The Vapor Deposition and Oxidation of Platinum- and Yttria-StabilizedZirconia Multilayers

Zhuo Yu,z Hengbei Zhao,w,z and Haydn N. G. Wadley

Department of Materials Sciences and Engineering, University of Virginia, Charlottesville, Virginia 22904

Attempts to increase the gas temperature within gas turbine en-gines are driving the development of thermal barrier coatings thatreduce superalloy component oxidation rates by disrupting thermaltransport processes. Novel metal–ceramic multilayer’s combiningthin metal layers with low thermal conductivity oxide ceramicsoffer a potential approach for impeding both the radiative andconductive transport of heat to a component surface. A gas jet-assisted vapor deposition technique has been modified and used toexperimentally explore the deposition model thermal protectionsystem consisting of platinum- and yttria-stabilized zirconia (YSZ)multilayers. Coatings containing one, three and four platinum lay-ers in 7YSZ have been deposited on NiCoCrAlY bond-coatedHastelloy X substrates and compared with conventional 7YSZmonolayers deposited on the same bond-coated substrates. Themultilayer samples have been thermally cycled to 11001C andfound to be less susceptible to delamination failure than the con-ventional coatings. Their bond coat oxidation rate at 11001C wasalso measured and discovered to decrease as the number of plat-inum layers was increased. The observations are consistent with aretarded inward diffusion of oxygen by the platinum layers.

I. Introduction

THE remarkable increase in efficiency of gas turbine enginesover the last 60 years has been achieved in significant mea-sure by elevation of the engine gas-operating temperature. Thiswas enabled by (i) the development of superalloys that were in-creasingly resistant to creep, hot corrosion and oxidation, (ii) theinvention of novel blade cooling techniques integrated intosingle crystal airfoil fabrication processes, and (iii) the emer-gence of oxidation and hot corrosion-resistant metallic bondcoats and thermal barrier coatings (TBCs) that reduced theirtemperature.1–3 Improved TBC systems for hot section air foilsare of critical importance as the turbine inlet gas temperaturecontinue to rise.2,4

Conventional TBC systems consist of a trilayer made up of a100–200-mm-thick low thermal conductivity ceramic outer layer(the top coat), a 10–20-mm-thick, aluminum-rich metallicbond coat applied directly to the superalloy air foil, and a ther-mally grown oxide (TGO) layer at the interface between thebond coat and the outer ceramic layer.5 During high-tempera-ture service, aluminum, chromium, and yttrium in the bond coatreact with ambient oxygen to form this TGO layer. Delamina-tion of the coatings during cyclic thermal exposure occurs whenthe TGO layer reaches a critical thickness of about 5 mm where-upon residual strain energies are able to drive delaminationfracture at the lowest toughness interface.6 The primary source

of the strain energy arises from the mismatch in expansioncoefficient of the TGO layer and the other components of thesystem.7 The growth rate of the TGO layer therefore plays asignificant role in determining the thermal cyclic life of the coat-ing system.6–8 The lowest oxide growth rates occur when a pure,large grain size a-phase aluminum oxide is formed directly onthe bond coat surface with no intermediate metastable oxideprecursor phase.9

Yttria-stabilized zirconia containing 6–8 wt% Y2O3 (7YSZ)for stabilization of the tetragonal phase is typically used as thethermal barrier layer. 7YSZ is chemically inert with the com-bustion environment inside a gas turbine engine10 and is alsothermochemically stable with the a-phase of alumina that formson the bond coat.1 However, oxygen is easily transportedthrough 7YSZ coatings due to the high ionic diffusivity ofYSZ at the operational temperature and the presence of inter-connected pores from the outer to inner surface of the coating.11

In pore-free single crystals of ZrO2–4.9%Y2O3, the measuredoxygen ion diffusion coefficient is about 10�11 m2/s at 10001C.At this temperature, Fox and Clyne12 predict an oxygen ion fluxof B2� 10�4 mol � (m2 � s)�1 can be transported through a250-mm-thick 5YSZ coating. This transported by permeationthrough its pores is estimated to be an order of magnitudegreater (B2� 10�3 mol � (m2 � s)�1). If such a combined fluxcould be immediately converted to a-alumina, it is sufficient tosupport an oxidation growth rate of 4100 mm/h.

Fortunately, real TGO thickening rates are much slowerbecause the kinetics of bond coat oxidation are controlled bydiffusion processes within the TGO layer.9 While the very earlystages of oxidation sometimes exhibit linear oxidation kinetics,the oxide thickening quickly becomes parabolic with a rateconstant that increases exponentially upon temperature.7

Because TGO growth is thermally activated, the only role ofthe ceramic layer in current TBC systems is to reduce the bondcoats surface temperature. This temperature then establishesthe oxygen and aluminum (and other reactive metal) mass trans-port rates through the oxide and thus the overall rate of theoxidation reaction.13

In aero gas turbines where the cyclic thermal loading can besevere, failure of the TBC system is driven by the TGO layerresidual stress-induced buckling or edge delamination.6,14–17

Failure under prolonged thermal cycling then occurs eithernear the TGO/YSZ or at the TGO/bond coat interface.3,6

Recent experiments indicate that the failure mode and cycliclife depend upon the bond coat composition and its surfacemorphology,6,18 the structure and mechanical properties of theceramic coating,6 and the toughness of the interfaces.15 Whilethe high-temperature cyclic life clearly depends upon manyfactors, experiments and simulations indicate that the rate ofTGO formation plays a critical role in establishing the thermalcyclic life.7,8,19,20 Reducing the oxidation rate of a bondcoat surface is therefore likely to have beneficial consequencesfor the TBC system durability when thermomechanical failuremodes dominate.

One approach to slow the oxidation rate is to reduce the bondcoat surface temperature by development of new ceramic layercompositions with lower intrinsic thermal conductivities.1,21,22

However, these efforts have been complicated by the introduc-

N. Padture—contributing editor

This work was supported by Office of Naval Research (Dr. David Shifler, programmanager) through ONR Contract No. N00014-03-1-0297.

wAuthor to whom correspondence should be addressed. e-mail: hengbei@virginia.eduzJoint First Authors.

Manuscript No. 28488. Received August 26, 2010; approved January 3, 2011.

Journal

J. Am. Ceram. Soc., 94 [8] 2671–2679 (2011)

DOI: 10.1111/j.1551-2916.2011.04427.x

r 2011 The American Ceramic Society

2671

mailto:hengbei@virginia.edu

tion of new failure modes. For example, some of the mostpromising materials systems are thermochemically incompatiblewith the TGO phase requiring the use of intervening diffu-sion barriers.22,23 Continued increases in the inlet gastemperature also increase the role of radiative rather than con-ductive thermal transport processes because the oxide ceramiclayer is often semitransparent at infrared wavelengths.24 This isleading to interest in optically opaque TBC materials. However,efforts to create these novel TBC systems are constrained bythe significant limitations of current coating applicationprocesses.25,26

The other approach is a conceptual design of multilayeredTBC. A conceptual multilayered TBC system, shown in Fig. 1,offers promise for addressing several failure modes causedby ever increasing temperature, intense thermal cycling, impactby ingested particles of various diameters, and attack bycalcium–magnesium–alumina–silicate (CMAS). The systemconsists of periodically stacked ceramic/metal layers. If multi-source vapor deposition tools are used, it could be synthesizedfrom a variety of ceramics and metals with each layer designedfor some specific combination of functionalities. For example,the metal layers might allow reflection of radiatively transportedheat, or act as a barrier to the inward diffusion of oxygen27 or toinhibit infiltration of CMAS, or to provide toughening to reducethe effects of foreign object impacts. The chemistry of eachceramic layer might also be varied. For example, the layer nextto the primary bond coat (where TGO formation occurs) couldbe based upon 7YSZ, which exhibits good thermochemical ca-pability with alumina. At this location, its operating tempera-ture could be maintained below that where deleterious phasechanges occur.28 The other ceramic layers could then be selectedfor greater high-temperature phase stability, lower thermal con-ductivity, higher impact toughness, or for CMAS resistanceconsistent with thermochemical compatibility with their bond-ing metallic layers.

The metal layer of a multilayer system could be a platinumgroup metal (PGM), because they all have good oxidation resis-tance (though all their oxide species are volatile).29,30 To begin anexperimental investigation of such coating concepts, we haveselected elemental platinum as the metallic layer in a modelPt/7YSZmultilayer structure. Platinum is thermochemically com-patible with 7YSZ forming no reaction products at the temper-atures of relevance.1,3 It has a high melting point (17691C) and theformation rate of its volatile oxide is low at the temperaturesof interest (the metal is reported to be removed at a rate of6� 10�4 mg � (cm2 � h)�1 at 11001C in flowing air).29 Platinumalso has a very low oxygen diffusivity (3.7� 10�17 m2/s at10001C) compared with that of 5YSZ (10�11 m2/s at 10001C).12,31

Here, we describe a dual-source, electron beam-directed va-por deposition method and its use to synthesize novel multilayercoatings that incorporate thin refractory metal layers in a 7YSZcoating. We demonstrate the growth of TBC multilayers withone, three, and four platinum interlayers, and show a reductionin their TGO layer growth rate that appears to be consistentwith retarded oxygen diffusion by the metal layers.

II. Multilayer Coating Deposition

A directed vapor deposition (DVD) technique has been adaptedfor multilayer synthesis. The detailed DVD technique has beendescribed elsewhere.32,33 The multilayer coatings were depositedby first evaporating some of the 7YSZ. The resulting ceramicvapor plume was transported to a substrate located 15 cm abovethe source using supersonic helium—10 mol% oxygen gas jetscreated by gas expansion through the nozzle. The pressure up-stream of the nozzle was 58 Pa, while that in the chamber wasmaintained at 11 Pa (the oxygen partial pressure in the chamberwas, therefore, 1 Pa). The upstream to downstream pressure ra-tio influences the jet speed. For the pressure ratio of 5.3 usedhere, the estimated gas jet speed was 1400 m/s at the annularnozzle opening. The total gas flow rate through the annularnozzle was 7 slm and resulted in a 7YSZ deposition rate of 7 mm/min. The substrate temperature was maintained between 10001and 10201C during ceramic layer deposition.

The metal layer was deposited at a temperature of 10501C.The platinum vapor plume was entrained in a pure (nonoxygen-doped) helium gas jet for transport to the substrate. Thegas pressures and flow rates were otherwise identical to thoseused for the deposition of the 7YSZ layers. The platinumdeposition rate under these conditions was 1.5 mm/min. Thissequence was then repeated to create the desired multilayeredcoating architecture.

The coatings were applied to commercial purity, 25.4-mm-di-ameter Hastelloy X coupons (provided by GE Aircraft Engines,Cincinnati, OH). They had been overlay coated with a 100–200-mm-thick Ni0.36Co0.18Cr0.16Al0.29Y0.005 bond coat. The bond-coated surfaces were polished using SiC grinding paper to a 800grit finish followed by ultrasonic cleaning in acetone and thenmethanol solutions. During deposition, a flat-plate heater was usedto heat the substrate from the backside. The substrates were firstpreheated to 4501C for 1 h to clean their surface, and then heatedto 10001C for 7YSZ deposition at a deposition rate of 7 mm/min.For the multilayer samples, platinum deposition required the re-moval of the residual oxygen from the deposition chamber. Thisresulted in an approximately half-hour hold of the sample at10001C while the deposition chamber was purged with helium.The substrate temperature was then increased to 10501C for about30 min while the platinum layer was deposited. The substrate tem-perature was then reduced to 10001C for deposition of the nextYSZ layer and the thermal sequence repeated. Table I summarizesthe thermal cycles experienced for each of the samples. The sub-strates were not rotated during deposition. After deposition, thesamples were cooled to ambient within the deposition chamberunder helium at a pressure of 3 Pa. A set of reference 7YSZmonolayer coatings were also grown using the same thermal con-ditions used to grow the four metal layer sample. About a half ofthe 7YSZ coating was first deposited at 10001C, the sample wasthen held in vacuum at 10501C for 2 h followed by the depositionof the remaining YSZ at 10001C.

Cyclic oxidation experiments were conducted to explore thegrowth of the multilayer coatings. The cycle consisted of a60-min isothermal holding period in air at 11001C followed byforced air cooling for 10 min to the ambient temperature. Thesamples were removed, cross-sectioned, polished, and examined

Fig. 1. An example of a notional multilayer coating in which refractorymetal layers are distributed within the ceramic layer.

Table I. Processing Duration for the Growth of MultilayerCoatings

Sample

Heating time

to 10001C (h)

Deposition duration (h)

Helium

purging time

at 10001C (h)7 YSZ at

10001CPt at

10501C

YSZ layer 2 0.5 2w 0One metal layer 2 0.5 0.5 0.5Three metal layer 2 0.5 1.5 1.5Four metal layer 2 0.5 2 2

wNo Pt deposited.

2672 Journal of the American Ceramic Society—Yu et al. Vol. 94, No. 8

by scanning electron microscopy (SEM) after 50, 200, 380, and800 cycles. The TGO thickness was measured in the polishedSEM images of as-deposited and thermally cycled samples.

III. Experimental Results

(1) Metal Deposition on Columnar Ceramic Structures

For the deposition temperature used here, 7YSZ forms a col-umnar microstructure with 1-mm-diameter columns and verynarrow intercolumnar pores. The surface of the coating has asignificant surface roughness and occasional wide, deeply pen-etrating intercolumnar voids. The surface roughness and porewidth of a columnar coating can be controlled by selection ofthe upstream to downstream pressure ratio (which effects theangular incidence distribution of the incident vapor and there-fore the degree of shadowing that is responsible for both coatingmorphology features).24,34,35 We selected deposition conditionsthat reduce the intercolumn gap width in the 7YSZ layers. Theeffectiveness of platinum layer sealing of these intercolumnargaps also depended upon the process conditions through theireffect upon the angular incidence distribution of the incidentflux.36 Incident fluxes with broad angle of incidence distribu-tions promote gap coverage and the relatively high chamberpressures used here enhanced this effect.37

To explore the feasibility of depositing a coating that bridgedthe surface intercolumnar gaps, a 7YSZ/Pt/7YSZ trilayer struc-ture was made using a platinum layer thickness of 7 mm and apressure ratio of 5.2. The metal layer coverage over the ceramiccoatings largest intercolumn gaps is shown in Fig. 2. The coatingcross sections indicate that the width of the vertical gaps deter-mines the continuity of the metal layer. For the chamber pres-sure, jet flow and substrate temperature conditions used here,intergrowth column gaps with widths Wg43 mm could not be

fully bridged by a 7-mm-thick metallic layer (Fig. 2(a)). When thegap width was reduced to 2 mm (Fig 2(b)), a thin ‘‘funnel-like’’void was present in the metallic layer. As the width of theintercolumnar gap was decreased to 1.5 mm, the metallic layerwas able to fully cover the gap although a funnel-like top metalsurface was still created (Fig. 2(c)). Thinner voids were fullycovered with no funnel-like surface features (Fig. 2(d)). It is alsoevident in Fig. 2 that new intercolumn gaps were only formed ina second 7YSZ layer deposited upon the platinum when the firstlayer gap coverage was incomplete (Wg41.5 mm).

A series of experiments were then conducted to identifythe deposition conditions that ensured intercolumnar gaps ofo1.5 mm. For a substrate temperature of 10001C, this constraintcould be achieved using a pressure ratio below 5.5. Under theseconditions, a large volume fraction of fine pores was formed inthe 7YSZ layer, while the fraction of intercolumn pores withdiameters 41.5 mm was limited.34

(2) As-Deposited Morphology

Multilayer TBC samples were made using the DVD conditionsidentified in Section III above. Reference 7YSZ monolayer layersamples containing no platinum layers were also made under thesame conditions to investigate the effect of the platinum layersupon the oxidization rate. Because the oxidation rate is sensitiveto the bond coat thermal history during deposition of the coat-ing, we deposited the 7YSZ samples using a similar thermal se-quence to that used for the multilayers (Table I).

The cross-section morphologies of multilayer samples areshown in Figs. 3(a) and 4(a). The total thickness of the multi-layer coatings was about 120710 mm. The thickness of eachmetallic layer was 3–7 mm, while that of ceramic layers was inthe range of 25–35 mm. Even though quite wide columnar gapswere formed in the porous ceramic layers, they were almost

(a) Wg > 3μm

Pt

7YSZ

(b) Wg ~ 2μm

(c) Wg ~ 1.5μm (d) Wg ~ 1μm

GrowthColumnFacets

2μm

2μm

2μm

2μm

Fig. 2. Micrographs showing platinum coverage of vertical gaps (intercolumn pores) of width Wg in the TBC. (a) Wg43 mm, as grown metallic layerdoes not completely cover wide columnar gaps. Intercolumnar porosity is renucleated in the next deposited ceramic layer. (b)WgB2 mm, crack-like voidextends through the metallic layer. A columnar gap is again formed at this location in the next deposited ceramic layer. (c)WgB1.5 mm, funnel-like voidformed but does not fully penetrate metallic layer. A new columnar gap was nucleated at the metal depression. (d) Wgo1 mm, complete metal coveragewith no surface imperfection. No columnar gap was created at this location in the next ceramic layer.

August 2011 Vapor Deposition and Oxidation of Pt/YSZ Multilayers 2673

always fully bridged by a metal layer. Very occasionally, a muchwider column gap was randomly formed but even these wereeventually bridged by either the second or the third metalliclayer. Careful examination of the micrographs in Fig. 2 reveal

that the platinum layers have a much rougher interface at themetal on ceramic (lower) interface than vice versa. This rough-ness at the lower interface had a short wavelength of about 1 mmand corresponds to the facets formed on top of the growth

(a) As deposited (b) 50 cycles

(c) 380 cycles (d) 800 cycles

Pt

TGO

Bond Coat

7YSZ

20μm 20μm

20μm

7YSZ

20μm

Fig. 3. Cross-section morphologies of thermally cycled one metal layer samples. (a) As-grown, (b) after 50, (c) after 380, and (d) after 800 thermal cycles(from ambient to 11001C and back to ambient).

(a) As deposited (b) 50 cycles

(c) 380 cycles (d) 800 cycles

20μm

20μm 20μm

20μm

Fig. 4. Cross-section morphologies of thermally cycled three metal layer samples. (a) As-grown, (b) after 50, (c) 380, and (d) 800 thermal cycles.

2674 Journal of the American Ceramic Society—Yu et al. Vol. 94, No. 8

columns of the first ceramic layer. The much higher adatommobility during deposition of the metal layer has eliminated thisshort length scale roughness and only long wavelength varia-tions in metal height are evident.

The TGO layer that formed on the bond coat during depo-sition of the single metal layer sample had a thickness of 0.3 mm.This was a little thinner than that of YSZ sample (Figs. 5(a) and(b)), because of the shorter holding period at 10501C (as residualoxygen was pumped from the deposition chamber) (Table I).The multilayer sample with three metal layers had a thicker(0.4 mm) initial TGO layer (Fig. 5(c)), about equal in thicknessto that of the YSZ coating.

(3) Thermal Cycling

The single YSZ layer sample (with no platinum interlayer) hadalmost completely spalled after 720 cycles. The delaminationcrack had formed at the TGO/bond coat interface consistentwith prior observations in this system.6 However, none of themultilayer samples showed spallation failure even after 800 ther-mal cycles. All the coatings exhibited significant ceramic layersintering, eventual platinum layer fracture, and slow TGO layergrowth (Figs. 3 and 4). Ceramic layer sintering during the ther-mal cycling widened the preexisting columnar gaps and as thehigh-temperature exposure time increased, these gradually ex-tended through the coating. At first these layers were bridged bythe metallic layers but the metal layers eventually failed in ten-sion at the location of the large vertical voids in the ceramiclayer (Fig. 6). The failure of the platinum layers was sometimesaccompanied by shear fracture in the platinum layer near theplatinum/YSZ interface (Fig. 6(c)).

(a) 7YSZ top coat

(b) One metal layer

(c) Three metal layers

7YSZ

TGO

Bond coat 1 μm

1 μm

1 μm

β-NiAl phaseγ-Ni phase

Fig. 5. Coating cross-sections of the TGO layer for three different asdeposited samples. (a) YSZ sample (no metal layer), (b) one metal layersample, and (c) three metal layer sample.

(a) Initial stage

10 μm

(b) Middle stage

10 μm

(c) Final stage

10 μm

Fig. 6. Intercolumnar pore and metallic layer morphology during ther-mal cycling. (a) Initial (as-deposited) structure showing complete gapcoverage by the metal layer. (b) Mid-stage showing widened intercol-umnar gap bridged by a metal layer. Note the local thinning of the metalnear the site of the bridge. (c) Eventual tensile fracture of metallic layerwith a shear delamination crack extending along the interface.

August 2011 Vapor Deposition and Oxidation of Pt/YSZ Multilayers 2675

Figure 7 shows the change in the bond coat TGO layer thick-ness during thermal cyclic oxidation. In order to analyze the sta-tistical significance of the test results, over 100 TGO thicknessmeasurements were made on every sample and the mean andstandard deviation of the TGO thickness were determined. TheTGO thickness differences between the as-deposited samples arosefrom the different times used to fabricate the coatings (Table I).The data indicate that the initial growth rate of the TGO duringcyclic oxidation decreased as the number of metal layers was in-creased. However, even after B720 cycles, the YSZ layer samplehad aB5.1-mm-thick TGO layer, while that of the one, three, andfour metal layer samples were 3.9, 3.7, and 3.6 mm, respectively.

The TGO thickness data could be fitted to a parabolic oxidethickening law of the form:

ðx� xoÞ2 ¼ kpt

where x and xo are the oxide layer thickness at time t and at thecompletion of the deposition process and kp is a parabolic rateconstant. The YSZ layer sample had the highest parabolic rateconstant of 0.015 mm2/h. This was similar to the 0.014 mm2/hvalue measured previously for samples coated at 10001C.6

The one, three, and four metal layer samples had smallerparabolic rate constants of 0.009, 0.008, and 0.008 mm2/h,respectively (Fig. 7).

Nickel- and cobalt-based bond coats containing high alumi-num atomic fractions form metastable g and y alumina poly-morphs during low temperature (8001–11001C) oxidation.20,38,39

These metastable phases eventually convert to the a phase asthe oxidation temperature increases above 11001C.40 The tran-sient phases grow faster than a alumina,20 and so a potentiallycontributing factor to the reduction in TGO growth rate inthe platinum-containing TBC is a transformation of the as-deposited metastable TGO to a-alumina during the depositionprocess. However, the YSZ-coated samples were subjected tothe same deposition thermal cycle as the three metal layer sam-ples. It therefore appears unlikely that differences in transientoxide transformation could have accounted for the loweroxidation rates of the multilayer samples.

In all samples, the growth of the TGO layer during thermalcycling was accompanied by a change in the g-Ni and b-NiAlphase volume fractions in the NiCoCrAlY bond coat adjacentto the TGO layer (Fig. 5). As the high-temperature exposuretime increased, the aluminum-rich phase adjacent to the

TGO/bond coat interface disappeared as the aluminum contentin this region of the alloy was consumed by formation of thealumina layer. Figure 8 clearly shows the gradual thickening ofthe b-depleted region as the thermal exposure was increased.Y–Al oxide (yttrium aluminate) pegs in the TGO layer near theinterface between TGO and primary bond coat were alsoobserved as early as 50 cycles in the YSZ sample (Fig. 8(a)).The one and three metal layer samples contained similar pegsafter 200 cycles. Figure 8 also shows high-magnification views ofthe TGO region after 380 thermal cycles. A significant delam-ination crack at the TGO/BC interface of the YSZ coatingoccurs after 720 thermal cycles. The voids that eventually lead tocoating spallation in YSZ coatings6 are not evident in thesamples with intermediate metal layers. The bond coat oxida-tion characteristics are clearly dependent on the architectureof the multilayer coating as well as the cyclic thermal exposureconditions.41,42

IV. Discussion

(1) Platinum Oxidation

The equilibrium relationships between the PGM, M, and theirvolatile oxides, MxOy can be express as:

xMðsÞ þy

2O2ðgÞ ,MxOyðgÞ

Platinum’s oxidation behavior has been carefully investi-gated.30,43 It loses mass during high-temperature oxidation dueto the formation of a volatile platinum oxide and no oxide scalesare observed. Experiments indicate that platinum has a linear ox-idation rate that is dependent upon the local environment abovethe oxidizing surface.30 Conditions that promote gas phase mixingand removal of the oxide vapor lead to the fastest rate of oxida-tion. The oxidation rates reported by different groups exhibit widescatter due to the different experimental setups.29,44 The data ofKrier and Jaffee29 appears representative and they report a massloss rate of 6� 10�4 mg � (cm2 � h)�1 at 11001C in flowing air.

To verify the oxidation rate of platinum under our experi-mental conditions, a 40-mm-thick platinum foil was used for amass loss test at 11001C. The foil was placed in the thermal cy-cling furnace and subjected to 230 h of oxidation at 11001C. Theresulting mass loss rate, Dm5 1.06� 10�3 mg � (cm2 � h)�1. Thecorresponding thickness recession rate

D _h ¼ D _mr¼ 1:06� 10

�3mg=cm2 � h21:45� 103mg=cm3 ¼ 4:9� 10

�4mm=h

where r is the metal density. This result is a factor of 2 greaterthan that reported by Krier and colleagues. The difference maylie in the vertical design of the furnace and the use of a fan for airventilation in our experiment. This permitted a continuous air-flow over the oxidizing surface, replenishment of oxygen at themetal surface and removal of the oxide reaction product.

The initial thickness of the platinum layer and the kinetics ofits evaporative oxidation determine an upper bound lifetime fora coating containing platinum layers. For a 7-mm-thick platinumlayer tested under the conditions used here, the predicted lifewould be between 14000 h when ideally oxidized from one sideof the platinum layer and 7000 h for double side oxidation. Thisis comparable with that required of a TBC system applied to anaircraft engine airfoil.44

(2) Oxygen Diffusion Kinetics

The TGO growth rate depends on diffusion of aluminum andoxygen to and through the TGO. Platinum has a very low ox-ygen diffusion coefficient. Grain-boundary oxygen diffusion isexpected to be the dominating transport mechanism through theplatinum layer.12 Velho and Bartlett’s31 approach can be used toestimate the inward rate of diffusion through a hermetic plati-num layer over an experimental temperature range of 14351–

YSZ layerkp = 0.015μm2/hr

1 metal layerkp = 0.009 μm2/hr

Initial stage

4 metal

YSZ

3 metal 1 metal

layer4 metal layerskp = 0.008 μm2/hr

3 metal layerskp = 0.008 μm2/hr

Fig. 7. The TGO layer thickness versus number of 1-h thermal cyclesfor a YSZ TBC and three multilayer structures containing one, three,and four metal layers. The error bars correspond to the standard devi-ation in the thickness measurements. The initial stage of thermal cycleswas amplified in the insertion.

2676 Journal of the American Ceramic Society—Yu et al. Vol. 94, No. 8

15041C. Under steady-state conditions, the diffusion coefficient,D, of oxygen in pure platinum can be expressed as:

D ¼ ð9:3� 1:8Þ exp � 78 000� 25 000RT

� �ðcm2=sÞ

The equilibrium solubility of oxygen in pure platinum, Cos , is

proportional to the square root of the oxygen partial pres-sure, P

1=2O2

:

CSO ¼ ð0:27� 0:13Þ � 1012

� exp � 1 17 000� 34 000RT

� �P1=2O2ðmol=cm3Þ

To extrapolate to our test temperature of 11001C, we notethat the oxygen flux, J, permeating a platinum metal layer isgiven by the approximation to Fick’s law:

J ¼ DCSO

h

where h is the platinum layer thickness. For ambient air, PO2 isabout 0.21. The Arrhenius expression for DCSO is given by

31

DCSO ¼ð2:4� 0:7Þ � 1012 exp �1 95 000� 59 000

RT

� �

�ffiffiffiffiffiffiffiffiffi0:21p

ðmol ðcm � sÞ�1Þ

The oxygen flux penetrating a platinum layer can therefore bewritten as

J ¼DCSO

h¼ 2:4� 0:7ð Þ � 10

12

hexp � 1 95 000� 59 000

RT

� �

�ffiffiffiffiffiffiffiffiffi0:21p

ðmol ðcm2 � sÞ�1Þ

For our multilayer samples, a typical platinum layer thicknessis 7 mm and the oxidation temperature was 11001C. The calcu-lated oxygen flux that can penetrate such a layer is then about1.4� 10�16 mol � (cm2 � s)�1. This is sufficient to support the for-

1 μm

1 μm

1 μm

(d) YSZ layer, 400 cycles

(e) 1 metal layer, 380 cycles

(f) 3 metal layers, 380 cycles

1 μm

1 μm

(a) YSZ layer, 50 cycles

(b) 1 metal layer, 50 cycles

1 μm

(c) 3 metal layers, 50 cycles

Fig. 8. Morphology of the TGO layers of samples with a YSZ layer (a and d), a single metal layer (b and e) and three metal layers sample (c and f) after50 (a, b, and c) and 380 1-h thermal cycles (d, e, and f). Note the emergence of a delamination crack at the bond coat –TGO interface in the samplecontaining no platinum layers (d).

August 2011 Vapor Deposition and Oxidation of Pt/YSZ Multilayers 2677

mation of about 10�8 mm of alumina per hour. It is clear thatalumina growth would be almost arrested under a 7 mm thick,perfectly hermetic platinum layer.

The model assumes a uniform thickness, fully dense,uncracked (hermetic) platinum layer. In reality, as depositedplatinum layers contain small isolated pores and regions that arelocally thinner that reduce the distance of oxygen diffusion inthe metal. Localized volatile oxide evaporation at regions ofhighest oxygen permeation through the YSZ (at intersectionswith intercolumn voids) can also lead to local thinning of themetal layers. The metal layers are also eventually fractured byshrinkage of the porous ceramic layer. Once cracks or locallythin regions are created in the outer metallic layer, oxygenquickly penetrates to the next metal layer through the verticalintercolumn separations. Because the unbroken layers might stillremain protective, the multilayer structure is likely to graduallylose its protective function as the layers progressively fail byvolatile oxidation or microfracture.

If the growth rate of the TGO is indicative of the rate atwhich oxygen diffuses through the multilayer coating to thebond coat, it is apparent from Fig. 7 that oxygen transport tothe TGO layer was significantly retarded by the platinum layers,and the retardation appears to be greatest for samples contain-ing the most metal layers. As the samples were thermally cycledto 400 cycles and beyond, the metal layers began to locally thinand to crack at the large intercolumnar gaps in the structure.This coincides with an increased rate of oxidation consistentwith the presence of a thinner TGO layer in the multilayer. Evenso, after 800 thermal cycles the multilayer’s TGO thicknesseswere in the 3.6–3.9 mm range, while the 7YSZ samples spalled atthe TGO thickness of 5.1 mm.

When platinum’s evaporative oxidation is not life limiting,other modes of coating failure become of interest. The thermalcycling experiments conducted here indicate that even after 800thermal cycles of heating to 11001C, no delamination failure ofthe multilayer samples had occurred, whereas spallation in theYSZ coating had happened afterB720 cycles. This is consistentwith recent work of Zhao and colleagues who thermally cycledEB-DVD 7YSZ coatings on the same substrate—bond coat sys-tem. They found delamination started at around 800 cycles.6

They experimentally confirmed that in a conventional TBC/TGO bilayer system, 7YSZ coatings spall when the TGO layerreached a critical thickness that was governed by the steady-state energy release rate Gss. The thermal cyclic lifetime could bepredicted from the thickness, residual strain, and Young’s mod-ulus of both the TGO and TBC layers and the toughness of theplane upon which spallation occurred. If it is assumed that forthe metal/ceramic multilayer samples synthesized here, the plat-inum layer(s) do not significantly contribute to strain energystorage (they have very low strengths), the critical thickness ofthe TGO layer in a 120-mm-thick 7YSZ multilayer coating withan elastic modulus of 30 GPa is B5.7 mm. Extrapolation of theTGO thickness–time relations for the metal multilayer data(Fig. 7) to this thickness predicts a 1600 cycle life for a onemetal layer system and a lifetime of more than 1900 cycles formultilayers with three or four metal layers.

V. Conclusions

Multilayer YSZ/Pt coatings have been made using a dual-sourceEB-DVD approach. By switching the electron beam between themetallic and ceramic source materials, YSZ/Pt multilayers withone to four Pt layers were deposited on overlay-coated HastelloyX substrates. By utilizing high-pressure deposition conditions, ithas been possible to fully cover the intercolumnar pores in theYSZ layers creating structures and therefore impede the subse-quent transport of oxygen through the coating while these layersremained uncracked. The presence of the metal layers was cor-related with a reduction in the oxidation rate of the underlyingbond coat with potentially significant implications for the du-rability of thermal protection coating concepts.

Acknowledgments

We would like to thank Professors Tony Evans, Carlos Levi, and David Clarkeof the University of California, Santa Barbara, and Dr. Derek Hass of DirectedVapor Deposition Technology’s International for helpful discussions. We are alsograteful to Dr. David Wortman of GE Aviation for kindly providing substrates.

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