















55 Summary 107




61 Introduction 109






63 Results 113



64 Discussion 122

65 Future Work 124

66 Summary 124

xii



72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES




























each 69



is also included 75

xiv








˚C for 24 h 80








96














xv

TABLE OF FIGURES





























100 h and (B) 200 h [36] 11



present) 23



xvi










respectively 26




were collected 28














were collected 32







34

xvii

























49










xviii



51

































xix


























L-R in (C) 74












xx










Table 17) 81



concept 85























xxi




21 105


































xxii

















1














2







temperature [46]




3
















119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)




1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4

















A B

5


















A B

6
















in Figure 4B








7




application [50]

123 EBC Failure


















8













A

B

C

D

E

F

9

















10























11












100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12












in CMAS was reached










13

14 Approach






















14



CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]










Al2O3 [104]







15















(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004


16








142 Objectives















17








18







21 Introduction


















19

















221 Processing






20























21

















PDF2 database







22












23 Results








powder processing

23



present)




pure γ-Y2Si2O7



A B

B A

24












composition





A B

Figure 12A

Figure 12B

25












1 18 23 23 31 5 CMAS Glass


3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)





26








respectively





2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27







28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29
















2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30
















31







1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)







layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33











1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass





A B

34



24 Discussion






criterion







35










reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)













36













25 Summary










37




38


MOLTEN CMAS








31 Introduction

















39






















the results [25]

40







321 Processing















41























42









33 Results







A B

43









B A

44

















A B

45



[116]







A

B C

Figure 23B

Figure 24A

Figure 24B

46





1 8 5 27 57 3 CMAS Glass



4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions


500 - - 500 - β-Yb2Si2O7 (Base)












A B

47








48







A B

C

D

Figure 25B

Figure 25D

Figure 27

49






2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass







50







Figure 28E








A

B

C

D

E

51












A B

52







1 9 6 31 50 4 CMAS Glass



4 51 - - 49 - β-Sc2Si2O7 (Base)




and 31C)

A B

53





A B

C

Figure 31B

Figure 31C

Figure 32A

54













Figure 32B

A

A

B

C

55




1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)











56










A

B

D

C

E

F G


Figure 34D

Figure 34F

57




1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7












58




A

B

C

Figure 35B

59







A

B C

60

34 Discussion











120574119866119861 gt 2120574119868 (Equation 6)












61






top dilated layer












A

B

62




Figure 39B









A B

63






A

B C

D

Figure 40B

Figure 40C

64







attractive
















65

35 Summary








A B

C D

66












67









41 Introduction

















68


[38153]















A B

69


each


NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044



421 Powders















70

















μm finish





71







out

43 Results











72





73













74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50


















76






Figure 45C

77







[116] respectively


78




are also included



3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions



45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7












79














Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass


15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7













81







Table 17)



82






1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass


4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2


8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion












83























84



45 Summary




















85






51 Introduction











concept




A B

C

86























87






Felsche [37])




512 Phase Stability








88










513 Solid solutions














89








section




II Si2O7)












90























91












92



119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)













119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)





93








v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182


from Ref [142]

94






95















119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)









96


ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin


YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099













temperature density

97












98







estimate




99



Temperature

(degC)




27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209











= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)








100




TEBC applications



















101























102





















procedures

103















104




cation basis)









105






Figure 56B

Figure 56C

106



EBC ceramic pellet


1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47












107












55 Summary



108








calculations








109





61 Introduction



















110






















111














622 Heat Treatments








112







finish








performed







113



63 Results














114



















115












116








Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity


Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5


Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3









117


the pure Yb1Y1Si2O7








APS coatings





CMAS penetration

118













119













3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions


500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120





CMAS penetration













121









122






2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions


250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion













123














the melt










124








65 Future Work










formation

66 Summary



125













126









[131617]















127













in Chapter 3










dilatation-gradient

128























129
















72 Future Work







130
















131


studied









facility shutdown

132

REFERENCES





httpsdoiorg1011791743280413Y0000000019

















2005










2016

133



29161993tb03684x



29161997tb02810x



8972(00)00889-6







29162003tb03466x




















134





































135















httpsdoiorg101007s11666-017-0574-1























136


































REVIEW (2013) 18




137

























2916200601209x







httpsdoiorg101111j1551-2916201004166x




138




































139









1ndash14
















httpsdoiorg101111j1744-7402200902373x












140







httpsdoiorg1010160022-3093(76)90027-2



29161997tb02999x






29161989tb06022x















29161997tb02999x






141


















5861(00)00380-1





Articles (2011)




httpsdoiorg101111j1551-2916201003916x









142
















Metallkunde v92 1083-1097 (2001) 92 (2001)




(2005) 103514 httpsdoiorg10106312128696







httpsdoiorg101111j1551-2916200701803x





httpsdoiorg102478s13536-012-0091-3






143























010-9672-1














144



httpsdoiorg101007s11666-017-0635-5





























httpsdoiorg101016S0065-2156(08)70164-9





145



















httpsdoiorg10108001411590412331282814


















146


1969










httpsdoiorg101016S0257-8972(02)00593-5























httpsdoiorg1010802166383120161220433



147






07160-7



httpsdoiorg101038s41598-018-26827-1













0319-3





httpsdoiorg101038s41578-019-0170-8











148








v

PUBLICATIONS
















vi

DEDICATION


vii

ACKNOWLEDGEMENTS






















viii










defense












ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi




TABLE OF FIGURES xv






123 EBC Failure 7




14 Approach 13


142 Objectives 16



21 Introduction 18


221 Processing 19



23 Results 22


x



24 Discussion 34

25 Summary 36


MOLTEN CMAS 38

31 Introduction 38


321 Processing 40



33 Results 42





34 Discussion 60

35 Summary 65




41 Introduction 67


421 Powders 69



43 Results 71





44 Discussion 82

45 Summary 84

xi


51 Introduction 85
















55 Summary 107




61 Introduction 109






63 Results 113



64 Discussion 122

65 Future Work 124

66 Summary 124

xii



72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES




























each 69



is also included 75

xiv








˚C for 24 h 80








96














xv

TABLE OF FIGURES





























100 h and (B) 200 h [36] 11



present) 23



xvi










respectively 26




were collected 28














were collected 32







34

xvii

























49










xviii



51

































xix


























L-R in (C) 74












xx










Table 17) 81



concept 85























xxi




21 105


































xxii

















1














2







temperature [46]




3
















119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)




1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4

















A B

5


















A B

6
















in Figure 4B








7




application [50]

123 EBC Failure


















8













A

B

C

D

E

F

9

















10























11












100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12












in CMAS was reached










13

14 Approach






















14



CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]










Al2O3 [104]







15















(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004


16








142 Objectives















17








18







21 Introduction


















19

















221 Processing






20























21

















PDF2 database







22












23 Results








powder processing

23



present)




pure γ-Y2Si2O7



A B

B A

24












composition





A B

Figure 12A

Figure 12B

25












1 18 23 23 31 5 CMAS Glass


3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)





26








respectively





2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27







28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29
















2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30
















31







1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)







layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33











1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass





A B

34



24 Discussion






criterion







35










reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)













36













25 Summary










37




38


MOLTEN CMAS








31 Introduction

















39






















the results [25]

40







321 Processing















41























42









33 Results







A B

43









B A

44

















A B

45



[116]







A

B C

Figure 23B

Figure 24A

Figure 24B

46





1 8 5 27 57 3 CMAS Glass



4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions


500 - - 500 - β-Yb2Si2O7 (Base)












A B

47








48







A B

C

D

Figure 25B

Figure 25D

Figure 27

49






2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass







50







Figure 28E








A

B

C

D

E

51












A B

52







1 9 6 31 50 4 CMAS Glass



4 51 - - 49 - β-Sc2Si2O7 (Base)




and 31C)

A B

53





A B

C

Figure 31B

Figure 31C

Figure 32A

54













Figure 32B

A

A

B

C

55




1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)











56










A

B

D

C

E

F G


Figure 34D

Figure 34F

57




1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7












58




A

B

C

Figure 35B

59







A

B C

60

34 Discussion











120574119866119861 gt 2120574119868 (Equation 6)












61






top dilated layer












A

B

62




Figure 39B









A B

63






A

B C

D

Figure 40B

Figure 40C

64







attractive
















65

35 Summary








A B

C D

66












67









41 Introduction

















68


[38153]















A B

69


each


NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044



421 Powders















70

















μm finish





71







out

43 Results











72





73













74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50


















76






Figure 45C

77







[116] respectively


78




are also included



3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions



45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7












79














Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass


15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7













81







Table 17)



82






1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass


4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2


8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion












83























84



45 Summary




















85






51 Introduction











concept




A B

C

86























87






Felsche [37])




512 Phase Stability








88










513 Solid solutions














89








section




II Si2O7)












90























91












92



119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)













119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)





93








v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182


from Ref [142]

94






95















119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)









96


ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin


YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099













temperature density

97












98







estimate




99



Temperature

(degC)




27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209











= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)








100




TEBC applications



















101























102





















procedures

103















104




cation basis)









105






Figure 56B

Figure 56C

106



EBC ceramic pellet


1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47












107












55 Summary



108








calculations








109





61 Introduction



















110






















111














622 Heat Treatments








112







finish








performed







113



63 Results














114



















115












116








Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity


Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5


Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3









117


the pure Yb1Y1Si2O7








APS coatings





CMAS penetration

118













119













3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions


500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120





CMAS penetration













121









122






2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions


250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion













123














the melt










124








65 Future Work










formation

66 Summary



125













126









[131617]















127













in Chapter 3










dilatation-gradient

128























129
















72 Future Work







130
















131


studied









facility shutdown

132

REFERENCES





httpsdoiorg1011791743280413Y0000000019

















2005










2016

133



29161993tb03684x



29161997tb02810x



8972(00)00889-6







29162003tb03466x




















134





































135















httpsdoiorg101007s11666-017-0574-1























136


































REVIEW (2013) 18




137

























2916200601209x







httpsdoiorg101111j1551-2916201004166x




138




































139









1ndash14
















httpsdoiorg101111j1744-7402200902373x












140







httpsdoiorg1010160022-3093(76)90027-2



29161997tb02999x






29161989tb06022x















29161997tb02999x






141


















5861(00)00380-1





Articles (2011)




httpsdoiorg101111j1551-2916201003916x









142
















Metallkunde v92 1083-1097 (2001) 92 (2001)




(2005) 103514 httpsdoiorg10106312128696







httpsdoiorg101111j1551-2916200701803x





httpsdoiorg102478s13536-012-0091-3






143























010-9672-1














144



httpsdoiorg101007s11666-017-0635-5





























httpsdoiorg101016S0065-2156(08)70164-9





145



















httpsdoiorg10108001411590412331282814


















146


1969










httpsdoiorg101016S0257-8972(02)00593-5























httpsdoiorg1010802166383120161220433



147






07160-7



httpsdoiorg101038s41598-018-26827-1













0319-3





httpsdoiorg101038s41578-019-0170-8











148








vi

DEDICATION


vii

ACKNOWLEDGEMENTS






















viii










defense












ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi




TABLE OF FIGURES xv






123 EBC Failure 7




14 Approach 13


142 Objectives 16



21 Introduction 18


221 Processing 19



23 Results 22


x



24 Discussion 34

25 Summary 36


MOLTEN CMAS 38

31 Introduction 38


321 Processing 40



33 Results 42





34 Discussion 60

35 Summary 65




41 Introduction 67


421 Powders 69



43 Results 71





44 Discussion 82

45 Summary 84

xi


51 Introduction 85
















55 Summary 107




61 Introduction 109






63 Results 113



64 Discussion 122

65 Future Work 124

66 Summary 124

xii



72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES




























each 69



is also included 75

xiv








˚C for 24 h 80








96














xv

TABLE OF FIGURES





























100 h and (B) 200 h [36] 11



present) 23



xvi










respectively 26




were collected 28














were collected 32







34

xvii

























49










xviii



51

































xix


























L-R in (C) 74












xx










Table 17) 81



concept 85























xxi




21 105


































xxii

















1














2







temperature [46]




3
















119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)




1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4

















A B

5


















A B

6
















in Figure 4B








7




application [50]

123 EBC Failure


















8













A

B

C

D

E

F

9

















10























11












100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12












in CMAS was reached










13

14 Approach






















14



CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]










Al2O3 [104]







15















(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004


16








142 Objectives















17








18







21 Introduction


















19

















221 Processing






20























21

















PDF2 database







22












23 Results








powder processing

23



present)




pure γ-Y2Si2O7



A B

B A

24












composition





A B

Figure 12A

Figure 12B

25












1 18 23 23 31 5 CMAS Glass


3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)





26








respectively





2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27







28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29
















2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30
















31







1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)







layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33











1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass





A B

34



24 Discussion






criterion







35










reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)













36













25 Summary










37




38


MOLTEN CMAS








31 Introduction

















39






















the results [25]

40







321 Processing















41























42









33 Results







A B

43









B A

44

















A B

45



[116]







A

B C

Figure 23B

Figure 24A

Figure 24B

46





1 8 5 27 57 3 CMAS Glass



4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions


500 - - 500 - β-Yb2Si2O7 (Base)












A B

47








48







A B

C

D

Figure 25B

Figure 25D

Figure 27

49






2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass







50







Figure 28E








A

B

C

D

E

51












A B

52







1 9 6 31 50 4 CMAS Glass



4 51 - - 49 - β-Sc2Si2O7 (Base)




and 31C)

A B

53





A B

C

Figure 31B

Figure 31C

Figure 32A

54













Figure 32B

A

A

B

C

55




1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)











56










A

B

D

C

E

F G


Figure 34D

Figure 34F

57




1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7












58




A

B

C

Figure 35B

59







A

B C

60

34 Discussion











120574119866119861 gt 2120574119868 (Equation 6)












61






top dilated layer












A

B

62




Figure 39B









A B

63






A

B C

D

Figure 40B

Figure 40C

64







attractive
















65

35 Summary








A B

C D

66












67









41 Introduction

















68


[38153]















A B

69


each


NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044



421 Powders















70

















μm finish





71







out

43 Results











72





73













74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50


















76






Figure 45C

77







[116] respectively


78




are also included



3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions



45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7












79














Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass


15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7













81







Table 17)



82






1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass


4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2


8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion












83























84



45 Summary




















85






51 Introduction











concept




A B

C

86























87






Felsche [37])




512 Phase Stability








88










513 Solid solutions














89








section




II Si2O7)












90























91












92



119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)













119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)





93








v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182


from Ref [142]

94






95















119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)









96


ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin


YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099













temperature density

97












98







estimate




99



Temperature

(degC)




27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209











= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)








100




TEBC applications



















101























102





















procedures

103















104




cation basis)









105






Figure 56B

Figure 56C

106



EBC ceramic pellet


1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47












107












55 Summary



108








calculations








109





61 Introduction



















110






















111














622 Heat Treatments








112







finish








performed







113



63 Results














114



















115












116








Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity


Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5


Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3









117


the pure Yb1Y1Si2O7








APS coatings





CMAS penetration

118













119













3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions


500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120





CMAS penetration













121









122






2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions


250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion













123














the melt










124








65 Future Work










formation

66 Summary



125













126









[131617]















127













in Chapter 3










dilatation-gradient

128























129
















72 Future Work







130
















131


studied









facility shutdown

132

REFERENCES





httpsdoiorg1011791743280413Y0000000019

















2005










2016

133



29161993tb03684x



29161997tb02810x



8972(00)00889-6







29162003tb03466x




















134





































135















httpsdoiorg101007s11666-017-0574-1























136


































REVIEW (2013) 18




137

























2916200601209x







httpsdoiorg101111j1551-2916201004166x




138




































139









1ndash14
















httpsdoiorg101111j1744-7402200902373x












140







httpsdoiorg1010160022-3093(76)90027-2



29161997tb02999x






29161989tb06022x















29161997tb02999x






141


















5861(00)00380-1





Articles (2011)




httpsdoiorg101111j1551-2916201003916x









142
















Metallkunde v92 1083-1097 (2001) 92 (2001)




(2005) 103514 httpsdoiorg10106312128696







httpsdoiorg101111j1551-2916200701803x





httpsdoiorg102478s13536-012-0091-3






143























010-9672-1














144



httpsdoiorg101007s11666-017-0635-5





























httpsdoiorg101016S0065-2156(08)70164-9





145



















httpsdoiorg10108001411590412331282814


















146


1969










httpsdoiorg101016S0257-8972(02)00593-5























httpsdoiorg1010802166383120161220433



147






07160-7



httpsdoiorg101038s41598-018-26827-1













0319-3





httpsdoiorg101038s41578-019-0170-8











148








vii

ACKNOWLEDGEMENTS






















viii










defense












ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi




TABLE OF FIGURES xv






123 EBC Failure 7




14 Approach 13


142 Objectives 16



21 Introduction 18


221 Processing 19



23 Results 22


x



24 Discussion 34

25 Summary 36


MOLTEN CMAS 38

31 Introduction 38


321 Processing 40



33 Results 42





34 Discussion 60

35 Summary 65




41 Introduction 67


421 Powders 69



43 Results 71





44 Discussion 82

45 Summary 84

xi


51 Introduction 85
















55 Summary 107




61 Introduction 109






63 Results 113



64 Discussion 122

65 Future Work 124

66 Summary 124

xii



72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES




























each 69



is also included 75

xiv








˚C for 24 h 80








96














xv

TABLE OF FIGURES





























100 h and (B) 200 h [36] 11



present) 23



xvi










respectively 26




were collected 28














were collected 32







34

xvii

























49










xviii



51

































xix


























L-R in (C) 74












xx










Table 17) 81



concept 85























xxi




21 105


































xxii

















1














2







temperature [46]




3
















119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)




1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4

















A B

5


















A B

6
















in Figure 4B








7




application [50]

123 EBC Failure


















8













A

B

C

D

E

F

9

















10























11












100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12












in CMAS was reached










13

14 Approach






















14



CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]










Al2O3 [104]







15















(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004


16








142 Objectives















17








18







21 Introduction


















19

















221 Processing






20























21

















PDF2 database







22












23 Results








powder processing

23



present)




pure γ-Y2Si2O7



A B

B A

24












composition





A B

Figure 12A

Figure 12B

25












1 18 23 23 31 5 CMAS Glass


3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)





26








respectively





2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27







28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29
















2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30
















31







1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)







layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33











1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass





A B

34



24 Discussion






criterion







35










reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)













36













25 Summary










37




38


MOLTEN CMAS








31 Introduction

















39






















the results [25]

40







321 Processing















41























42









33 Results







A B

43









B A

44

















A B

45



[116]







A

B C

Figure 23B

Figure 24A

Figure 24B

46





1 8 5 27 57 3 CMAS Glass



4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions


500 - - 500 - β-Yb2Si2O7 (Base)












A B

47








48







A B

C

D

Figure 25B

Figure 25D

Figure 27

49






2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass







50







Figure 28E








A

B

C

D

E

51












A B

52







1 9 6 31 50 4 CMAS Glass



4 51 - - 49 - β-Sc2Si2O7 (Base)




and 31C)

A B

53





A B

C

Figure 31B

Figure 31C

Figure 32A

54













Figure 32B

A

A

B

C

55




1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)











56










A

B

D

C

E

F G


Figure 34D

Figure 34F

57




1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7












58




A

B

C

Figure 35B

59







A

B C

60

34 Discussion











120574119866119861 gt 2120574119868 (Equation 6)












61






top dilated layer












A

B

62




Figure 39B









A B

63






A

B C

D

Figure 40B

Figure 40C

64







attractive
















65

35 Summary








A B

C D

66












67









41 Introduction

















68


[38153]















A B

69


each


NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044



421 Powders















70

















μm finish





71







out

43 Results











72





73













74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50


















76






Figure 45C

77







[116] respectively


78




are also included



3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions



45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7












79














Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass


15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7













81







Table 17)



82






1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass


4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2


8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion












83























84



45 Summary




















85






51 Introduction











concept




A B

C

86























87






Felsche [37])




512 Phase Stability








88










513 Solid solutions














89








section




II Si2O7)












90























91












92



119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)













119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)





93








v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182


from Ref [142]

94






95















119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)









96


ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin


YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099













temperature density

97












98







estimate




99



Temperature

(degC)




27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209











= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)








100




TEBC applications



















101























102





















procedures

103















104




cation basis)









105






Figure 56B

Figure 56C

106



EBC ceramic pellet


1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47












107












55 Summary



108








calculations








109





61 Introduction



















110






















111














622 Heat Treatments








112







finish








performed







113



63 Results














114



















115












116








Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity


Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5


Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3









117


the pure Yb1Y1Si2O7








APS coatings





CMAS penetration

118













119













3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions


500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120





CMAS penetration













121









122






2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions


250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion













123














the melt










124








65 Future Work










formation

66 Summary



125













126









[131617]















127













in Chapter 3










dilatation-gradient

128























129
















72 Future Work







130
















131


studied









facility shutdown

132

REFERENCES





httpsdoiorg1011791743280413Y0000000019

















2005










2016

133



29161993tb03684x



29161997tb02810x



8972(00)00889-6







29162003tb03466x




















134





































135















httpsdoiorg101007s11666-017-0574-1























136


































REVIEW (2013) 18




137

























2916200601209x







httpsdoiorg101111j1551-2916201004166x




138




































139









1ndash14
















httpsdoiorg101111j1744-7402200902373x












140







httpsdoiorg1010160022-3093(76)90027-2



29161997tb02999x






29161989tb06022x















29161997tb02999x






141


















5861(00)00380-1





Articles (2011)




httpsdoiorg101111j1551-2916201003916x









142
















Metallkunde v92 1083-1097 (2001) 92 (2001)




(2005) 103514 httpsdoiorg10106312128696







httpsdoiorg101111j1551-2916200701803x





httpsdoiorg102478s13536-012-0091-3






143























010-9672-1














144



httpsdoiorg101007s11666-017-0635-5





























httpsdoiorg101016S0065-2156(08)70164-9





145



















httpsdoiorg10108001411590412331282814


















146


1969










httpsdoiorg101016S0257-8972(02)00593-5























httpsdoiorg1010802166383120161220433



147






07160-7



httpsdoiorg101038s41598-018-26827-1













0319-3





httpsdoiorg101038s41578-019-0170-8











148








viii










defense












ix

TABLE OF CONTENTS

TITLE PAGE i

COPYRIGHT PAGE ii

SIGNATURE PAGE iii

CURRICULUM VITAE iv

PUBLICATIONS v

DEDICATION vi




TABLE OF FIGURES xv






123 EBC Failure 7




14 Approach 13


142 Objectives 16



21 Introduction 18


221 Processing 19



23 Results 22


x



24 Discussion 34

25 Summary 36


MOLTEN CMAS 38

31 Introduction 38


321 Processing 40



33 Results 42





34 Discussion 60

35 Summary 65




41 Introduction 67


421 Powders 69



43 Results 71





44 Discussion 82

45 Summary 84

xi


51 Introduction 85
















55 Summary 107




61 Introduction 109






63 Results 113



64 Discussion 122

65 Future Work 124

66 Summary 124

xii



72 Future Work 129

REFERENCES 132

xiii

TABLE OF TABLES




























each 69



is also included 75

xiv








˚C for 24 h 80








96














xv

TABLE OF FIGURES





























100 h and (B) 200 h [36] 11



present) 23



xvi










respectively 26




were collected 28














were collected 32







34

xvii

























49










xviii



51

































xix


























L-R in (C) 74












xx










Table 17) 81



concept 85























xxi




21 105


































xxii

















1














2







temperature [46]




3
















119878119894119862 + 3

21198742 (119892) = 1198781198941198742 + 119862119874 (119892) (Equation 1)




1198781198941198742 + 21198672119874 (119892) = 119878119894(119874119867)4 (119892) (Equation 2)

4

















A B

5


















A B

6
















in Figure 4B








7




application [50]

123 EBC Failure


















8













A

B

C

D

E

F

9

















10























11












100 h and (B) 200 h [36]

A B ndash 4 h

C ndash 24 h

A ndash 100 h

B ndash 200 h

12












in CMAS was reached










13

14 Approach






















14



CaO 100 [103]

MgO 078 [103]

Al2O3 060 [103104]

SiO2 048 [103]

Gd2O3 118 [105]

Y2O3 100 [100]

Yb2O3 094 [105]

La2O3 118 [105]

Sc2O3 089 [100]










Al2O3 [104]







15















(Λ = 064)

Gd4Al2O9 [112] 099 035

Y4Al2O9 [112] 087 023

GdAlO3 [112] 079 015

LaAlO3 [112] 079 015

Y2SiO5 [69113] 079 015

Yb2SiO5 [114] 076 012

YAlO3 [115] 070 006

Y2Si2O7 [2569] 070 006

Yb2Si2O7 [25114] 068 004

Sc2Si2O7 [25] 066 002

Lu2Si2O7 [25] 066 002

Yb18Y02Si2O7 -- 069 005

Yb1Y1Si2O7 -- 068 004


16








142 Objectives















17








18







21 Introduction


















19

















221 Processing






20























21

















PDF2 database







22












23 Results








powder processing

23



present)




pure γ-Y2Si2O7



A B

B A

24












composition





A B

Figure 12A

Figure 12B

25












1 18 23 23 31 5 CMAS Glass


3 34 45 8 11 2 Y-Al-Ca YAG (ss)

4 54 46 - - - Y-rich YAP (Base)


6 36 43 7 12 2 Y-Al-Ca YAG (ss)

7 46 43 11 - - Y-Al-Ca YAG (ss)

8 55 45 - - - Y-rich YAP (Base)

9 55 45 - - - Y-rich YAG (Base)

10 46 54 - - - Y-rich YAG (Base)

11 45 55 - - - Y-rich YAP (Base)

Ideal Compositions

500 500 - - - YAlO3 (YAP)

500 - - 500 - γ-Y2Si2O7

500 - 125 375 - Y8Ca2(SiO4)6O2 Apatite

375 625 - - - Y3Al5O12 (YAG)





26








respectively





2 27 53 7 11 2 Y-Al-Ca YAG (ss)

3 33 61 4 - 2 Y-Al-Ca YAG (ss)

4 33 62 3 - 2 Y-Al-Ca YAG (ss)

5 30 62 3 - 2 Y-Al-Ca YAG (ss)

6 31 63 6 - - Y-Al-Ca YAG (ss)

7 32 63 5 - - Y-Al-Ca YAG (ss)

B

A

27







28




were collected

A

Figure 13B

B

C

D

Figure 14A

Figure 14B

29
















2 50 11 16 23 - Y-Ca-Si Apatite (ss)

3 37 48 5 9 1 Y-Al-Ca YAG (ss)

4 49 13 16 22 - Y-Ca-Si Apatite (ss)

5 37 48 5 9 1 Y-Al-Ca YAG (ss)

6 53 47 - - - Y-rich YAP (Base)

B A

30
















31







1 8 8 19 61 4 CMAS Glass


3 9 6 16 65 4 CMAS Glass

4 49 13 16 22 - Y-Ca-Si Apatite (ss)







layer

32




were collected

A B

C

D

Figure 17B

Figure 18A

Figure 18B

33











1 8 7 14 68 3 CMAS Glass


3 6 8 14 68 4 CMAS Glass





A B

34



24 Discussion






criterion







35










reaction [135]

2119862119886119874 2119862119886119884prime + 119881119874

∙∙ (Equation 5)













36













25 Summary










37




38


MOLTEN CMAS








31 Introduction

















39






















the results [25]

40







321 Processing















41























42









33 Results







A B

43









B A

44

















A B

45



[116]







A

B C

Figure 23B

Figure 24A

Figure 24B

46





1 8 5 27 57 3 CMAS Glass



4 46 - - 54 - β-Yb2Si2O7 (Base)

Ideal Compositions


500 - - 500 - β-Yb2Si2O7 (Base)












A B

47








48







A B

C

D

Figure 25B

Figure 25D

Figure 27

49






2 46 - - 54 - β-Yb2Si2O7 (Base)

3 10 11 21 53 5 CMAS Glass







50







Figure 28E








A

B

C

D

E

51












A B

52







1 9 6 31 50 4 CMAS Glass



4 51 - - 49 - β-Sc2Si2O7 (Base)




and 31C)

A B

53





A B

C

Figure 31B

Figure 31C

Figure 32A

54













Figure 32B

A

A

B

C

55




1 11 12 13 62 2 CMAS Glass

2 47 - - 53 - β-Sc2Si2O7 (Base)











56










A

B

D

C

E

F G


Figure 34D

Figure 34F

57




1 55 - - 45 - β-Lu2Si2O7

2 55 - - 45 - β-Lu2Si2O7

3 11 7 24 55 3 CMAS Glass

4 10 7 26 54 3 CMAS Glass

5 6 9 32 50 4 CMAS Glass

6 16 9 24 49 3 CMAS Glass

7 55 - - 45 - β-Lu2Si2O7

8 55 - - 45 - β-Lu2Si2O7












58




A

B

C

Figure 35B

59







A

B C

60

34 Discussion











120574119866119861 gt 2120574119868 (Equation 6)












61






top dilated layer












A

B

62




Figure 39B









A B

63






A

B C

D

Figure 40B

Figure 40C

64







attractive
















65

35 Summary








A B

C D

66












67









41 Introduction

















68


[38153]















A B

69


each


NAVAIR CMAS [116117128] 376 50 79 495 076

NASA CMAS [61] 266 50 79 605 044



421 Powders















70

















μm finish





71







out

43 Results











72





73













74





L-R in (C)

Figure 44B

75



is also included

Region Yb Y Si

1 30 25 45

2 30 23 47

3 amp 4 28 23 49

Ideal Composition

25 25 50


















76






Figure 45C

77







[116] respectively


78




are also included



3 amp 4 4 1 28 4 8 55 CMAS Glass

5 41 4 - - - 55 Yb18Y02Si2O7

6 3 1 28 5 8 55 CMAS Glass

7 amp 8 39 5 - - - 56 Yb18Y02Si2O7

9 20 20 13 - - 47 Y-Y-Ca-Si Apatite

10 amp 11 4 4 22 3 5 62 CMAS Glass

12 4 3 21 3 5 64 CMAS Glass

13 22 20 12 - - 46 Yb-Y-Ca-Si Apatite

14 2 3 24 4 6 61 CMAS Glass

15 amp 16 23 18 - - - 59 Yb1Y1Si2O7

Ideal Compositions



45 5 - - - 50 Yb18Y02Si2O7

25 25 - - - 50 Yb1Y1Si2O7












79














Figure 46G

Figure 46H

80




˚C for 24 h


1 44 - - - - 56 Yb2Si2O7

2 18 - 15 3 3 61 CMAS Glass

3 25 - 10 3 1 61 CMAS Glass

4 44 - - - - 56 Yb2Si2O7

5 40 4 - - - 56 Yb18Y02Si2O7

6 3 1 26 4 6 60 CMAS Glass

7 40 4 - - - 56 Yb18Y02Si2O7

8 5 1 23 3 6 63 CMAS Glass

9 23 18 - - - 59 Yb1Y1Si2O7

10 3 2 24 4 6 61 CMAS Glass

11 22 18 - - - 59 Yb1Y1Si2O7

12 3 2 24 4 5 62 CMAS Glass


15 - 15 15 4 6 60 CMAS Glass

16 - 45 - - - 55 Y2Si2O7













81







Table 17)



82






1 - - - - - 100 SiO2

2 4 - 17 7 11 61 CMAS Glass


4 44 - - - - 56 Yb2Si2O7

5 3 1 16 7 12 61 CMAS Glass

6 - - - - - 100 SiO2


8 38 5 - - - 57 Yb18Y02Si2O7

9 2 3 17 7 11 60 CMAS Glass


11 - - - - - 100 SiO2

12 17 25 - - - 58 Yb1Y1Si2O7

13 - - - - - 100 SiO2

14 - 5 12 5 10 68 CMAS Glass

15 amp 16 - 45 - - - 55 Y2Si2O7

44 Discussion












83























84



45 Summary




















85






51 Introduction











concept




A B

C

86























87






Felsche [37])




512 Phase Stability








88










513 Solid solutions














89








section




II Si2O7)












90























91












92



119896119904119904 = 119896119875119906119903119890 (120596119900

120596119872) tanminus1 (

120596119872

120596119900) (Equation 7)

where

(

120596119900

120596119872)

2

= 119891(119879) (41205951205742119898119896119861

31205871205831198863) 119879 [119888 (

Δ119872

119872)

2

]

minus1

(Equation 8)













119891(119879) =

300 times 119896119875119906119903119890|300

119879 times 119896119875119906119903119890|119879 (Equation 9)





93








v 031para 032 031 032

Ave μ (GPa) 77 65 62 68

Ave E (GPa) 202 170 162 178

a3 (x 10-29 m2) 115 133 127 127

m () 11 11 11 11

γ 3373para 3491 3477 3487

v (mmiddots-1) 4762 4067 3180 3322

Min E (GPa) 153 102 102 114

MW (gmiddotmol-1) 2582 3460 5142 5182


from Ref [142]

94






95















119896119872119894119899 rarr 087119896119861119873119860

23 119898231205881611986412

(119872119882)23 (Equation 10)









96


ternary) ceramics



x

ρ

(Mgmiddotm-3)

Min E

(Gpa)

MW

(gmiddotmol-1)

kMin


YxYb(2-x)Si2O7 104 500 102 4266 099

YxLu(2-x)Si2O7 079 534 109 4505 100

YxSc(2-x)Si2O7 172 388 109 3337 107

YbxSc(2-x)Si2O7 134 523 119 4294 115

LuxSc(2-x)Si2O7 167 578 120 4756 102

LuxYb(2-x)Si2O7 200 625 114 5181 099













temperature density

97












98







estimate




99



Temperature

(degC)




27 420 507 361 447

200 351 405 302 342

400 304 335 264 276

600 263 280 231 229

800 247 258 216 210

1000 247 252 212 209











= 20) Sr2+ (ZSr = 38) or Ba2+ (ZBa = 56)








100




TEBC applications



















101























102





















procedures

103















104




cation basis)









105






Figure 56B

Figure 56C

106



EBC ceramic pellet


1 11 8 11 8 10 52

2 11 8 11 8 11 51

3 11 8 11 8 10 52

4 12 9 12 9 11 47












107












55 Summary



108








calculations








109





61 Introduction



















110






















111














622 Heat Treatments








112







finish








performed







113



63 Results














114



















115












116








Coatings Density

(Mgm-3)

Theoretical

Density (Mgm-3)

Relative

Density

Open

Porosity


Yb2Si2O7 HT (1300 degC 4 h) 612 615 99 5


Yb1Y1Si2O7 HT (1300 degC 4 h) 481 5045 95 3









117


the pure Yb1Y1Si2O7








APS coatings





CMAS penetration

118













119













3 62 - 38 Yb2SiO5

4 44 - 56 Yb2Si2O7

5 61 - 39 Yb2SiO5

6 45 - 55 Yb2Si2O7

7 61 - 39 Yb2SiO5

Ideal Compositions


500 - 500 Yb2Si2O7

667 - 333 Yb2SiO5

120





CMAS penetration













121









122






2 24 18 - 58 Yb1Y1Si2O7


4 24 18 - 58 Yb1Y1Si2O7

5 22 20 - 58 Yb1Y1Si2O7

6 33 25 - 42 Yb1Y1SiO5

7 22 20 - 58 Yb1Y1Si2O7

8 30 27 - 43 Yb1Y1SiO5

Ideal Compositions


250 250 - 500 Yb1Y1Si2O7

333 333 - 334 Yb1Y1SiO5

64 Discussion













123














the melt










124








65 Future Work










formation

66 Summary



125













126









[131617]















127













in Chapter 3










dilatation-gradient

128























129
















72 Future Work







130
















131


studied









facility shutdown

132

REFERENCES





httpsdoiorg1011791743280413Y0000000019

















2005










2016

133



29161993tb03684x



29161997tb02810x



8972(00)00889-6







29162003tb03466x




















134





































135















httpsdoiorg101007s11666-017-0574-1























136


































REVIEW (2013) 18




137

























2916200601209x







httpsdoiorg101111j1551-2916201004166x




138




































139









1ndash14
















httpsdoiorg101111j1744-7402200902373x












140







httpsdoiorg1010160022-3093(76)90027-2



29161997tb02999x






29161989tb06022x















29161997tb02999x






141


















5861(00)00380-1





Articles (2011)




httpsdoiorg101111j1551-2916201003916x









142
















Metallkunde v92 1083-1097 (2001) 92 (2001)




(2005) 103514 httpsdoiorg10106312128696







httpsdoiorg101111j1551-2916200701803x





httpsdoiorg102478s13536-012-0091-3






143























010-9672-1














144



httpsdoiorg101007s11666-017-0635-5





























httpsdoiorg101016S0065-2156(08)70164-9





145



















httpsdoiorg10108001411590412331282814


















146


1969










httpsdoiorg101016S0257-8972(02)00593-5























httpsdoiorg1010802166383120161220433



147






07160-7



httpsdoiorg101038s41598-018-26827-1













0319-3





httpsdoiorg101038s41578-019-0170-8











148








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