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中 華 大 學
博 士 論 文
水溶性溶膠凝膠保護膜層的濕潤特性及在玻
璃模造之應用
The Study of Wettability of Water Based Sol-Gel Protective Coatings and It’s Applications in
Glass Molding
系 所 別：工 程 科 學 博 士 學 位 學 程 學號姓名：D09324008 SV PRABHAKAR
VATTIKUTI (阿偉) 指導教授：簡錫新 博士 共同指導：馬廣仁 博士
中 華 民 國 100 年 1 月
The Study of Wettability of Water Based Sol-Gel Protective Coatings and It’s Applications in Glass Molding
By
SV Prabhakar Vattikuti
Under supervision of
Dr. Hsi-Hsin Chien and Dr. Kung-Jeng Ma
DISSERTATION
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Nano Materials and Coatings in Institute of Science
and Engineering, Chung Hua University in Hsinchu, Taiwan, 2010
I
ABSTRACT
The glass molding process is considered to have a great potential for the mass production
of aspherical and free form glass lenses with high precision and lower cost. In glass molding
process, the die surfaces are exposed to chemically active glass and also subjected to mechanical
and thermal cyclic operations, which leads to glass sticking and premature failure of the die. This
thesis concentrates on the fabrication of glass anti-sticking coatings on dies and glass preforms to
solve above mentioned problems via the water based sol-gel dipping approach. The water based
sol-gel coatings were selected because of their chemical stability, without shape limitation, high
uniformity and low cost. Particular attention was paid to the optimization of the deposition
process and post-deposition heat treatment. High temperature glass wetting experiment was carried
out to investigate the effects of coatings on high temperature interfacial reaction between the glass
gobs and stainless steel substrates.
The results show that both the Al2O3 coated stainless steel substrate and glass preforms
demonstrated an excellent anti-sticking behavior compared to that of uncoated ones. The Al2O3
film is thermodynamic stable phase and exhibits dense structure which can effectively hinder the
out diffusion of active elements from stainless steel and low Tg glasses at high temperature. The
time and temperature dependent glass wetting and sticking behavior were investigated. The
effect of Al2O3 film coated glass preform on glass lens forming behavior was discussed.
Key words: sol-gel coating, Al2O3, glass molding, wettability
II
摘要
玻璃模造製程最有潛力量產低成本且高精度的非球面或自由曲面的光學玻璃鏡片。玻
璃模造過程中模具表面和高化學活性的玻璃接觸，並在機械應力及週期熱應力下操作，導
致模具發生玻璃沾黏及過早失效。本論文著重於探討以水性溶膠浸鍍法在模具及玻璃預形
體表面製作抗玻璃沾黏膜層以解決此問題。選擇水性溶膠鍍膜的原因是因為具有化學穩定
性，無形狀限制，均勻性佳及價格低廉等優點。研究中特別強調鍍膜及後處理製程最佳化
之重要性，也同時完成高溫玻璃濕潤試驗來評估膜層對於玻璃預形體及不鏽鋼介面反應的
效應。
研究結果顯示製備氧化鋁薄膜於不鏽鋼基板及玻璃預形體表面都具有極佳的抗玻璃沾
黏效果，氧化鋁薄膜具有極佳的熱力學穩定性及緻密的結構，可有效抑制不鏽鋼及低轉移
點玻璃內之活性元素往外擴散。研究中對於時間及溫度對玻璃濕潤及沾黏的行為做了探
討，玻璃預形體表面施鍍氧化鋁薄膜對玻璃鏡片成形的影響也做了討論。
關鍵字：溶膠鍍膜，氧化鋁，玻璃模造，濕潤測試
III
ACKNOWLEDGEMENT
I would like to dedicate this PhD work to my parents, Kondala Rani and Bhaskar Rao
who have provided me endless love, support and encouragement throughout my entire education
and career life. I love you amma. I could not have come all the way to this stage without your
blessing Amma.
First of all, I would like to express my great gratitude to both of my advisors, Prof. Hsi-
Hsin Chien and Prof. Kung-Jeng Ma for their guidance and advice during the course of this study.
I cannot think of any other advisors from whom I could have learned as much as I did. They are
not only available when I needed counsel regarding the direction of my research, but also there
when I needed career and even personal advice. I never forgot their help and care when I met
traffic accident, I’m deeply thankful. I Hope that this work is up to their expectations. Both of
them are great human being, and I hope that I not only learned from their vast knowledge in the
field of nanomaterials but also from their even greater knowledge of life and human character.
Technically, I dedicated this thesis to Prof. Hsi-Hsin Chien and prayed to god for his health from
my bottom heart.
I thank to head of department prof. Lin Yuli for all the help that he gave me during my
study and moral support. I would also like to thank the other members of my department.
Special humble thanks to my lovely brother, sister and brother-in-law for their co-
operation and encouragement of my PhD study. I wish to express my sincerest appreciation to
Ajay veerenki and Vita without whose financial support and continuous encouragement during
the earlier stages of my studies. I would not have been able to complete my Ph.D. I am also
thankful to Shinning Optics Co., for the great opportunity and funding they provided for an
industrial internship during the 2006-2009 year of this Ph.D. I also owe added thanks to all of my
IV
juniors by name to name, for their valuable helps in the part of group work and analysis during
the course of this research.
Finally, I wish to express my warmest thanks to Kalyan, Ch.Venkata Reddy, Samadiya
Durgesh, Manik Kumar, Robin, Yun Peng, Ariel, Jerry, Eden, Telvin, Augusto, Suway, Kiran,
Kevin and Wang Ma whose friendship and support during my Ph. D. years were invaluable and
will always stay in my memories.
V
Parts of this thesis were published in the following places:
Hsi-Hsin Chien, Kung-Jeng Ma, SV Prabhakar Vattikuti, Chien-Hung Kuo, Zen-Bong Huo and
Choung-Lii Chao, “High Temperature Interfacial Reaction between Glass Gobs and Sol-Gel
Coated Al2O3 Films” Advanced Materials Research Vols. 76-78 (2009) pp 708-712.
Kung-Jeng Ma, Hsi-Hsin Chien, SV Prabhakar Vattikuti, Chien-Hung Kuo, Zen-Bong Huo and
Choung-Lii Chao, “Thermal Stability of Al2O3 Coated Low Transition Temperature Glass”
Defect and Diffusion Forum Vols. 297-301 (2010) pp 875-880.
VI
Dedicated to My Beloved Parents
VII
The Study of Wettability of Water Based Sol-Gel Protective Coatings and It’s Applications in Glass Molding
Contents
ABSTRACT………………………………………………………………………………………...………I
ACKNOWLEDGEMENT………………………………………………………………………………...III
DEDICATION……………………………………………………………………………………………VI
LIST OF TABLES ………………………………………………………………………………… …...XII
LIST OF FIGURES ………………………………………………………………………………... …..XIII
TABLE OF SYMBOLS……………………………………………………………………………. ….XXI
INTRODUCTION ……………………………………………………………………………………........1
1.1 Overview…………………………………………………………………………………..1
1.2 Research goals …………………………………………………………………………....2
II. LITERATURE SURVEY ………………………………………………………………………....5
2.1 Glass molding process (GMP)………………………………………………………………….….5
2.2 General limitations and problems of optical molds…………………………………………... …..7
2.3 The approaches to extend the service life of optical molds…………………………………. …..11
2.4 Necessities and requirements of protective coatings for molds……………………………......... 12
2.5 Necessities and requirements of protective coatings for glass preforms…………………............13
2.6 An overview of the existing protective coatings and related facilities… ……..…………............14
2.6.1 Ni and Cr based coatings………………………………………………………………...16
2.6.2 Precious metal based coatings……………………………………………………….......17
VIII
2.6.3 Single and multi-layer nitride and oxide based coatings………… ……………………..20
2.6.4 Diamond and diamond like carbon coatings…………………………………………….23
2.6.5 Boride and other coatings ……………………………………………………………….26
2.7 State of the art on sol-gel technology…………………………………………………………….28
2.7.1 Organic and inorganic sols………………………………………………………………28
2.7.2 Methods of deposition…………………………………………………………………. .29
2.7.3 Role of solvents in in-situ solution……………………………………….. ……………..32
2.7.4 Advantages of sol-gel coating process…………………………………………………..33
2.7.5 Physical properties of sol-gel thin film ………………………………………………….34
2.7.6 Importance of sol-gel Al2O3 coating……………………………………………………..36
2.8 Wettability and interfacial reactions……………………………………………………………...36
2.8.1 Fundamental of wetting theory…………………………………………………………..36
2.8.1.1 Factors affect on the wetting process……………………………………………...40
2.8.1.2 Reactive and nonreactive wetting: Thermodynamic point of view……………….47
2.8.1.3 Dynamic wetting: Effects of surface roughness…………………………………..48
2.8.2 Kinetics of wetting: Reactive Vs nonreactive wetting…………………………………...50
2.8.3 Modeling of spreading…………………………………………………………… ……..52
2.8.4 Surface properties………………………………………………………………………..54
2.8.4.1 Surface energy and surface tension ………………………………………. ……..54
2.8.4.2 Hydrophilic and hydrophobic surfaces……………………………………………55
2.8.5 Contact in glass –to-metal system……………………………………………… ……....56
2.8.6 Contact in glass-to-ceramic system……………………………………………………...59
2.8.7 Thermodynamic and kinetics of glass-metal/ceramic system…………………………...60
2.8.8 Nanoscale thermal transport at solid-liquid interface……………………………………61
IX
III. EXPERIMENTAL DETAILS……………………………………………………………............62
3.1 Glass materials……………………………………………………………………………………62
3.1.1 L-BAL 42 glass preforms ……………………………………………………………….62
3.1.2 Chalcogenide glass preforms…………………………………………………………….65
3.2 Mold materials and Preprocessing ……………………………………………………………….67
3.3 Procedures………………………………………………………………………………………...68
3.3.1 Details of the sols and coating preparation ……………………………………………...68
3.3.2 Characterization of precursors…………………………………………………………...70
3.3.2.1 Viscosity and pH value……………………………………………………………70
3.3.2.2 Differential thermogravimetric (DTG)……………………………………………70
3.4 Approaches……………………………………………………………………………………….71
3.4.1 Coating on mold surface…………………………………………………………………71
3.4.2 Coating on L-BAL 42 and Chalcogenide glass preforms………………………………..71
3.4.3 Heat treatment of coated samples………………………………………………… …….71
3.5 Characterization of developed coatings for mold and glass preforms……………………………72
3.5.1 Characterization of the film……………………………………………………………...72
3.5.1.1 Surface morphology and uniformity ……………………………………………...72
3.5.1.2 Thickness of the film………………………………………………………………73
3.6 Wetting equipment………………………………………………………………………………..73
3.7 Wetting test and analysis…………………………………………………………………………74
3.7.1 Analysis of capillary wetting phenomenon of glass with contact surface……………….75
3.7.1.1 Wetting and spreading rate of glass preform with Al2O3 coated mold……………75
3.7.1.2 Wetting and spreading rate of Al2O3 coated glass preform with mold……………76
3.7.2 Molding test at high temperature ………………………………………………………..76
3.7.2.1 Molding conditions for Al2O3 coated L-BAL 42 preforms……………………….76
3.7.2.2 Molding conditions for Al2O3 coated chalcogenide preforms…………………….76
X
3.7.3 Theoretical Analysis……………………………………………………………………..77
3.7.3.1 Thermal Expansion of Glass………………………………………………………77
3.7.3.2 Analysis of heat transfer in between glass to mold surface ……………………...78
3.7.3.3 High temperature viscosity of glass……………………………………………….79
IV. RESULTS AND DISCUSSIONS………………………………………………………………...80
4.1 Variation of Al2O3 coating morphologies with withdrawal speed………………………………..80
4.2 Thickness of Al2O3 coating Vs withdrawal speed………………………………………………...82
4.3 Differential thermogravimetry (DTG) analysis of Al2O3 coatings…………………………...85
4.4 Physico-optical properties of Al2O3 coatings……………………………………………………..86
4.4.1 Transmittance of Al2O3 Coated Glass Preform ….............................................................86
4.4.2 Al2O3 Coated Glass After Scratch Test …………………………………………………87
4.5 Glass wetting test…………………………………………………………………………………88
4.5.1 Glass on mold……………………………………………………………………………88
4.5.1.1 Spreading Kinetics of glass preform on mold surface……………………………88
4.5.1.2 Effect of temperature on the final contact angle…………………………………..94
4.5.1.3 Influence of ridge formation on the spreading kinetics…………………………...95
4.5.1.4 Formation of Oxide layer ……………………………………………… ..…….. 97
4.5.2 Glass on Al2O3 coated mold ……………………………………………………………99
4.5.3 Glass on SiO2 coated mold …………………………………………………………….102
4.5.4 Al2O3 coated glass on mold…………………………………………………………….105
4.5.5 Al2O3 coated glass on Al2O3 coated mold………………………………………………110
4.6 Molded lens analysis…………………………………………………………………………….111
4.6.1 Al2O3 coated L-BAL 42 molded lens…………………………………………………..111
4.6.2 Al2O3 coated chalcogenide molded lens..........................................................................115
4.6.2.1 Surface morphology of molded Al2O3 coated lens………………………………115
XI
4.6.2.2 XPS analysis of molded Al2O3 coated lens………………………………………117
V. SUMMARY…………………………………………………………………………………….120
VI. FUTURE WORK……………………………………………………………………………….125
VII. REFERENCES………………………………………………………………………………….126
XII
LIST OF TABLES
Table: 2-1 Mold materials and their properties. Representation: O – Good; Δ – Average and
× – Poor………………………………………………………………………………...10
Table: 2-2 Mechanical and thermal properties of different coating materials………………............15
Table: 2-3 Coatings properties comparison …………………………………………............16
Table: 2-4 Properties of carbon coating materials……………………………………...........25
Table: 3-1 Composition of glass……………………………………………………………… ……64
Table: 3-2 Properties of glass…………………………………………………………………. ……64
Table: 3-3 Physical properties of chalcogenide (Ge28Sb12Se60) glass…………………. ……66
Table: 3-4 Composition of Substrate………………………………………………………… …….67
Table: 3-5 Details of different elements involved in the precursors and their atomic mass
and mass percent…………………………………………………………............69
Table: 4-1 Representation of standard free energy of redox reactions in stainless steel/glass
interface at 1098K………………………………………………………………… …...91
Table: 4-2 Calculated results from the wetting test…………………………………………… …..119
XIII
LIST OF FIGURES
Figure: 1-1. Schematic illustration of to measure the wettability of molten glass on sol-gel
coatings by different approaches, such as, approach-1: pair of sol-gel coated mold
with glass preform, approach-2: pair of coated glass preform with mold,
approach-3: both mold and glass preform are coated, approach-4: multi layers
coated on both mold and glass preform…………………………………................
Figure: 2-1 Robotic features of (a) glass molding process (GMP), (b) molding setups (a):
heating, (b): pressing, (c): annealing and (d): cooling……………………………...
Figure: 2-2 Diagram of possibilities of volatile matter transfer or diffusion between the mold
and glass materials. At high temperature, inter-diffusion or fusion between
glass/film/substrate will accelerate adhesion wear………………………………..
Figure: 2-3 Optical microscopic images of (a) glass stick mark and coating delamination on
Plano side of the mold, (b) glass sticking on aspheric side of the mold…………..
Figure: 2-4 (a) Cross-sectional SEM images of TaN/Pt-Ir multilayer coated mold (b) XRD
results of TaN/Pt-Ir coated substrate after 700°/6hrs annealing treatment………..
Figure: 2-5 SEM micrographs of (a) Pt/Ir with Ta buffer layer coatings of 37 layers with 296
nm thickness and (b) residual reactant from glass (S2-type Al2O3, BaO, Na2O, F
are main compounds in the glass) on same substrate after wetting test…… …....
3
7
8
9
19
20
XIV
Figure: 2-6 SEM pictures of (a) TiAlN coatings on WC (thickness about 300nm), (b) and (c)
glass adhesions on TiAlN-coated WC mold after pressing……………………...
Figure: 2-7 SEM image of (a) Large-area of the surface of the a-C films prepared at 400°C,
(b) water contact angle data measured on the surface of the a-C films with varying
deposition temperature. The insert exhibit the sharp of water on the surfaces of the
a-C films, the upper image is super-hydrophobic surface with a contact angle of
152°; and the lower image is hydrophilic surface with a contact angle of
40°.………………………………………………………………………………………
Figure: 2-8 Classification of DLC coatings………………………………………………………….
Figure: 2-9 The effect of temperature on the wetting angle for glass gobs contact with various
ceramic substrates………………………..........................................................................
Figure: 2-10 Representation of dip coating process…………………………………………………....
Figure: 2-11 Representation of thin film deposition mechanism……………………………………....
Figure: 2-12 Complexity of sol-gel coating ……………………………………………………. ……
Figure: 2-13 Schematic sketch of (a) contact angle between the solid and contact liquid/glass; (b)
hydrophilic; (c) hydrophobic contact angle………………………………………. ……..
Figure: 2-14 Schematic drawing of a sessile drop. Both (A) advancing and (B) receding
angle.……………..............................................................................................................
Figure: 2-15 Schematic drawing of the advancing and receding contact angle versus. The spreading
velocity (v) of the triple line is moving along two directions. The advancing angle
exceeds the receding angle. This is called contact-angle hysteresis...................................
22
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31
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35
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40
XV
`Figure: 2-16 Different factors influencing on the wettability at high temperature…………………….
Figure: 2-17 Hypothetical variation of the interfacial energies (σSL, σSV, σLV) versus the activity a due
to adsorption effects (for example oxygen or carbon adsorption)…………………………
Figure: 2-18 Illustration of a) contact angle between the solid and contact liquid (b) the dihedral angles
in the case of ridge formation ………………………..........................................................
Figure: 2-19 The geometry of a liquid drop on a substrate depends on time. In Regime 1 (A) the
spreading velocity of the liquid is faster than the ridge formation. The liquid spreads on a
flat surface. Regime 2+3 (B): a ridge can form, depending on the ratio of the height of the
ridge compared to the curvature of the liquid one differentiates between Regime 2 and 3.
Regime 4 (C): full equilibrium is obtained. The curvature of the drop is constant.............
Figure: 2-20 Experimental contact angles for pure metal M/ionocovalent oxide systems versus the
calculated equilibrium mole fraction of oxygen in liquid M resulting from dissolution of
the oxide. …………................................................................................................. ………
Figure: 3-1 Plot of volume change against temperature for a typical optical glass L-BAL42.
This is showing strongly temperature-dependent thermal expansion characteristics.
-transition………...................................................................................................
Figure: 3-2 Flow charts for Al2O3 sol preparation process…………………………………… ………
Figure: 3-3 Flow chart of heat treatment process for Al2O3 coating on both stainless steel substrate
and glass preforms..............................................................................................................
Figure: 3-4 Schematic illustration of the high temperature’s wetting equipment………………
Figure: 3-5 Schematic diagram of the IR source high temperature wetting equipment...............
41
42
44
46
59
63
69
72
74
75
XVI
Figure: 3-6 Molding parameters of chalcogenide glass……………………………………........
Figure: 4-1 Effect of drawing speed on the surface morphologies of Al2O3 films (a) 20 m/min
(b) 100 mm/min (c) 200 mm/min…………………………………………………..
Figure: 4-2 Effect of withdrawal speed on the thickness of Al2O3 films (a) 20 mm/min (b) 100
mm/min (c) 200 mm/min…………………………………………………..............
Figure: 4-3 Graph between the withdrawal speed (mm/min) and film thickness (nm)…………
Figure: 4-4 SEM micrographs of (a) SiO2 and (b) Al2O3 coated substrates after heat treatment
process carried out at 650°C……………………………………………………….
Figure: 4-5 Topography of coated mold surface after wetting test……………………………..
Figure: 4-6 Differential thermogravimetry (DTG) spectra of Al2O3 coated glass ball…………
Figure: 4-7 UV-spectra of transparency of glass gob before and after Al2O3 sol-gel coating;
The traces are very similar, indicating that the transmittance remains unaffected
by deposited Al2O3 sol-gel coating…………………………………………..…….
Figure: 4-8 Surface mophologies of Al2O3 film coated glass after scratch test………..............
Figure: 4-9 Variation of (a) Contact angle and (b) contact area radius as a function of time for
molten glass on the uncoated stainless steel substrate at 800°C for a 5- minute
holding time………………………………………………………………..............
Figure: 4-10 Cross-sectional views at the interface of the molten glass/uncoated stainless steel
substrate……………………………………………………………………………
77
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84
86
87
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89
92
XVII
Figure: 4-11 Cross-sectional views at the interface of the molten glass/uncoated stainless steel
substrate and EDX results………………………………………………………….
Figure: 4-12 Cross-sectional views at the interface of the molten glass/uncoated stainless steel
substrates with element mapping results……………………………………………
Figure: 4-13 Variation of Contact angle with respective to temperature profile as a function of
time for molten glass on the uncoated stainless steel substrate at 800°C for 5-
minute holding time…………………………………………………………….......
Figure: 4-14 Analysis of interface conditions between the glass and uncoated stainless steel
substrate; (a) microscopic image of glass adhesion at interface: chemical reaction
takes place at edge of interface between the glass and uncoated substrate, (b) ridge
formation indentified by optical microscopy, (c) SEM image: width of ridge
formation at interface, (d) SEM image: ring of small glass islands formed at
surrounding interface………………………………………………..……………..
Figure: 4-15 Graph represents relationship between the net weight of oxidation with respective
to holding time……………………………………………………………………...
Figure: 4-16 Relationship between average thickness of oxide layer on uncoated mold and
isothermal holding time at 800°C ……………………………………….................
Figure: 4-17 Micrograph of oxide layer of uncoated mold substrate treated at 800°C…………...
Figure: 4-18 The behavior of (a) Contact angle and (b) contact area radius as a function of time
for molten glass on the sol-gel Al2O3 coated substrate at 800°C for a 5-minute
holding time…………………………………………………………………….......
93
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98
97
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100
XVIII
Figure: 4-19 SEM /EDX results of Al2O3 coated substrate after wetting test……………………
Figure: 4-20 SEM/EDX results of tested glass surface after wetting test when contacted with
Al2O3 coated substrate……………………………………………………………...
Figure: 4-21 The variation of (a) contact angle and (b) contact area radius as a function of
holding time for molten glass on SiO2 coated substrate at 800°C for a 5-minute
holding time………………………………………………………………………...
Figure: 4-22 SEM /EDX results of SiO2 coated substrate after wetting test…………………….
Figure: 4-23 SEM/EDX results of tested glass surface after wetting test when contacted with
SiO2 coated substrate…………………………………………………………........
Figure: 4-24 The variation of contact angle as a function of holding time for sol-gel Al2O3 -
coated glass ball on the stainless steel substrate with respect to temperature
profile………………………………………………………………………….......
Figure: 4-25 Variation of (a) Contact angle and (b) contact area radius as a function of time for
sol-gel Al2O3 coated glass ball on the stainless steel substrate at 800°C for 5
minutes holding period……………………………………………………………
Figure: 4-26 Images of the final contact angle of sol-gel Al2O3 coated glass ball on the stainless
steel at 800°C…………………………………………………………………......
Figure: 4-27 The variation of contact angle as a function of holding time and temperature for
uncoated and sol-gel Al2O3 coated glass ball on stainless steel………………......
101
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108
XIX
Figure: 4-28 a) Appearance of Al2O3 coated glass ball (b) SEM image of stainless steel and (c)
EDX results after wetting test…………………………………………….. ….......
Figure: 4-29 Variation of Contact angle as a function of time for the Al2O3 coated substrate and
Al2O3 coated glass preform at 800°C for 5- minute holding time……………........
Figure : 4-30 Elements depth profile of Al2O3 coated glass lens produced by molding process at
580°C………………………………………………………………………….......
Figure: 4-31 (a) Appearance of molded lens (b) SEM surface image of molded lens and (c)
high magnification SEM image near the edge of the molded lens… ……………..
Figure: 4-32 SEM surface images of (a) molded lens near the edge of the molded lens and (b) at
magnification image of near the edge of the molded lens at the molding
temperature of 304℃………………………………………...……………………..
Figure: 4-33 Represents (a) Appearance of molded lens with protective Al2O3 film on the
surface (b) SEM surface image of molded lens and (c) high magnification SEM
image near the edge of the molded lens………………………………………......
Figure: 4-34 XPS –elements depth profile of molded glass lenses (SCHOTT - Ge28Sb12Se60)
after molding test at molding temperature of 305℃ and applied load of 800
N……………………………………………………………………………. …….
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XX
TABLE OF SYMBOLS
Tg Glass transition temperature
η Viscosity
Uo Withdrawal speed
C1 Constant
ρ Density
g Gravitational force
Tc Critical thickness
E Young’s modulus
A Dimensionless proportionality constant
Gc Energy require to form two new crack surfaces
θ Contact angle
σSV Solid-vapor interface energies
σSL Solid-liquid interface energies
σLV Liquid-vapor interface energies
ρL Density of molten materials
H Final height of droplet
θadv Advancing angle
XXI
θrec Receding angle
θ0 or θe Equilibrium angle
acr Activity of adsorption rate
Tm Melting point
φS Equilibrium dihedral angles in the solid
φL Equilibrium dihedral angles in the liquid
φv Equilibrium dihedral angles in the vapor
ΔGs Change in surface free energy
ΔA Change in area of surface
Δθ Equilibrium contact angle
Wa Work of adhesion
σSLD Solid-liquid interfacial tension or energy
C Proportional constant
τw Viscous shear stress
V Drop volume
σ Surface tension or energy
θd Capillarity
XXII
R Radius of the wetted spot grows
θw Wenzel angle
r Average roughness ratio
Ac Liquid-solid contact area
Af Final equilibrium value of the normalized wet area
τ Dimensionless time
k, n Empirical constants
θd Dynamic or instantaneous contact angle
Fd Reactive wetting driving force
Fc Capillary force
Fg Gravity force
Fv Viscous force
t Time
Wc Work of cohesion
d Drop base diameter
StP Strain point
At Yield point
1
1 INTRODUCTION
1.1 Overview
The glass molding process is considered to have a great potential for the mass production
of aspherical and free form glass lenses with high precision and lower cost. In glass molding
process, the die surfaces are exposed to the chemically active glass and also subjected to
mechanical and thermal cyclic operations, which leads to three critical problems including
sticking/adhesion of glass to the die surface, oxidation and wear of the die. These problems result
in imperfections in the glass products, loss of dimensional control of glass products and limited
service life of dies.
There are several approaches to improve glass sticking problems including (1) choosing
the low transition point glasses, (2) applying protective coatings on the mold, (3) shortening the
process time, and (4) applying anti-stick coating on the glass performs etc.
The glass with low transition temperature (Tg) has the advantage of extending the service
life of molding dies because the molding temperature is significantly reduced. However, most of
low Tg glasses have high content of alkali metal oxides and tend to be decomposed at high
temperature and may induce severe glass sticking problems. Furthermore, the low Tg glasses
normally demonstrate poor chemical durability and scratch resistance. As a result, the yields of
fabricating the glass-preforms are frequently rather low.
To develop protective coatings on the mold surface is the most popular approaches to
improve glass sticking problems. The precious metal alloys and amorphous carbon based
coatings have been widely used as protective coatings on the molds to improve glass contact
2
induced sticking problems. However, the coating facilities and target materials are very
expensive and difficult to achieve a good surface coverage for a mold with high aspect-ratio
structure. Even the precious metal alloys are unable to resist oxygen diffusion induced the
oxidation of substrate materials at molding temperature over 650℃. Furthermore, protective
coating on the mold is unable to inhibit volatize or unstable elements evaporating from glass and
redeposited on the mold surface.
To shorten the process time is benefit improve service life of optical molds due to shorten
the contact time between the glass and molds. However, it may lead to a lower production yield
and the molded lenses with a lower refractive index.
Recent studies proposed to apply a very thin carbon or carbon-hydrogen based coating on
glass preforms to suppress the unstable elements diffusion and hence improve glass sticking
problems. However, some drawbacks still existed for these coatings: (1) thermal decomposed C:
H coatings with hydrogen trapped in the film may result in the reduction of oxide glass. (b) the a-
C or C: H film is unstable at high temperature. Both effects may cause glass sticking on the mold.
I.2 Research Objective and Goals
This objective of this research is to develop water based sol-gel process to apply a
protective coating on both optical molds and glass performs, which can effectively prevent glass
sticking problems at high temperature. The novel water based sol-gel technology has other
advantages over traditional coating technologies :(1) good surface coverage and thickness
uniformity (2) water based oxide film without organic residues and is stable at high temperature
(3) A thin layer of Al2O3 or SiO2 film is optical transparent and strong adhesion on substrate (4)
3
low temperature and cheap process. Applying a protective coating on glass performs is able to
solve chemical durability and scratch problems occurred in low Tg glass. It favors improving the
production yield in molded glass components. In this study, we measured wettability of molten
glass on water based sol-gel coatings by different approaches (i.e. various situations or
combination of mold/coating/glass preform) as mentioned in below Figure1-1.
Figure: 1-1. Schematic illustration of to measure the wettability of molten glass on sol-gel
coatings by different approaches, such as, approach-1: pair of sol-gel coated mold with glass
preform, approach-2: pair of coated glass preform with mold, approach-3: both mold and glass
preform are coated, approach-4: multi layers coated on both mold and glass preform.
4
Fundamental theoretical work and experimental tests were carried out to clarify glass
sticking mechanism and optimize protective coating material and coating process. The major
tasks were as follows:
- Understand the mechanisms of molten glass sticking at high temperature.
- Investigate time and temperature dependent glass wetting and sticking behavior.
-Develop and optimize water based sol-gel coating process to apply protective coatings on
stainless steel molds.
-Develop and optimize water based sol-gel coating process to apply protective coatings on glass
performs.
-develop test procedure to assess the durability of the protective coatings.
-Investigate the effect of protective film on glass sticking behavior.
- Study the effect of protective film on glass visco-elastic flow or lens forming behavior.
- Study the effect of protective film on glass optical transmission properties.
5
2 LITERATURE SURVEY
2.1 Glass Molding Process (GMP)
In practically, the usage of glass lenses offers substantial advantages over the plastic
lenses on aspects of mechanical strength, refractive index, light permeability, stability to
environmental changes in terms of temperature and humidity [1-5]. Conventionally, glass lenses
have been made up by different material removal processes, such as grinding, lapping and
polishing which requires a long production cycle with machinery and results in very high
production cost [2-6]. The most advanced technology to replace grinding, lapping and polishing
is precision glass molding; it was first introduced in Japan in the late 70’s [2].
Precision glass molding is state-of-the-art technology for efficiently mass production of
complex shaped lenses, such as aspherical lenses, Fresnel lenses, micro lens arrays, diffractive
optical elements (DOEs) and so on [7]. This technology permits ready-to-use optical elements to
be manufactured, without the need for expensive and time-consuming finishing operations.
Molded glass lenses have been widely used in a variety of applications, such as digital to mobile
phone cameras, digital camcorders, digital projectors, CD/DVD players and recorders, laser
pointing and aiming, laser diode to fiber coupling, medical devices and micro optics systems etc.
[1-7]
GMP is under highly repeatable process, which can be accomplished by heating and press
forming of preforms using ultra precision tooling and molds. Small pressing load is maintained,
the formed lens is slowly cooled down to release the internal stress, namely, annealing. Then, the
6
glass lens is cooled rapidly to ambient temperature and release from the molds which are
schematically shown in Figure 2-1.
Figure: 2-1 (A) Robotic features of glass molding process (GMP); (B) molding setups (a):
heating, (b): pressing, (c): annealing and (d): cooling [1, 2].
7
GMP is usually carried at a temperature between the glass transition temperature ( ) and
softening point (SP) of glass, the glass material shows significant viscoelasticity in deformation.
The ratio between the resultant deformation and applied force or pressure is related to viscosity
at pressing stage. The glass behavior during the cooling is more complex due to structural
changes and stress relaxation.
The quality of molded products depends on the mold qualities and optimization of
molding process. The low cost relies on the material cost, tooling time, yield and service life of
molding dies. This study mainly focuses on the topic of service life of molding dies in glass
molding technologies.
2.2 General Limitations and Problems of Optical Molds
Problems in Glass Molding Process
The contacts of the hot glass with the handling and forming tools often generate defects
in the glass and tool surfaces. In glass molding process, the die surfaces are exposed to the
chemically active glass and also subjected to mechanical and thermal cyclic operations, which
leads to three critical problems:
(a) Sticking/adhesion of glass to the die surface
(b) Oxidation of the dies
(c) Accelerate wear of the dies
8
Figure: 2-2 Diagram of possibilities of volatile matter transfer or diffusion between the mold
and glass materials. At high temperature, inter-diffusion or fusion between glass/film/substrate
will accelerate adhesion wear.
The possibilities of various volatile matter transfer or diffusion either from mold itself or
coating part or glass is shown in Figure 2-2. Due to above mentioned problems, the molded
glass products have different defects such as tear defect, sticking mark, scale-rust and feather etc.
The definition of these defects is as following:
-tear defect: a place where a small fragment of glass has been torn out by sticking to mold
surface
-sticking mark: small surface defect, often a matt patch, caused by local sticking of glass to
mold during forming
-scale, rust: flake of metal oxide or graphite included in the glass or stuck to its surface during
forming
9
-feather: cluster of very fine bubbles caused by the deposition of foreign matter on the hot glass
during forming
These defects may cause the molded lenses loss of form accuracy and shorten the service
life of the molding die. After 1000 shots, the conditions of the novel metal coated precision mold
as shown in Figure 2-3. As can be seen, glass stick marks on Plano side of the mold and coating
flaking/cracking. Other hand, large amount of glass species stick on aspheric side of the mold
after 1000 shots was observed. Clouding, fogging and stick marks on mold are needed to control
by optimized process parameters.
Figure: 2-3 Optical microscopic images of (a) glass stick mark and coating flaking/delamination
on Plano side of the mold, (b) glass sticking on aspheric side of the mold after 1000 shots.
Selection of the appropriate material for mold is one of the most important issues. The
selected material should be thermally stable, having low thermal expansion and high thermal
10
conductivity, mechanical strong enough and no chemical interactions even at high temperature
environmental conditions [8-10]. In addition, mold materials should be economically advantages.
Generally selected materials are used as mold such as stainless steel, silicon carbide (SiC),
tungsten carbide (WC), tungsten carbide with cobalt (WC/Co), hex boron nitride (hBN),
zirconium di-oxide (ZrO2), plated steel , nickel-phosphorous (NiP),titanium carbide (TiC) and
amorphous carbon (GC) etc,. In practice, Ni base alloys have been used as the mold material for
molding very low Tg glass optical components. Sintered tungsten carbide (WC) based materials
were widely used for molding normal low glass components. The glassy carbon can be used
for molding high Tg or silica glass components.
Some of mold materials and their properties as shown in Table: 2-1; among these,
amorphous carbon (GC) is one the best mold material for molding Quartz glass [9, 10]. High
thermal conductivity and heat transfer rate of mold is main criteria of material selection [9].
Table: 2-1 Mold materials and their properties. Representation: O – Good; Δ – Average and ×–
Poor [9].
Stability Strength Defect Density adhesionSiC O O O ×hBN O × × ΔZrO2 O Δ × ×C O × × ΔGC O O O O
Ceramic molds are more suitable for optical elements processing with low porosity and
binder phase at excellent attributes for example of Silicon Nitride. Contaminated ceramic molds
11
are re-used to removal of unwanted materials from surface of the mold by magnestoreheological
finishing (MRF) technique [9]. However, the molds are tough and brittle materials, which are
more complex and complicated to finish.
2.3 The Approaches to Extend the Service Life of Optical Molds
In generally the molds have a short lifetime, which can be attributed to the high thermal
and mechanical stresses and to the chemical interactions between the hot glass and the mold
surface [11-19]. In practice, there are few approaches to extend life span of the molds and avoid
sticking/adhesion of inorganic molten glasses with mold surface, such as: (1) use of low
glasses (2) lowering the molding temperature (3) glass molding carried out in an inert
environment (4) applying protective coatings on the mold and (5) applying anti-stick coating on
the glass performs etc.
The glass with low transition temperature (Tg) has the advantage of extending the service
life of molding dies because the molding temperature is significantly reduced. However, most of
low Tg glasses have high content of alkali metal oxides and tend to be decomposed at high
temperature and may induce severe glass sticking problems [11]. Furthermore, the low Tg glasses
normally demonstrate poor chemical durability and scratch resistance. As a result, the yields of
fabricating the glass-preforms are frequently rather low. Decreasing the molding temperature is
benefit for the improving the glass sticking; however, it will extend the heating duration and
influence the production efficiency. Most commercial glass molding machines have changed
chamber design and can be operated in an inert environment. However, even with very small
amount of oxygen residue in the chamber will cause the oxidation of ceramic molds and trigger
glass sticking. To develop protective coatings on the surface of mold and glass perform has
12
become the most popular and effective approaches to improve glass sticking problems. The
requirements of the protective coatings and related coating technology will be reviewed in the
following section.
2.4 Necessities and Requirements of Protective Coatings for Molds
It is essential to apply a protective coating on the molds to improve the surface quality of
molded components and service life of molds. There are several materials which can be utilized
as protective coatings including Cr plating, Diamond-like carbon (DLC), various nitrides,
carbides, oxides and noble metal coatings, mostly deposited by using of the physical vapor
deposition (PVD), chemical vapor deposition (CVD) process and sputtering deposition methods.
The lifetime of the mold with above mentioned coatings has been increases from 10 to 50 times
approximately, but their performance is inconsistent [14, 20-23].
Normally, the requirements of protective coating for mold are as following:
Surface qualities--no scratch, no particles
Surface roughness (Ra) < 5 nm
Film thickness uniformity < 5%
No influence on the form accuracy of molds
Service life of molds > 3000 times
2.5 Necessities and Requirements of Protective Coatings for Glass Preforms
The protective coatings on the molds are able to improve glass contact induced sticking
problems. However, some of very low Tg glass is unstable when contacts with reducing agents
13
such as hydrogen and carbon, which normally comes from the contaminants. The redox reaction
at glass surface may trigger glass sticking on the coated mold. Furthermore, the protective
coatings on the molds are unable to inhibit volatize or unstable elements evaporating from glass
and redeposited on the mold surface [24]. It is essential to applying a protective coating on glass
performs to improve above mentioned glass sticking problems. The protective coating on glass
performs has to satisfy the following requirements:
Surface qualities--no scratch, no particles
Surface roughness (Ra) < 5 nm
Film thickness < 30 nm
Film thickness uniformity < 20 %
With limited influence on the molding parameters
No influence on glass transparency
Recent studies proposed to apply a very thin carbon or carbon-hydrogen based coating on
glass preforms to suppress the unstable elements diffusion and hence improve glass sticking
problems [25-27]. However, some drawbacks still existed for these coatings: (1) thermal
decomposed C: H coatings with hydrogen trapped in the film may result in the reduction of oxide
glass, (b) the a-C or C: H film is unstable at high temperature. Both effects may cause glass
sticking on the mold. It is essential to develop a new coating material and technology for glass
performs to solve above mentioned problems.
14
2.6 An Overview of the Existing Protective Coatings and Related Facilities
Protective coating on the mold surface is a widely used approach to solve the sticking
action since a decade. Not only occurred in glass molding process, sticking is major task for
making high revolution patterned surfaces by nanoimprint lithography, hot pressing and injection
molding methods in field of MEMS, microelectronics and diffractive optical devices etc.. In low
temperature operation condition polymer based coatings are most suitable to prevent the sticking;
for good example is non-stick fry pan. Dipping of the master mold into chain length fluorinated
molecules, which has strong adherence and good anti-sticking performance of fluorinated layers
were developed [16]. However, these films are not suitable for high temperature environment.
The materials have been selected for the protective coatings for molding glass coatings
can be divided into five groups including [28-37]: (1) single layer carbides, nitrides, oxides and
borides such as TiN, BN, TiAlN, NiAlN, TiBC, TiBCN, NiCrSiB and Al2O3 (2) nitrides or
carbides based gradient and multilayer’s, (3) nitrides based superlattice films, (4) amorphous
carbon or diamond-like carbon and (5) precious metal based alloys. Several factors need to be
considered for the design of protective coatings including die materials, glass composition
operation temperature and applied load etc. The mechanical and thermal properties of different
coating materials are shown in Table 2-2 [38].
15
Table: 2-2 Mechanical and thermal properties of different coating materials [38].
16
Table: 2-3 Coatings Properties Comparison [14].
Several existing surface coatings have been utilized for high temperature glass molding
application to be reviewed as following: In general, there are three most commonly utilized types
of coatings in the glass molding industry such as noble metal based coatings, ceramic coatings,
and carbon based coatings [29-35].
2.6.1 Ni and Cr Based Coatings
The Ni and Cr based alloys are the most popular protective coating materials used for
traditional glass industry which normally operated at very high temperature but at a lower
pressure. These coatings are relatively not expensive and exhibit acceptable thermal and
oxidation resistance at evaluated temperature [14, 39, 40]. The surface qualities and form
accuracy are not so critical in commercial glass components. However in the case of optical
17
glass components, the surface qualities and form accuracy have strict requirements. The Ni and
Cr based alloys exhibits a higher thermal expansion coefficient and a lower strength at high
temperature, which is not good to control shape accuracy of molded optical components.
Besides, they are unable to avoid oxidation and to react with glass material at high temperature,
which will affect the surface qualities of molded optical components.
2.6.2 Precious Metal Based Coatings
Noble metal based coatings are being used by most of mass producers of optical
components in Asia due to the excellent oxidation resistance and anti-sticking behavior. The
cost of theses coating is extremely high and needs additional polishing to clean up the
contaminants from the coating surface of old mold.
Japan Patent No: 60-246,230 reported mold surface coated with multi layer Pt group
alloy used to produced micro-optical elements, the main drawback of the coating is too soft; the
coating conditions cannot control easily and exhibits columnar structure of surface easily[37].
Often difficulty with Pt group alloy coating is large chances of flaking due to adhesive failure
between coating and mold, because of stress relaxation process. Rhenium –Iridium (Re-Ir)
coating (with 240nm thick) was deposited by DC magnetic sputtering method on Tungsten
carbide (WC) mold. It shows the demolding performance and service life of the mold were
improved, and also the form accuracy and roughness of the molded components were enhanced
[41, 42].
Protective film including any one of high melting point metals or metal alloys of Pt, Ir, W,
Re, Ta, Rh, Ru and Os are developed and described in Japan patent no 2003-26429 and these
18
coating combinations perform good releasability of glass lens up to 700°C and at high
temperature (i.e. more than 700°C), releasability of glass lens is reduced. In addition, the metal
forming the film is expensive, resulting in increase in material cost [43]. Y.I. Chen reported Mo–
Ru coatings with the Ni interlayer on tungsten carbide (WC) achieved satisfactory thermal
stability with respect to phase evolution and surface characteristics including roughness and
hardness [45]. However, due to dual phase distribution in Mo–Ru coatings exhibited which
improves the roughness thoroughly. The surface roughness of the mold was improved by
application of Re–Ir coating on the surface of the tungsten carbide mold [45]. Re-Ir coating on
the mold surface which improve of the demolding performance between the lens and molding
core during the molding process and the mold lifetime [45].
H.H. Chien et al reported that TaN was deposited on the WC/Co substrate as the
diffusion barrier using a magnetron sputtering system, and followed by the deposition of Pt-Ir
film as the protective layer, TaN act’s as thermal barrier layer and inter-diffusion layer [46]. Pt-Ir
alloy layer is a thermodynamic stable phase which can avoid oxidation at 700°C. However, the
oxygen from the ambient diffused through the Pt-Ir layer and reacted with nanocrystallined TaN
to form Ta2O5 complex compounds, it confirmed from XRD results as shown in Figure: 2-4.
19
Figure: 2-4 (a) Cross-sectional SEM images of TaN/Pt-Ir multilayer coated mold (b) XRD
results of TaN/Pt-Ir coated substrate after 700°C/6hrs annealing treatment [46].
C.L. Chao et al demonstrated multi layered Pt/Ir protective coating (up to 388 nm
thicknesses) on the WC/Co mold with Ta as a buffer layer [47] and results proved that number of
layers, total thickness of protective coating not depends on better performance of anti-stick effect.
The anti-stick effect mostly depends on glass composition (either network formers or modifier or
intermediates) rather than thickness of the film. However tendency of glass sticking is not yet
solved by this type of coatings and fabricated cost of this type of coating much high. Sticking
20
behavior that could result in the surface quality deterioration of the molds and potentially destroy
the mold has not been properly understood. So that, looking forward for alternative coating
solutions and optimized glass materials.
Figure: 2-5 SEM micrographs of (a) Pt/Ir with Ta buffer layer coatings of 37 layers with 296 nm
thickness and (b) residual reactant from glass (S2-type Al2O3, BaO, Na2O, F are main
compounds in the glass) on same substrate after wetting test [47].
2.6.3 Single and Multi-Layer Nitride and Oxide Based Coatings
In the 1980’s hard ceramic TiN, TiC and Al203 coatings were commercially introduced as
protective layers on tools in the production industry. In practice, TiN/CrN and BN are usually
used to mold applications. These coatings are under the “ceramic” type category with good
thermal stability [14]. TiN/CrN coatings have poor oxidation resistance and easy to reactions
21
with glasses. The BN coatings normally exhibit too much internal stress which causes premature
failure of coatings.
Ceramic coatings such as silicon nitride (Si3N4), titanium nitride (TiN), chromium nitride
(CrN), chromium tungsten nitride (CrWN) and titanium aluminum nitride (TiAlN) have all been
applied to WC molds with varying degrees of success [48- 51]. Pits /holes appears on TiAlN
coated mold after pressing, hole are caused by small gas bubbles formed at the contact area of
glass. These bubbles appeared because of out gassing of elements from the glass or because of
chemical reactions between the counter parts. Very thin layer of glass adheres on TiAlN-coated
mold as shown in Figure 2-6 (c). However, residual reactants deposited on mold are sufficiently
high and sticking problem is not resolved.
22
Figure: 2-6 SEM pictures of (a) TiAlN coatings on WC (thickness about 300nm), (b) and (c)
glass adhesions on TiAlN-coated WC mold after pressing [48].
23
2.6.4 Diamond and Diamond Like Carbon Coatings
In the 1990s very low friction diamond and diamond-like carbon (DLC) surface layers
were investigated and some of them were introduced commercially [53]. The friction and wear
properties were one to two orders of magnitude lower than that of nitride or oxide coatings.
They are suitable for components in engines and mechanical elements requiring both low friction
and low wear. Kim et al demonstrated that DLC coating on the mold can improve the demolding
performance and mold life. The optical properties of molded products are improved at the same
time [53-57]. Y. Zhou et al developed superhydrophobic series of amorphous carbon films with
novel bionic nanostructured surfaces on Si (1 0 0) and glass by a simple sputtering technique as
shown in Figure 2-7, (a) Large-area of the surface of the a-C films prepared at 400°C, (b) water
contact angle data measured on the surface of the a-C films with varying deposition temperature.
The insert exhibits the shape of water on the surfaces of the a-C film is super-hydrophobic
surface with a contact angle of 152° (upper picture); and the hydrophilic surface with a contact
angle of 40° (lower picture) [58].
24
Figure: 2-7 SEM image of (a) Large-area of the surface of the a-C films prepared at 400°C, (b)
water contact angle data measured on the surface of the a-C films with varying deposition
temperature. The insert exhibit the sharp of water on the surfaces of the a-C films, the upper
image is super-hydrophobic surface with a contact angle of 152°; and the lower image is
hydrophilic surface with a contact angle of 40° [58].
25
Table: 2-4 Properties of carbon coating materials [59]
Diamond coatings, demonstrates good temperature stability, oxidation resistance and
lower chemical interactions with glass. The main drawback of this coating is uncertain stress
distribution inside the coating [53].
26
Diamond like carbon coatings is having very poor oxidation resistance and high reactivity
properties. The lifetime of the diamond like carbon coatings is limited due to the oxidation and
internal stress problems [53, 58]. Different impurity atoms like Si, F and N may be integrated to
modify the surface chemistry of the hard coatings as shown in Figure 2-8.
Figure: 2-8 Classification of DLC coatings [23].
27
2.6.5 Boride and Other Coatings
Boride coatings have very good anti-oxidation, chemical and wear resistance properties.
These coatings are widely used for cutting tools has improved lifetime and as diffusion barriers
[60].The excellent hardness of boride coatings is due to a high degree of covalent bonding and
among these coatings (example: TiB2, ZrB2 and CrB2 are the most popular materials used as the
protective coatings). Because of the mismatch of thermal expansion coefficients of coating and
substrate, several networks of cracks are developed inside these coatings [61-63]. It is suggested
that post-deposition treatment is required for releasing stresses.
The high temperature (up to 900 °C) wetting angles of glass gobs on pure ceramics (such
as Si3N4, WC, Si, and SiC) are higher than that of the sputtered coatings on M42 steel substrates
as shown in Figure: 2-9. This can be attributed to a higher chemical stability can be obtained in
pure ceramics compared to ceramics with metallic binders or ceramic coatings/M42 steel
combinations. The glass wetting phenomenon was very severe for using WC/Co (8%) and quartz
as the contact substrate materials, because Co is unstable at high temperature and quartz (SiO2)
easily reacts with P2O5 glass, which degrades the surface tension of glass. Although the wetting
angle was increased for glass in contact with most of the pure ceramics, they are still unable to
achieve satisfied anti-stick purpose at 900 °C [17].
28
Figure: 2-9 The effect of temperature on the wetting angle for glass gobs contact with various
ceramic substrates [17].
2.7 State of the Art on Sol-Gel Technology
Sol-gel technology has proven to be highly versatile technique with well -controlled
physical and chemical properties. Understanding about the reactivity of the precursors is main
criteria for the preparation of homogeneous sols.
2.7.1 Organic and Inorganic Sols
In the 80’s, H. Schmidt reported successful preparation of a new family of sol-gel based
materials named as “Ormocers”, organically modified ceramics [64]. It is obvious that the
29
constitution of the coating solution is great importance. For example, containing of fluorine in
Ormocers coatings act as anti-sticking constituent for glass containers application [65].
“Sol” – a colloidal dispersion of practices in a liquid; on the other hand, “sols” – are
typically multi component systems consisting of an inorganic phase dispersed in a solvent
mixture. “Gel” – is a giant aggregate or molecule that extends throughout the sols [66, 67]. Sol-
gel materials are peculiar because they often contain more than one solvent, each solvent
differing in volatility and surface tension [66]. The precursors could be classified as inorganic or
metal organic precursors which participate in a polymerization (gelation) process.
The organic and inorganic components can interpenetrate each other on a nanometer
scale. Depending on the interaction between organic and inorganic components, hybrids are
divided into two classes: (1) hybrids consisting of organic molecules, oligomers or low
molecular weight polymers embedded in an inorganic matrix to which they are held by weak
hydrogen bond or van der Waals force and (2) in those, the organic and inorganic components
are bonded to each other by strong covalent or partially covalent chemical bonds [68-71]. The
physical properties of coatings varied from brittle and hard to rubbery and soft depending on the
ratio of the organic to the inorganic constituents [65].
30
2.7.2 Method of Depositions
In practically, there are several methods available for applying liquid coatings to
substrates; the best choice depends on several factors including solution viscosity, coating speed
and desired coating thickness. Most commonly used methods for sol-gel deposition are dip
coating and spin coatings [68-71]. The film microstructure depends on the size and extent of
branching (or aggregation) of the solution species prior to film deposition and relative rates of
condensation and evaporation during film deposition. Physics of film formation examines the
dipping an spinning processes with respect to such parameters as withdrawal rate, spin speed,
viscosity, surface tension, and evaporation rate. The reactions which occur during this sol-gel
process can be classified in two categories: hydrolysis and condensation reactions.
Dip Coating:
In the dip coating process, sol-gel materials involves more than one competition between
viscous, capillary and gravitational forces; the mechanisms which control final film thickness
and microstructure very complex as shown in figure below :
Film thinning by gravitational draining is assisted by vigorous evaporation. Differential
evaporation may trigger several events at and beneath the liquid/gas interface. First it may lead to
concentration variations along the gas-liquid interface; theses variations leads to surface tension
gradients, which contribute to the surface stress and alter the flow. Second, differential
evaporation leads to diffusion of the volatile species towards the surface and non-volatile ones
away from the surface [65]. The representation of dip coating process is shown in Figure 2-10.
31
As the substrate is withdrawn upwards, a layer of solution is deposited, and a
combination of viscous drag and gravitational forces determine the film thickness, H [68, 69]:
H = c1 (ηUo/ρg) 0.5 (1)
Where - is the viscosity, the withdrawal speed and is a constant. The thickness of a dip
coated film commonly in the range of 50-500nm [68, 69]. For sol-gel coatings, the formation of
critical coating thickness, has been defined
Tc = EGc/ Aσ2 (2)
Where E is young’s modulus of the film, A is a dimensionless proportionality constant, and
the energy required to form two new crack surfaces. The mechanism of sol gel thin film
deposition is shown Figure 2-11.
32
Figure: 2-10 Representation of dip coating process. [68]
33
Figure: 2-11 Representation of thin film deposition mechanism [66].
2.7.3 Role of Solvents in In-situ Solution
Solvent acted as the coating carrier. The removal of solvent or drying of the coating
proceeds simultaneously with continues condensation and solidification of the gel network. The
origin of stress developed during drying of a solidified coating is due to the constrained
shrinkage and low rate of solvent loss after solidification [68-71]. The solvent content at
solidification should be minimized in order to lower the stress in the coating [69]. It is very
34
important to limit the condensation reaction rate during the removal of solvent upon drying, so
that the volume fraction of solvent at solidification is kept small.
The drying rate plays a very important role in the development of stress and formation of
cracks particularly in the late stages and depends on the rate at which solvent or volatile
components diffuse to the free surface of the coating and the rate at which the vapor is
transported away in the gas[68, 69].
2.7.4 Advantages of Sol-Gel Coating Process
Through the sol-gel method well control microstructural (e.g. high surface areas and
small pore size) films obtained directly from the gel state. Porous structures created in solution
are preserved, which lead to the application in filtration, insulation, separations, sensors and
antireflective surfaces. The advantages of sol-gel process are summarized as following.
Better homogeneity and purity from raw materials
Lower temperature of preparation:
• Save energy;
• Minimize evaporation losses;
• Minimize air pollution;
• No reactions with containers, thus purity;
• Bypass phase separation;
• Bypass crystallization.
35
However, the disadvantages of sol-gel processing include the cost of raw materials,
shrinkage that accompanies drying and sintering, and processing times.
2.7.5 Physical Properties of Sol-Gel Thin Film
The physical properties of a coating include hardness, residual stresses, tensile strength
and Poisson’s ratio, expansion coefficient and elastic module etc. These properties can be
manipulated by the porosity, residual OH, chemistry, structure, unreacted organics, thickness and
the uppermost temperature and duration of the heat-treatment [65, 72]. The expansion coefficient
and elastic module of the substrate also have an influence on the properties of the coatings. The
complexity of sol-gel film is shown in Figure 2-12. The relationship between physical
properties of coatings and other interdependent variables such as sol gel chemistry, process
parameters and thickness of coating are still unclear.
36
Figure: 2-12 Complexity of sol-gel coating [65].
2.6.6 Importance of Sol-Gel Al2O3 Coating
The Al2O3 coatings prepared by sol-gel process have been used for mechanical, optical,
semi-conductor and microelectronic applications, because of its excellent properties such as good
mechanical strength, high hardness, high resistance to radiation, corrosion resistance, excellent
antiwear ability, high abrasive nature, chemical inertness, insulating and optical properties
(transparency) [73-80]. In additional, it also has excellent adhesion property on substrate surface
37
achieved through low temperature sol-gel process. Selection of metaloxide and solvent is very
important for a desired high quality of nano scale film in sol-gel processing. The tribological
behavior of water based (aqueous sols) sol-gel Al2O3 coatings was evaluated by Zhang et al [74].
The results show sol-gel Al2O3 coatings exhibit better wear resistance, toughness and long life
with low coefficient of friction [74, 77].
Kim et al, reported that sol-gel derived Al2O3 buffer layer (< 10nm) acted as diffusion
barrier between the substrate and Pt film which improved the microstructural and electrical
properties of PZT ferroelectric films for nonvolatile memory devices [75]. The Inter-diffusion of
reactive elements was effectively prevented from the substrate by the Al2O3 diffusion barrier. In
practice, aluminum oxide film can be obtained by different techniques such as physical vapor
deposition (PVD), chemical vapor deposition (CVD), Atomic layer deposition (ALD) and
electroplating methods [74, 81]. However, the above mentioned methods are difficult to deposit a
film with good surface coverage and thickness uniformity for the samples with complex profile
or microstructured surface.
The optical and mechanical property of amorphous Al2O3 film is shown in Table 2-2.
The Transmittance range is in the range of 0.2~7 μm which is very suitable utilized as the
transparent protective coating. The high melting temperature and hardness Al2O3 film can
significantly improve thermal stability and wear resistance for both of metallic molds and glass
performs.
38
2.8 Wettability and Interfacial Reactions
2.8.1 Fundamental of Wetting Theory
Wetting or spreading is a physical process through which liquid or glass sticks the
surface. Spreading means that the coverage area by the liquid increases with holding time. The
ability of liquid to spread on a solid substrate within certain period of time is called “wettability”.
Wetting or spreading can be classified into two categories, such as reactive wetting and non-
reactive wetting. The wettability is usually characterized by the contact angle (θ ) which is
determined by the condition that the contact line between the three phases is at rest on perfectly
smooth and homogeneous solid surface, as shown in Figure 2-13.
Figure: 2-13 Schematic sketch of (a) contact angle between the solid and contact liquid/glass;
(b) hydrophilic; (c) hydrophobic contact angle.
39
The Young's equation, which is related θ to the surface energies σ of the three, interfaces [82]
LV
SLSVCosσ
σσθ
−= (3)
Where SVσ , SLσ and LVσ are the solid-vapor, solid-liquid and liquid-vapor interface energies,
respectively. There are two major parameters to characterize the wettability of drop of liquid or
glass on surface, such as degree of wetting (which indicated by the contact angle formed at the
interface) and rate of wetting( which indicates that how fast the liquid or glass wets the surface
and meantime spreads over the surface. Degree of wetting followed by surface and interfacial
energies involved at interface and it is governed by law of thermodynamics. The value of LVσ
can be obtained from the sessile drop method by the following equation [83, 84]
⎥⎦
⎤⎢⎣
⎡+−= f
fgdL
LV 0481.01227.00520.04
2ρσ (4)
4142.02−⎟
⎠⎞
⎜⎝⎛=
dhf (5)
Η−⎟⎠⎞
⎜⎝⎛=
2dh (6)
Where Lρ the density of molten materials is, g is the gravitational acceleration, Η is the final
height of the drop at a particular temperature and d is the drop base diameter. Rate of wetting is
governed by various factors such as capillary forces, viscosity of the liquid, chemical reactions at
interface and thermal conditions of the system.
40
Figure 2-14 illustrates an experimental approach used to measure equilibrium advancing
and receding angles using the sessile drop method in high temperature systems [85, 86]. The
sample is shaped either as a vertical piece or as a flat plate placed on the substrate. Because, the
sample has a lower melting temperature than the substrate, it becomes liquid during heating
whereas the substrate remains solid. If the sample is a vertical piece, the liquid advance across
the surface of the substrate until the velocity of the liquid front becomes zero. The final angle in
such a measurement is called “advancing contact angle” (Figure 2-14-A). When the initial
sample is a flat plate (Figure 2-14-B), then the liquid retracts and the final angle is the “receding
contact angle”.
Figure: 2-14 Schematic drawing of a sessile drop. Both (A) advancing and (B) receding angle
are shown [34]
The dependence of θ on the velocity of the wetting line as shown in Figure 2-15. The
contact angle varies with speed and direction of motion. In the advancing case, the angle
increases with increasing magnitude of the velocity. If the velocity is equal to 0, θ reaches the
41
static advancing contact angle, θadv. In the receding case the angle decreases with increasing
magnitude of velocity. For this case θ reaches the static receding contact angle θrec if the velocity
is equal to zero. The difference between θadv and θrec can be quite large—as much as 50° for
water on mineral surfaces. This is generally attributed to surface heterogeneity or surface
roughness. The equilibrium angle θ0, which would occur if no surface roughness or chemical
inhomogeneity on the substrate existed, lies in between these two values [87, 88].
Figure: 2-15 Schematic drawing of the advancing and receding contact angle versus. The
spreading velocity (v) of the triple line is moving along two directions. The advancing angle
exceeds the receding angle. This is called contact-angle hysteresis [87, 88].
2.8.1.1 Factors Influencing the Wettability
Different factors influencing the wettability at high temperature and the related
applications are shown in Figure 2-16.
42
Figure: 2-16 Different factors influencing the wettability at high temperature.
Absorption:
The adsorption of atoms (as for example, oxygen or carbon) on solid and liquid surfaces
and at solid–liquid interfaces leads to a reduction in the surface and interface energies. The
hypothetical dependence of the different surface energies of a solid-liquid-vapor system on the
43
activity of an active element are shown in Figure 2-17 [85, 88]. Since the equilibrium amount
of adsorbate depends on the activity, it can be used as a measure of the amount of adsorbate on
the surface. A critical activity acr exists where the surface interface energy decreases with
increasing activity as shown in Figure 2-17. Depending on the amount of adsorbate the
equilibrium contact angle decreases or increases. This can be derived from Young’s equation
(1) by introducing activity dependent surface energies.
Figure: 2-17 Hypothetical variation of the interfacial energies (σSL, σSV, σLV) versus the activity
a due to adsorption effects (for example oxygen or carbon adsorption) [85, 88].
Ridge Formation
Young’s equation only applies to systems where the substrate (should be homogeneous)
is perfectly rigid and insoluble, and where the triple line can only move in the direction parallel
44
to the substrate. The Young equation is derived by balancing the horizontal force components
and vertical force components can be neglected. This approximation is valid for many low-
temperature systems like organic liquids on hard, high-cohesive energy substrates such as most
metals or ceramics [85, 89-91]. When liquids are in contact with soft solids pronounced local
elastic deformation and formation of a triple line ridge on the substrate surface may occur. The
ridges can be several tens of nanometers tall and they can affect the dynamics of wetting [92-96].
For most high-temperature systems (e.g., molten metals or oxides on ceramics or metals)
the temperatures during the experiment are typically ≥0.5 Tm, and therefore, local atomic
diffusion can occur. This provides a mechanism for ridge formation even for hard substrates. In
the case of ridge formation, spreading requires motion of the triple line both horizontally and
vertically, which leads to two independent relations:
45
Figure: 2.-18 Illustration of (a) contact angle between the solid and contact liquid (b) the
dihedral angles in the case of ridge formation [85].
SL
V
SV
LS
σφ
σφ
σφ sinsinsin
LV
== (7)
where øS, øL, øV are equilibrium dihedral angles in the solid, liquid and vapor respectively.
These dihedral angles are visualized in Figure 2-18.
The ridge will evolve until complete equilibrium is reached. If a ridge is present one has
to differentiate between microscopic and macroscopic angles. The microscopic angles are the
aforementioned dihedral angles øS, øL, øV. The macroscopic angle is the angle between the
tangent of the liquid/vapor interface at the triple line and the unperturbed substrate surface. The
presence of a ridge can strongly influence the spreading kinetics and the equilibrium angle. The
observed spreading rates can be orders of magnitudes lower than for liquids, where the flow is
just controlled by capillarity and viscosity Saiz et al [85, 91].
46
The spreading process divided into four stages, depending on the degree of ridge growth
Saiz et al [85]. In the first stage, the deformation which occurs at the triple line is due to elastic
strains in the solid and capillary forces drive the contact angle towards the one defined by
Young’s equation. In the case of metallic or ceramic substrates this distortion was calculated
and found sufficiently small such that no plastic deformation is expected. This stage is found at
short times when the liquid spreads very fast with a high driving force so that a triple line ridge
would be unstable Saiz et al [85, 91]. In the second stage some diffusion processes or solution
precipitation is allowed to occur. The substrate will deform at the triple line and capillary forces
will drive θ towards a value close to Young’s angle but spreading kinetics will be dictated by
the velocity at which the attached ridge moves. There exists a certain time where the ridge will
be small compared to the radius of curvature of the liquid. A ridge can form, depending on the
ratio of the height of the ridge compared to the curvature of the liquid one differentiates
between second and third stage (as shown in Figure2-19). The fourth stage describes complete
equilibrium, which means macroscopic 2D equilibrium and constant curvature. To reach full 2D
equilibrium, times much longer than the experimental ones might be necessary in many
practical systems.
47
Figure 2-19 The geometry of a liquid drop on a substrate depends on time. In Regime 1 (A) the
spreading velocity of the liquid is faster than the ridge formation. The liquid spreads on a flat
surface. Regime 2+3 (B): a ridge can form, depending on the ratio of the height of the ridge
48
compared to the curvature of the liquid one differentiates between Regime 2 and 3. Regime 4 (C):
full equilibrium is obtained. The curvature of the drop is constant [85, 88, 91].
2.8.1.2 Reactive and Non-Reactive Wetting: Thermodynamic Point of View
Non-Reactive Wetting:
Spreading of liquid or glass on surface without local reaction /adsorption at interface is
known as non-reactive wetting. In case of non-reactive wetting higher contact angles can be
obtained and for this type of wetting, only thermodynamic principle of energy reduction of
system should be under considerations. The change in surface free energy ( ) following a
small displacement of liquid is [97-100].
= ΔA (σSL – σSV) + ΔA σLV Cos (θ – Δθ) (8)
Where change in area of surface covered by liquid. At equilibrium condition, and
values are unity and surface free energy becomes zero at uniform temperature and chemical
potentials. Therefore Eq. (5) obeys young’s equation:
σSL – σSV + σLV Cosθ = 0 (9)
Reactive Wetting:
In reactive wetting, uncertain stabilize conditions exist at interfaces. It means a new solid
compound formation at interface due to the mainly chemical reactions or mass transports
between the liquid or glass and substrate. The wettability and reactivity is determined by the
strength or quality of chemical bonding between the mating surfaces. Thermodynamic approach
49
for reactive wetting is more sensitive and many deviations exhibits to analysis the unique
concept. On the other hand the concept of thermodynamics of wetting is limited only to non-
reactive systems [88, 98]. From Gibbs free energy of wetting; wetting is still enhancing in
systems, where bulk reactions are occurred between the phases even may not be
thermodynamically feasible.
In reactive systems, the strength of chemical bonding between mating surfaces can be
assessed by determining the work of adhesion aW . The thermodynamic work of adhesion aW
defined as follows by Dupré: [85, 88, 91-98]
Wa = σSV + σLV - σSL (10)
aW is the work per unit area that must be performed to separate a solid- liquid interface to obtain
a solid/vapor and a liquid/vapor interfaces. Combing equations (1) and (10), the Young-Dupré
equation can be obtained: [85, 88, 91-98]
( )θσ cos1+= LVaW (11)
Hence, the condition for complete wetting in a reactive system is at contact angle is
0 .
2.8.1.3 Dynamic Wetting: Effects of Surface Roughness
In dynamic wetting process, the dynamic contact angle increase with the increasing
wetting velocity, the stress should increase . It was reversed in dewetting process. The
change of the solid-liquid interfacial tension is proportional to the viscous shear stress at triple
line from below mentioned equation [101]
50
(12)
(13)
Where is solid-liquid interfacial tension in the dynamic wetting process, c proportional
constant having dimension of length and is viscous shear stress in N.m-2.
In generally, capillarity (θd) and gravity (drop weight) are two forces that drive spreading (W).
For drop volume V, viscosity η, density ρ and surface tension σ, the radius of the wetted spot
grows as [101]
1/10 (14)
For small drops and as for large drops follows:
1/8 (15)
For a drop of maximum height H, the viscous force is proportional to ηUR/H, where
U=dR/dt is the velocity of the contact line. However, there is no involvement of spreading
parameters (for example, spreading coefficient) in above laws and not well -defined value of R
of the triple line formation at interface.
Surface roughness improves the surface energy value by providing an additional
interfacial area for the spreading liquid over on it. Wenzel theory [101] discusses the effect of
roughness on the equilibrium contact angle and apparent contact angle on a rough surface as
given by
51
Cos = r Cosθ (16)
Where is Wenzel angle (also called as apparent contact angle on rough surface), is the
equilibrium contact angle and is the average roughness ratio (i.e. ratio of actual wetted surface
area to projected or geometric surface area). - Value always greater than unity and equal to
unity for smooth surfaces.
2.8.2 Kinetics of Wetting: Reactive Vs Non-Reactive Wetting
For understanding the kinetics of wetting, the forces action in the particular systems
should be considered instead of thermodynamics or simple energetic. For example, when a liquid
is placed on the substrate, capillary forces drive the interface spontaneously towards equilibrium,
i.e. from the contact angle of maximum value to its equilibrium value with respective to time.
Particularly those related to the kinetics of contact line moments [86, 88, 98-101]. Many
empirical approaches as well as molecular kinetic and hydrodynamics approaches are reminds
the dynamic contact angle has been correlated with the velocity of the contact line [88, 98,101].
In kinetic studies, experimental results deals about the different dimensionless parameters such
as dimensionless radius, spread factor, Reynolds number, bond number, Weber number and
capillary number and discuss regarding to kinetic spreading by utilizing forced spreading and
spontaneous spreading.
52
Non-Reactive Wetting:
For non-reactive wetting, wetting kinetics can be simply determined by liquid surface
tension and viscosity [85, 98, 100, 101].The rapid spreading state is under the non-reactive
wetting process and which is explained by Young’s equation.
Power law equation is a common tool to measure the spreading kinetics of non-reactive
systems. The partial spreading of organic liquids was successfully determined by using the
exponential power law [99] given as
Ac\Af = 1- exp {- K/Af (τn)} (17)
where A is the measured liquid-solid contact area (normalized with respect to (V)2/3), is the
final equilibrium value of the normalized wet area, is the dimensionless time( equal to
t/(ηV1/3), t is the time, k and n are empirical constants, V is drop volume, σ is surface tension
and η is liquid viscosity. The linear decrease of parameter K with the ratio (η/σ) was also
observed indicating quicker kinetics at lower viscosities and higher surface tensions [98-102].
Reactive Wetting:
Up to now, theoretical models are still unable to fully explain complete reactive wetting
phenomenon, but some of empirical relations are used to describe the reactive wetting situation.
The main reason of the this difficulties are a single function can hardly describe the full range of
relaxation behavior of the system because of reactive wetting processes consists with different
actions (such as diffusion, chemical reaction and fluxing and their possible combinations affects)
moderate the wetting rate.
53
From existing studies, five stages are identified in a reactive wetting: (1) an initial rapid
spreading stage, (2) an initial quasi equilibrium stage, (3) an interfacial front advancing stage, (4)
no advancing but continuous decrease in drop height stage and (5)a final wetting equilibrium
stage. A decrease in contact angle may be due to: (1) advancing interface (an improvement in
wetting), or (2) consumption of spreading liquid by reaction with active substrate (no
improvement in wetting). N.Eustathopoulos et.al reported modified equation for reactive wetting
driving force from equation (18) is [103]
Fd (t) = σLV [ Cos θe – Cos θd] (18)
Where is the equilibrium contact angle of the liquid on the reaction product surface and
dynamic or instantaneous contact angle θd is defined as contact angle with respect to time (i.e.
time dependent contact angle). Spreading will occur in a reactive system through either local
chemical reactions (rate is limiting the diffusion within the droplet is comparatively rapid) or
transport (diffusion and convection, rate is limiting the local reaction rates are comparatively
rapid) mechanism [98,102-104].
2.8.3 Modeling of Spreading:
High-temperature spreading occurs if the spreading experiment is performed at T > 0.5
Tm. Spreading of a liquid on a solid can be classified into complete and incomplete wetting
systems. Traditionally the models work better for complete wetting systems.
However, the theoretical models of low-temperature spreading assume that the substrate
is smooth, homogeneous, inert, and non-deformable. If a liquid drop contacts a substrate, the
system (liquid, solid and vapor) is generally not in its equilibrium state of minimum free energy.
54
Capillary forces drive the system towards its equilibrium by spreading of the liquid over the
substrate. During spreading the contact angle θ changes its value from its initial maximum value
of 180° (at the moment of contact), towards its equilibrium angle θ0 in the case of partial wetting,
and 0° in the case of complete wetting. As the drop spreads, the radius of the contact area
changes from its initial value of 0 mm to its equilibrium base radius R0. Since the system
changes its energy state from a state of higher energy to a state of minimum energy, energy has
to dissipate during spreading. The energy can be dissipated in viscous dissipation in the bulk
drop and at the triple line. In principle energy is dissipated in both ways. However a model that
treats simultaneously both sources of dissipation has not been developed yet. Consequently there
exist two models which consider different main sources of energy dissipation. Both describe the
motion of the triple line of the liquid drop while the system equilibrates [105].The molecular
kinetic model considers the friction at the triple line as main source of energy dissipation
whereas the fluid flow model considers the viscous dissipation in the bulk drop as main source of
energy dissipation.
Capillary and gravity forces can be considered in the complete spreading. The capillary
force, gravity force and viscous forces can be derived approximate by following equations
Fc = σθ2 (19)
and
Fg = H2ρg (20)
and
Fv = ηUR/H (21)
55
Where h is height of drop, R is radius, U is contact line velocity (given by dR/dt). An ideal
surface the force that opposes spreading is the viscous force. From Tanner’s law, radius of the
drop proportional to relaxation time as followed by
R α tn (22)
Where the value of is 0.1 and 0.125 for small and large drops respectively.
2.8.4 Surface Properties
2.8.4.1 Surface Energy and Surface Tension
The surface tension of a solid/liquid system can be determined via contact angle
measurements. The contact angle of a sessile drop of liquid resting on a solid provides the
surface tension of the liquid/solid system. In addition, contact angle measurements can be used to
assess the hydrophilicity/hydrophobicity of a solid surface. Contact angle measurements have
been correlated with surface properties of tension over the past four decades. The glass tends to
stick to materials that come in contact with the glass. This is similar to a liquid wetting a solid,
which is controlled by the differences in surface tensions (or surface energies) between the
materials.
The contact angle is one of the most important parameters describing interfacial
phenomena in a wide variety of practical applications. The well-known Young’s equation very
well describes the relationship between static contact angle and interfacial tensions on an ideal
solid surface, which is chemically homogeneous, rigid and flat, not perturbed by adsorption or
other chemical interactions with the wetting liquid.
56
Cassie theory
According to the Cassie model air can remain trapped below the drop, forming “air
pockets”. Thus, hydrophobicity is strengthened because the drop sits partially on the air. On the
other hand, according to the Wenzel model the roughness increases the surface area of the solid,
which also geometrically modifies hydrophobicity [106]. It is conventional to relate so-called
moderate hydrophobicity to the Wenzel regime, whereas Cassie scenario results in strong water-
repellent surface properties. However, the Cassie regime has been reported recently for slightly
hydrophobic interfaces, moreover coexistence of Cassie and Wenzel regimes at the same
surfaces has been reported [4, 66, 106]. Lack of the reproducible experimental data in the field
has to be emphasized [107].
2.8.4.2 Hydrophilic and Hydrophobic Surfaces
The biological expedience of the phenomenon, called the lotus effect, consists in the
possibility of self-cleaning of plant leafs due to the rolling of drops without water spreading on
the leaf surface. The underlying physical problem was how hydrophobicity can develop on
materials which are partially wettable [106-111]. This phenomenon has been explained by
forming the large water-air interface under a water droplet in consequence of the air trapping in
pockets of a highly textured substrate. Since the surface energy of the water-air interface is large,
the droplet tends to decrease the underlying area increasing the contact angle.
Contact angle measurements can be used to assess the hydrophilicity/hydrophobicity of a
solid surface. In sol-gel deposition, porosity is another important property of sol-gel film.
57
Porosity of coating can also be controlled as well as the hydrophobic and hydrophilic balance
[107, 109-114].
2.8.5 Contact in Glass –to-Metal System
Understanding of glass-to metal interaction concept is mostly important in various field
applications such as solar cell encapsulation, pressure transducer fabrication (joining glass to
silicon), opto-electronic devices and the attachments of glass fibers to silicon [115-120]. Metal
has higher thermal expansion coefficient than that of the glass. At the high temperature, metal
will expand more than glass. While cooling stage the metal tends to contract more than glass.
This will lead to residual stresses at interfaces. The quality of the interface for two dissimilar
materials mainly depends on the material properties like elastic modulus, the coefficient of
thermal expansion and roughness etc. Glasses are suitable for solid oxide fuel cells (SOFCs) as
sealing since the physical and chemical properties of the glass can be tailored within a wide
range [117].
Earlier investigation of glass-to-metal system was carried out in 70’s. In glass-to-metal
adhesion, the glass at the interface is saturated with the oxide of the lowest-valence cation of the
metal substrate and once saturation is achieved at interface, there is strong chemical reaction
occurred [115-117, 119-121]. A continuous electronic transition zone developed through metallic
bonding in the substrate and the ionic –covalent bonding in the glass. Final contact angle of glass
on iron substrate is 10° was reported by C.E.Hoge et al. at 1000°C and 10-8atm pressure for 2hr
holding time [122-124]. The degree of adherence is related to the degree of saturation of the
glass with the substrate oxide.
58
Tomsia et al., investigated glass -to-metal interaction, according to the oxygen activity of
system which can be classified into nonreactive and reactive system. Due to the oxidation effect
and surface diffusion at interface, strong radical chemical reaction occurred between the glass
and metal [124].
In the case of dewetting/nonwetting, the interlayer formation at interface should be
avoided. It’s a great challenge to explain how ions exchange influences the dissolution or
dissociation of interlayer through diffusion mechanism [124, 125] There is very limited literature
discussing anti sticking mechanism between glasses and coatings.
Bonding between the metal and glass materials can be obtained by oxidation of metal i.e.
oxide of the metal that shares the oxygen ions. Other reason of the bonding formation is redox
reaction without oxidation layer on the metal. The reaction layer is formed by redox reaction
between the glass and the stainless steel substrate was conformed through the element mapping
analysis, which shown there is a quantified diffusion of Ni2+ in to glass at interface [115-125].
When Fe3+ ions are reaction with glass material, the color of the glass changed into bluish
and greenish-blue. FeO, NiO, Cr2O3 are precipitated near the glass and metal interface [126,127].
The oxidation of the substrate at interface by reduction of Ba, Si in the glass was the principle
reaction observed. The oxidation-reduction reactions observed can be classified in to 3 categories:
two involve the reduction of a cation in the glass to the atomic state and the third reduction of a
cation to a lower valence. The first two types can be represented by
MS + MO (glass) = MSO (glass) + M (23)
Where Ms -represents substrate metal and M is glass material. Most alloying elements will be
oxidize when they exposure to high temperature environment.
59
Ison et al explained how to achieve strong adhesion or bond at interface. A satisfied
bonding of the glass- to- metal system can be achieved by a thin metal oxide interlayer [127].
Excessive dissolution or diffusion will result in unacceptable level of corrosion and lose of
bonding (i.e. lose of metal oxide layer). N.H. Menzler et al. investigated the interaction of glass-
to-metal (as steel) under air, humified air and hydrogen atmosphere conditions for fuel cell
application and conclude that the strong adhesion enhanced between the glass-to-metal interfaces
by the formation of the oxide layer [117]. Dissociation pressure of oxides will depend on
reaction ability with interface materials. If dissociation pressure of oxides higher than the oxygen
partial pressure, the individual metallic form of particular elements will be in stable position
[117].
For several metals composed of ferrous elements (Ni, Co, Fe, and Pd) on SiO2 and Al2O3
as shown in Figure 2-16. For instance, the calculated value of XO ( oxygen mole fractions ) for
Fe/Al2O3 at 1853 K is 2x10-5 which puts this couple in the reactive range although the contact
angles are typical of non-reactive systems (θ = 109°, θ = 130° values from different
researcher’s)[ 129-132]. If the dissolved oxygen and/or the dissolved Al have a significant effect
on q, any variation in the concentration of these elements would modify θ. The experimental
contact angles for pure metal M/ionocovalent oxide systems versus the calculated equilibrium
mole fraction of oxygen in liquid M resulting from dissolution of the oxide mentioned in Figure
2-20 [129-131].
60
Figure: 2-20 Experimental contact angles for pure metal M/ionocovalent oxide systems versus
the calculated equilibrium mole fraction of oxygen in liquid M resulting from dissolution of the
oxide [129-131].
2.8.6 Contact in Glass-to-Ceramic System
The main criteria of bonding of two dissimilar materials (for example: ceramic and glass),
the coefficient of expansion of ceramic member must be compatible with that of the glass.
Lopez-Esteban et al. [133] investigated the spreading kinetics of glass-to-ceramic system
(SiO2–CaO–Al2O3 on polycrystalline Mo) at an oxygen partial pressure of > 10-14 Pa by using a
drop transfer method was performed at 1200 °C [133]. Through SEM and EDX analyses, there is
61
no reaction between the glass and the metal was observed and also no interdiffusion at the
glass/metal interface. This system is a model system since the glass does not have any volatile
components and is stable under a wide range of temperatures and oxygen partial pressures. The
analyses of the spreading kinetics showed that the spreading velocity of the glass front is orders
of magnitudes slower than expected for spreading controlled by viscous impedance (fluid flow
model). Lopez-Esteban et al [133] were able to fit their experimental data to a theoretical model
introduced by Blake et al [134]. In their theory they suggested that both viscosity and local
dissipation at the triple line play a role in determining the wetting kinetics (molecular kinetic
model). The viscous effect can be included in the molecular kinetic model.
2.8.7 Thermodynamic and Kinetics of Glass-Metal/Ceramic System
Wetting has mainly been illustrated from the view point of thermodynamics. Because a
glass is generally consisting of a various elements and is complex as the system, hurdle to
understand a wetting mechanism has been exist when we consider wetting as the basic interface
science for bonding of glass-metal/ceramic system [135].
Both work of adhesion and work of cohesion are proportional with the surface tension:
Young-Dupré equation can be obtained [82, 83]
LVCW σ2= (24)
Where LVσ is liquid-vapor interface energy which is directly calculated by the following
equation [82, 83]:
⎟⎟⎠
⎞⎜⎜⎝
⎛Η+
Η=
)2/(641.1)2/(641.1
2
2
ddg
LVρσ (25)
62
where g is the gravitational acceleration, ρ is the density of molten metal, Η is the final height
of the drop at a particular temperature and d is the drop base diameter.
2.8.8 Nanoscale Thermal Transport at Solid-Liquid Interface
In the case of small length –to-time scales, the thermal transport is more sensitive to the
properties of interfaces. It means that thermal transport across interface is sensitive to atomic-
level structure. The possibility of phonons scatter at the interface is large at high temperature and
at sufficiently imperfect interfaces due to diffuse mismatch [136, 137].
63
3 EXPERIMENTAL DETAILS
3.1 Glass Materials:
3.1.1 L-BAL 42 Glass Preforms
Glass preforms (Ohara L-BAL 42) with 6 mm diameter and each mass of 3.05g were
chosen as test materials. The glass preforms were cleaned with piranha solution which contains
30% H2O2 and 98% H2SO4 mixture at a volume fraction of 3:7 at 75°C for 40 minutes then
rinsed with distilled water and acetone solution. Allow to dry the all samples in air at room
temperature. Cleaned samples were stored inside clean room environment.
The composition and thermal properties of glass are shown in Table 3-1 and 3-2. Silica
and barium oxides are major components in tested glass for this wettability study, which have
been used in high precision glass lens production. The main effect of silica in the glass, silica is
the major acidic oxide and easily mixes with the basic components. Benefits of large proportions
of silica increase the melting and softening point of glasses and lower the thermal expansion.
Moreover; initiating of barium gives a high brightness to glass. Sodium is added in most glass to
reduce the melting temperature during glass production.
The L-BAL42 (Ohara Corp...) glass preforms were selected as glass materials for wetting
test. The volume-temperature relationship of L-BAL42 glass is plotted in Figure 3-1.
64
Figure: 3-1 Plot of volume change against temperature for a typical optical glass L-BAL42. This
is showing strongly temperature-dependent thermal expansion characteristics. -transition point;
SP-softening point; -yielding point; AP-annealing point; StP-strain point [data provided by
O’Hara Corp., data][138].
Definitions:
Softening point (SP) - is defined as the temperature at which the glass deforms under its own
weight and behaves as liquid.
Yielding point ( -) - also called “deformation point”, is a temperature at which glass reaches its
maximum expansion and a relatively low plasticity and starts shrinking.
Annealing point (AP) - is the upper end of the annealing range for the pressed glass lens, at
which the internal stress is reduced to a practically acceptable value over a short period.
65
Strain point ( P) - represents the lower end of the annealing temperature range. It is also the
upper limit of service temperature of glass component. During annealing, glass is slowly cooled
down from AP to somewhat below P.
Table: 3-1 Composition of glass [138]
_______________________________________________________________
Content SiO2 BaO B2O3 Al2O3 ZnO Sb2O3
Composition in Wt. % 40-50 20-30 2-10 2-10 2-10 0-2
_______________________________________________________________
Table: 3-2 Properties of glass [138]
_________________________________________________________
Properties Values
_________________________________________________________
Transformation Temperature Tg ( °C ) 506
Thermal Expansion Coefficient α (10-7/°C) at 300°C 88
Young’s Modulus E (108N/m2) 891
Rigidity Modulus G (108N/m2) 357
Poisson’s Ratio σ 0.247
Refractive Index nd 1.58313
_________________________________________________________
66
3.1.2 Chalcogenide Glass Preforms
Chalcogenide and chalcohalide glasses are more attractive material for optics due to the
high refractive index, low cost, ultralow optical loss and easy fabrication processes for optical
devices. So they are useful for optoelectronic applications, especially for planar waveguide
application such as routers, switches, electro-optic modulators and non-linear optical parametric
converters [139,140].
Molded IR glass lenses opening up new applications in thermal imaging and smart
sensors. According to the published results, the molded IR lenses with the sizes up to 25 mm
and can be achieved and with a diffractive surface to make the systems achromatic [139-143].
There still exists some technical difficulty to molding high quality aspherical or free-form lenses
for chalcogenide glasses, which can be attributed to the following factors: (1) it is difficult to
fabricate suitable glass performs for molding chalcogenide glasses due to their fragile
characteristics. (2) The chalcogenide glasses with a higher thermal expansion coefficient tend to
produce a higher thermal stress and induce cracking on molded lenses. (3) The mechanical
properties of chalcogenide glasses are very sensitive to the applied load and temperature in
molding process. This restricts the molding conditions and makes the molding work very tough.
It is found the bigger the size of the molded lens, the more difficult to achieve optimized molding
condition. (4) Most of chalcogenide glasses have high content of unstable volatile elements,
these elements tends to diffuse out from glass performs at high temperature and deposit on the
surface of the molds which induce severe glass sticking problems. This has made the molding
process of these kinds of glasses very difficult indeed.
67
The glass balls of chalcogenide (Ge28Sb12Se60) with 5.82 mm diameter were chosen as
one of the glass preforms. The physical properties of the glasses are shown in Table 3-3.
Table: 3-3 Physical properties of chalcogenide (Ge28Sb12Se60) glass [138].
_________________________________________________________
Properties Values
_________________________________________________________
Density : g/cm3 4.72
Coefficient of Thermal Expansion :
10-6 K-1 at 20°C-100°C
10-6 K-1 at 100°C-200°C
14.1
15.5
Specific Heat : J/gK 0.34
Thermal Conductivity : W/mK 0.23
Transition Temperature : °C 285
Softening Point: °C at 107.6dPa.s 309
Annealing Temp: °C 285
Transmission: µm 0.9～15
68
3.2 Mold Material and Preprocessing:
Grade 304 stainless steel is used as a mold material. Stainless steel has been widely used
as the mold material in manufacturing commercial glass containers and optical components
because of its advantages such as high tensile strength, high heat resistance and low cost.
However, the main drawback of the stainless steel employed as a die or mold material is the
sticking between the glass and the mold surface during the molding process at elevated
temperatures. Design of particular protective coating on mold surface is often marginal. The
composition of substrate is mentioned in Table 3-4.
Mirror-polished Grade 304 stainless steel plates with dimensions 15mm x 15mm x 1 mm
were used as the substrates for the deposition of water based sol-gel Al2O3 coating. Before
deposition, these substrates were pre-cleaned using following steps: (1) ultrasonically cleaning
the substrate by immersion in acetone for 2-3 minutes; (2) substrates were dipped into diluted FP
(contains FeCl3, HCl, H2O2) solution for 30 seconds and then cleaned with de-ionized water; (3)
substrates were dipped into diluted FPS (contains (NH4)2HPO3, 12-alkane base- phosphorus
acid-amine) solution for 2-3 minutes, then again were cleaned with de-ionized water.
Table: 3-4 Composition of Substrate
The chemical composition of the AISI 304 Stainless Steel
Contents Fe Cr Ni Mn Si Mo C
Composition (wt %) 72 17.4 8.42 1.37 0.49 0.17 0.06
69
3.3 Procedure
3.3.1 Details of the Sols and Coating Preparation
An alumina sol was prepared by adding 2272 g AlCl3-6H2O to 10 liters of distilled water
(for the hydrolysis), and the mixture stirred vigorously (300 rpm) for 1hr until the suspension
turned to a clear sol. To obtain the pH value is 4 by quickly adding 20% HNO3 (so for called as
peptization process) with a stirred speed of 600 rpm and followed by the slowly adding 20%
HNO3 (2 ml/min) until the pH value to achieve required value (in between 5.5 and 8.5). The
solvent was filtered and dried by vacuum process until it turned into aggregated white powders
(orthoaluminic acid).
The second step is to add orthoaluminic acid powders into 110 liters of distilled water
and stir it for 1 hour with the speed of 300 rpm. Next, the NP (Nonyl phenol) was added into the
mixture and kept stirring for 30 min at room temperature. The mixture was then gradually heated
at 85~95°C and followed by adding 500 ml H2O2 into the mixture and kept stirring for 6 hours.
Finally, the mixture solution was cooled down and turned to clear Al2O3 sol. Flow charts for
Al2O3 sol preparation process as shown in Figure 3-2.
Overall shrinkage of Al2O3 sol during annealing of the gel material is negligible and does
not imply crack formation.
Details of Precursors:
Aqueous sol derived from AlCl3-6H2O. AlCl3-6H2O is used as a precursor. Molecular
weight of this precursor is: Molar mass of AlCl3-6H2O = 241.432218 g/mol. Different elements
involved and mass percent in precursors are shown in below Table 3-5.
70
Table: 3-5 Details of different elements involved in the precursors and their atomic mass and
mass percent.
Figure: 3-2 Flow charts for Al2O3 sol preparation process.
71
The diluted water is act as solvent for solution. The gelation and condensation of the
derive solution were carried out through suitable method. The pH value of water based Al2O3
sols is approximately 4~4.1. The Al2O3 sols were transparent at room temperature. No precipitate
was formed even after storing such a long term.
The cleaned substrates were dipped into the water-based sol-gel solution for 20 seconds,
and then withdrawn at speed range of 5 ~ 200mm/min by using (Sense company- programmability
control the parameters) dip coating machine and to be dried in air for 2 minutes inside the clean
bench.
3.3.2 Characterization of Precursors
3.3.2.1 Viscosity and pH Value
The pH of the sol-gel solution produced by alumina alkoxide was measured at 25°C
using pH meter electrode. 50cm3 of the sol-gel solution was collected into a beaker and the pH
value was determined by inserting the pH meter electrode into it. The pH value from the
instrument was recorded. Depending on the pH value, chain-like or branched networks will be
built [145]. The morphology and the size of the particles of coated films can be tailored by pH
value on the sol-gel process [145].
3.3.2.2 Differential Thermogravimetric (DTG)
Differential Thermogravimetric (DTG) analysis of a material at thermal treatment
coupled with mass spectrometry analysis of the gaseous species which evolve during heating.
DTG of the gel samples were carried out at a heating rate of 20 °C/min up to 900 °C, in air.
72
3.4 Approaches
3.4.1 Coating on Mold Surface
After ultrasonically cleaned mold substrates (stainless steel used as a mold material) were
bring to dip coating solution(contains Alumina metal alkoxide) bath and rise up the mold
substrate with withdrawal speed of 50mm/s from solution bath by using precision remote control
dip coater. Withdrawal mold substrate is kept in clean room environment for drying and
deposition of the films.
3.4.2 Coating on L-BAL 42 and Chalcogenide Glass Preforms
The properties of sol-gel coatings could be varied by different substrate materials due to
the change of bonding forces between two different materials. Coating procedure for glass
performs is the same as coating on mold surface. Thickness of coating depends upon the
withdrawal speed of substrate, viscosity and concentration of solution [64-69].
3.4.3 Heat treatment of Coated Samples
Flow chart of heat treatment process for Al2O3 coating on both stainless steel substrate
and glass preforms as shown in Figure 3-3
73
Figure: 3-3 Flow chart of heat treatment process for Al2O3 coating on both stainless steel
substrate and glass preforms.
3.5 Characterization of Developed Coatings for Mold and Glass Preforms
The morphology, reaction products and crystal structure were investigated by optical
microscope (OM), high resolution electron scanning microscope (HRSEM), EDX and XRD.
3.5.1 Characterization of the Film
3.5.1.1 Surface Morphology and Uniformity
74
The samples were preliminarily examined with optical microscopy to obtain
morphological information. We used an optical reflection microscope equipped by a digital
camera to observe the surface structure of all the specimens.
SEM is a well-established technique used for surface morphology characterization and
qualitative elemental analysis. The interfacial interaction between the substrate and molten glass
has been investigated through SEM and elements mapping.
3.5.1.2 Thickness of the Film
SEM instrument utilized to measure thickness of the film on glass perform and mold. The
thickness of Al2O3 film can be controlled by the drawing speed in dip coating process.
3.6 Wetting Equipment:
The schematic diagram of the wetting equipment is shown in Figure 3-4. The
experimental apparatus consists of a quartz tube which is heated externally by an infrared (IR)
heating furnace. The glass preform with a diameter of 6.0 mm was placed on the centre of
substrate. Wetting experiments were performed using sessile drop technique in N2 gas ambient
with heating temperature up to 800°C, and then kept at this temperature for 5 minutes in order to
study the time dependent wetting behavior. A CCD camera is fitted at the end of the quartz tube
to record the images of the wetting angles during the test. The morphology, reaction products
and interface structure were investigated by optical microscope (OM), high resolution electron
scanning microscope (HRSEM) and EDX.
75
Figure: 3- 4 Schematic illustration of the high temperature’s wetting equipment.
3.7 Wetting Test and Analysis
High temperature glass wetting experiment is normally used to investigate the high
temperature interfacial reaction between the coatings and glasses (laboratory IR source setup as
shown in Figure 3-5). Generally non-wetting and low reactivity are required for a mould surface
or coating materials in order to avoid the sticking between glass and mold surfaces, and also
enhance the service life of mold and qualities of the final molded products. This study mainly
focuses on investigating the effect of sol-gel derived oxide coatings on the wettability of glass
gobs at high temperatures.
76
Figure: 3-5 Schematic diagram of the IR source high temperature wetting equipment
3.7.1 Analysis of Capillary Wetting Phenomenon of Glass with Contact Surface
3.7.1.1 Wetting and Spreading State of Glass Preform with Al2O3 Coated Mold
The wetting experimental apparatus consists of a quartz tube which is heated externally
by an infrared (IR) heating furnace. The glass preform with a diameter of 6.0mm was placed on
the centre of Al2O3 coated mold. Wetting experiments were performed using sessile drop
technique in N2 ambient with heating temperature up to 800°C, and then kept at this temperature
for 5 minutes in order to study the time dependent wetting behavior. A CCD camera is fitted at
the end of the quartz tube to record the images of the wetting angles during the test.
77
3.7.2.1 Wetting and Spreading Rate of Al2O3 Coated Glass Preform with Mold
Time dependent spreading of Al2O3 coated glass preform with mold surface was
investigated in this research work. Al2O3 coated glass preform was placed on the centre of the
mold substrate and heated both of them up to 800°C for maintained 5 minute holding time. After
wetting test, both glass preform and mold substrates were examined by optical microscope, SEM
with mapping and EDX tools.
3.7.2 Molding Test at High Temperature
3.7.2.1 Molding Conditions for Al2O3 Coated L-BAL 42 Preforms
Molding glass optical lenses were carried out by Toshiba press molding machine at a
molding temperature of 580℃ to examine the plastic extension behavior of Al2O3 film.
3.7.2.2 Molding Conditions for Al2O3 Coated Chalcogenide Preforms
Molding glass lenses were carried out by Toshiba press molding machine using WC/Co
as the mold material. The molding conditions were shown in Figure 3-6, with the molding
temperature in the range from 298 to 308 °C at an applied load of 800 N.
78
Figure: 3-6 Molding parameters of chalcogenide glass.
3.7.3 Theoretical Analysis:
3.7.3.1 Thermal Expansion of Glass:
The form accuracy of the lens is strongly influence on the thermal expansion of particular
glass material. For low glasses, linear thermal expansion coefficient has been taken in account.
As for simplification, linear approximations are adopted to represent the changes of thermal
coefficient in different temperature ranges [1, 117]. The linear thermal expansion coefficient (∝)
can be given by Eq. (26)
79
(26)
Where is the overall length of material in the direction being measured; T is the instantaneous
temperature in °C; is the constant thermal expansion coefficient in the working temperature
range of glass; is the constant thermal expansion coefficient below and is the average
gradient of the increase of thermal expansion coefficient between and . For L-BAL42 glass,
= 7.2 × 10-6 1/°C, =8.8 × 10-6 1/°C, = 3.0× 10-8 1/°C2.
The volume thermal expansion coefficient (β) is approximately three times the linear thermal
expansion coefficient, thus it can be defined by Eq. (27)
β ˜ 3α (27)
3.7.3.2 Analysis of Heat Transfer in between Glass to Mold Surface:
The major heat transferred to the glass preform is from the lower mold and surrounding
insert gas during the soaking time. Glass is a compound material; the specific heat of glass is
known to vary with its composition and temperature. From Sharp-Ginther equation, the mean
specific heat ( ) of glass has been derived in Eq. (28)
= {(a3T + C0)/ (0.00146T+1)} x 4.186 x 103 (28)
80
Where is a constant of a glass material, = 0.000408 J/°C2; C0 is the true specific heat at
0°C, C0 = 0.144 J/°C2; both and C0 are both obtained by the glass composition. An
intermediate value of each composition was used to calculate the constants and C0 in Eq. (28).
The true specific heat can be easily calculated from the differential equation as follows:
= d (T )/dT (29)
The thermal conductivity k is varying with temperature in a complex manner due to the effects of
high temperature radiation. The thermal conductivity (k) of glass has been obtained by following
formula as shown in Eq. (30)
k (T) = 1.028 + 0.000624T (30)
The thermal conductivity coefficient (a) could be calculated [64] by following Eq. (31)
a = k/ ρ.Cp (31)
Where is density (x 1000kg/m3) and is specific heat (J/Kg K). The product of is called
volumetric heat capacity gives the thermal energy storage ability of the materials.
3.7.3.3 High Temperature Viscosity of Glass
From Vogel-Filcher-Tamman (VFT) equation, we can be estimate the glass viscosity at
particular temperature range is shown in Eq. (32)
Log η = A + [B/(T-To)] (32)
Where A=5.87, B =234.6 and T0=490 are constants.
81
4 RESULTS AND DISCUSSION
4.1 Variation of Al2O3 Coating Morphologies with Withdrawal Speed
Hydrolysis and polycondensation reactions of the alkoxy and hydroxyl group take place
during the deposition and drying stages at the room temperature. The cross-sectional SEM image
shows the dried Al2O3 film composed by aggregated nanoparticles with dense structure and
smooth surface, as shown in Figure 4-1. A lower drawing speed favors the deposited Al2O3 film
with a better uniformity and dense. The thickness of Al2O3 film was increased with the drawing
speed (Figure 4-2) and the variation of film thickness is less than 10 % with respect to
withdrawal speed was identified from the Graph between the withdrawal speed (mm/min) and
film thickness (nm) as shown in Figure 4-3. The XRD results present nanocrystallined structure
after annealed treatment. Annealing treatment allows the Al2O3 films to complete further
densification by diffusion.
82
Figure: 4-1 Effect of drawing speed on the surface morphologies of Al2O3 films (a) 20 mm/min
(b) 100 mm/min (c) 200 mm/min.
83
4.2 Thickness of Al2O3 Coating Vs Withdrawal Speed
Figure: 4-2 Effect of withdrawal speed on the thickness of Al2O3 films (a) 20 mm/min (b) 100
mm/min (c) 200 mm/min.
84
Figure: 4-3 Graph between the withdrawal speed (mm/min) and film thickness (nm).
Up to 2μm thickness of sol-gel derived films can be achieved through dip coating process.
Uniformity and cracks free surface coatings were observed from the OM analysis. These sol-gel
derived SiO2 and Al2O3 coatings exhibit amorphous-crystalline phase transformation at 300°C
and 350°C respectively. Heat treated at 650°C, both water based sol-gel SiO2 and Al2O3
coatings become fully crystalline structure and the grain size increases with increasing the
heating duration as shown in Figure 4-4. The grain size increased to 500nm when the sintering
temperature was up to 800°C; however, the surface becomes rough due to the crystal facet effect.
85
Figure: 4-4 SEM micrographs of (a) SiO2 and (b) Al2O3 coated substrates after heat treatment
process carried out at 650°C for 30minutes.
A 3D optical imaging profiler was used to evaluate the topographies of the Al2O3 coated
mold before and after wetting tested mold surface (shown in Figure 4-5). Based on the
measurement, the average roughness of Al2O3 Coated mold (Ra) is 45.41nm and average
roughness value (Ra) is 62.3nm at where the glass contacted with coated mold surface. Sharp
peaks intimate raising the surface roughness of coated mold which is in contact with molten
glass from left portion of Figure 4-5.
86
Figure: 4-5 Topography of coated mold surface after wetting test.
4.3 Differential Thermogravimetry (DTG) Analysis of Al2O3 Coatings
Hydrolysis and polycondensation reactions of the alkoxy and hydroxyl group take place
during the deposition and drying stages at the room temperature. pH value of water based Al2O3
sol-gel coating is approximately 4.1. From the DTG curve at 100°C, weight loss peak appeared
due to the evaporation of the solvent (water) from the Al2O3 coating surface as shown in Figure
4-6. From DTG spectra the temperature range 150 to 570°C, it has thermal effects due to
shrinkage of aggregated nanoparticles in the coating. Moreover, during this range,
polycrystallization processes associated inside the coating material. Above the 570°C
temperature range, DTG curve manifested crystallization peaks from Figure 4-6.
87
Figure: 4-6 Differential thermogravimetry (DTG) spectra of Al2O3 coated glass preform.
4.4 Physico-Optical Properties of Al2O3 Coatings
4.4.1 Transmittance of Al2O3 Coated Glass Preform
Coating’s light transmittance was evaluated by UV-vis spectrophotometer. UV-spectra of
transparency of glass preform before and after Al2O3 sol-gel coating. The traces are very similar,
indicating that the transmittance remains unaffected by deposited Al2O3 sol-gel coating as shown
in Figure 4-7.
88
Figure: 4-7 UV-spectra of transparency of glass gob before and after Al2O3 sol-gel coating. The
traces are very similar, indicating that the transmittance remains unaffected by deposited Al2O3
sol-gel coating.
4.4.2 Al2O3 Coated Glass after Scratch Test
The surface morphology of Al2O3 film after scratch test was shown in Figure 4-8. The
detached Al2O3 film presents obvious plastic deformation and ductile failure appearance. It is
believed that the Al2O3 film composed by aggregated nanoparticles, which are able to flow to
accommodate a large amount of ploughing and associated shear stress through densification and
shear deformation. There is no stress concentration at flaws in nanoparticle aggregated ultra-thin
film. The critical applied stress for fracture of ultra-thin Al2O3 film may become infinite or
failure approaches theoretical strength. In this case, the adhesion between the Al2O3 nano-
89
particles may dominate fracture behavior of Al2O3 film.
Figure: 4-8 Surface mophologies of Al2O3 film coated glass after scratch test.
4.5 Glass Wetting Test
4.5.1 Glass on Mold
4.5.1.1 Spreading Kinetics of Glass Preform on Mold Surface
Figure 4-9 shows the variation of contact angle and contact area radius with respect to
holding time for glass ball on uncoated stainless steel substrate at 800°C. The contact angle
rapidly decreases from initial and then gradually approaches a stable value after 2 minutes
holding time. However, the contact radius keeps increasing with the holding time and finally
spreads completely over the substrate. The final contact angle and contact radius are 29° and 5.5
mm respectively. The glass remains adhered with substrate after cooling down to room
90
temperature due to the formation of the oxide layer at interface; it constitutes strong chemical
bonds at interface.
0 50 100 150 200 250 3000
20
40
60
80
100
120
140
160
Con
tact
Rad
ius(
mm
)
Holding time ( Sec)
Con
tact
ang
le (
degr
ees)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
(b)
(a)
Figure: 4-9 Variation of (a) Contact angle and (b) contact area radius as a function of time for
molten glass on the uncoated stainless steel substrate at 800°C for 5- minute holding time.
The interfacial interaction between the substrate and molten glass has been investigated
through EDX analysis and elements mapping (Figure 4-11 and 4-12). The reaction products
mainly consist of Ni, Cr, Ba, Fe, Zn and O mixed compounds. This indicates that high affinity
reactive elements, Cr and Fe enhanced chemical reactions, resulting in the formation of Cr-O and
Fe-O based compound layer at interface. It has also observed that some of the reactive elements
Si and Ba diffused out from the molten glass into the substrate.
91
From elements mapping analysis of uncoated glass/substrate interface was good evidence
of noticeable reactive elements diffusion at interface (representative view as shown in Figure 4-
10). In generally, the nature of the interface is also influenced by external perturbations. Both
side of reactive elements are actively involved into reaction at interface and formed mixed
product through redox process. From EDX and mapping analysis, the redox processes enhanced
between the stainless steel (uncoated) and the glass materials at interface as given below;
From the substrate
4/3 Cr(s) + O2(g) = 2/3 Cr2O3(s) (33)
2 Ni(s) + O2(g) = 2 NiO(s) (34)
2 Fe(s) + O2(g) = 2FeO(s) (35)
From the glass
Si(s) + O2(g) = SiO2(s) (36)
2 Ba(s) + O2(g) = 2 BaO(s) (37)
However, in reality some oxides may carry covalent bonds only, no actual electron
transfer from the metal to the oxygen. In oxide systems, the change of standard Gibbs free
energy can be represented by a general equation of the form [1, 2]
(38)
In our case, according to redox reactions for stainless steel and glass interaction the above eq. (38)
was modified to [1, 2]
92
(39)
Where A, B and C are three experimentally determined constants for a particular system. The
values of these constants for a number of oxide system of redox reactions as mentioned above is
given in Table 4-1.
Table: 4-1 Representation of standard free energy of redox reactions in stainless steel/glass
interface at 1098K.
Reaction -A C - (calories)
178500 41.1 133372.2 61.131
116900 47.1 65184.2 29.877
124100 29.9 91269.8 41.833
215600 41.5 170033 77.935
271600 6.4 220652.8 101.136
The free energy change is related to the equilibrium constant of any chemical reaction by
the equation: [1, 2, 148]
ΔG°T = RT lnPO2 (40)
Thus by knowing of any oxidation reaction, the equilibrium oxygen pressure (lnPO2)
of the system is calculated. For ideal gas, R=1.987 cal/mol.K. From the Table 4-1, it is
93
estimated that standard Gibbs free energy of formatted SiO2 elements in interface at 1073K is
much smaller than the same at 298K (- =192100calories). In other hand, for barium
oxide (BaO) pieces in interface at 1073K is higher than at 298K. It believed that barium oxide
encourage the adhesion or strong hermetic bonding through diffusion at interface (example:
uncertainty of sodium (Na) and potassium (K) volatile elements in glass at high temperature). It
believed that by reduce or avoid the BaO percentage in main glass composition, which
discriminate the less tendency of glass sticking with substrate material from our study.
Figure: 4-10 Cross-sectional views at the interface of the molten glass/uncoated stainless steel
substrate.
It was observed that the ratio of the work of adhesion and cohesion is 98% for the case of
glass on the uncoated substrate at 800°C, indicating a strong adhesion, which is ensured by
effective chemical bonding through transfer of the mass or forces. The value of surface energy
for uncoated stainless steel substrate has been obtained from the sessile drop method is about
0.128 Nm-1 and the calculated work of adhesion is about 0.249 Nm-1. Molten glass completely
94
spread over the substrate and sticking is confirmed at room temperature. Spreading coefficient
has been obtained as -0.00856 Nm-1. The surface tension (σ) for the test carried out in oxygen–
free environment is higher than that of oxygen-containing ones [148]. The occurrence of inter-
diffusion usually results in good adhesion/wetting.
Figure: 4-11 Cross-sectional views at the interface of the molten glass/uncoated stainless steel
substrate and EDX results.
95
Figure: 4-12 Cross-sectional views at the interface of the molten glass/uncoated stainless steel
substrates with element mapping results.
96
4.5.1.2 Effect of Temperature on the Final Contact Angle
The variation of Contact angle with respective to temperature profile as a function of time
for molten glass on the uncoated stainless steel substrate at 800°C for 5- minute holding time as
shown in Figure 4-13. In this case, the temperature is inverse proportional to the contact angle.
During the holding stage, rapid decreased of contact angle curve has been observed. In cooling
stage, fluctuation of contact angle curve exists due to contraction mismatch of the substrate and
glass material during the cooling process and finally it reached to stable value at end of the
cooling process.
Figure: 4-13 Variation of Contact angle with respective to temperature profile as a function of
time for molten glass on the uncoated stainless steel substrate at 800°C.
97
4.5.1.3 Influence of Ridge Formation on the Spreading Kinetics
A strong reaction can lead to dissolution of the particles in terms of ridge formation.the
ridges form by diffusion or solution/precipitation of the solid atoms in response to the groove
formation at the intersection of a grain boundary and a free surface [85]. Through SEM
investigations, potential ridge formation was observed on uncoated substrates. Ridge formation
and ring of small glass islands formed at surrounding interface was observed by interface
analysis shown in Figure 4-14. Formed ridge is very small compared with the radius of liquid
curvature was observed at interface boundary and it controls the spreading kinetics.
98
Figure: 4-14 Analysis of interface conditions between the glass and uncoated stainless steel
substrate; (a) microscopic image of glass adhesion at interface: chemical reaction takes place at
edge of interface between the glass and uncoated substrate, (b) ridge formation indentified by
optical microscopy, (c) SEM image: width of ridge formation at interface, (d) SEM image: ring
of small glass islands formed at surrounding interface.
99
4.5.1.4 Formation of Oxide Layer
In this study, the oxide layer (FeO) formed on the uncoated stainless steel surface after
high-temperature oxidation treatment serves as a glass-to-metal interface layer. It is investigated
that the influence of oxide layer formation in between the uncoated mold substrate and glass
preform. Form an oxide layer on substrate under heat treatment process at O2 environment
(which is known as pre-oxidation process). The variation in rate of oxidation for uncoated mold
substrate oxidized at 800°C for different isothermal holding periods. The rate of oxidation is
defined by the difference in net weight per unit area of samples before and after heat treatment,
which is also called as “net weight of oxidation” [150]. The net weight of oxidation is increases
with increasing of isothermal holding as shown in Figure 4-15. Longer explores time helps
higher oxidation rate of the substrate and also net weight of oxidation is linear proportional to
holding time.
Figure: 4-15 Graph represents relationship between the net weight of oxidation with respective
to holding time.
100
The relationship between the thickness of oxide layer and isothermal holding time was
shown in Figure 4-16. The change of the oxide layer thickness with increase in the isothermal
hold time; it seems oxidations become saturated above 60min isothermal hold period, because of
formatted oxide layer has very loose structure and becomes mechanically brittle nature and it
will disappears or break easily during the handling process or long explore holding time at high
temperature. (Figure 4-15). Formed oxide layer is more dense and brittle nature during the
holding at 800°C (Figure 4-16). It can be concluded that oxide layer plays a crucial role in
adhesion or sticking process between the uncoated mold and glass. Wetting at the interface
indicates adhesion of glass to the substrate surface, which will facilitate good sealing and joining
with help of interface oxide layer. The proposed optimum thickness of the oxide layer should be
2–6.5 µm for good quality of sealing at interface [150]. The thickness of the oxide layer formed
on the pre-oxidized samples was observed using optical microscope. The obtained thickness of
oxide layer at 800°C of uncoated mold substrate is 18µm as shown in Figure 4-17.
101
Figure: 4-16 Relationship between average thickness of oxide layer on uncoated mold and
isothermal holding time at 800°C.
102
Figure: 4-17 Micrograph of oxide layer of uncoated mold substrate treated at 800°C.
4.5.2 Glass on Al2O3 Coated Mold
The wetting curve shows very interesting results for Al2O3 coated substrate. The
variation of the contact angles varied from 152 to 136° with minor change in contact radius,
presents anti-sticking or non wetting behavior (Figure 4-18).
103
0 50 100 150 200 250 300134
136
138
140
142
144
146
148
150
152
154
Con
tact
Rad
ius(
mm
)
Holding time ( Sec)
Con
tact
ang
le (
degr
ees)
0.0
0.5
1.0
1.5
2.0
2.5
(b)
(a)
Figure: 4-18 The behavior of (a) Contact angle and (b) contact radius as a function of time for
molten glass on the sol-gel Al2O3 coated substrate at 800°C.
The appearance of the area contacted by glass ball looks smooth and clean. Many
elements such as Fe, Ni, Cr, C, Al, and O were observed from the Al2O3 coated substrate (Figure
4-19).
104
Figure: 4-19 SEM /EDX results of Al2O3 coated substrate after wetting test.
However, there are no active elements such as Zn and Ba can be found on the glass
surface after wetting test. There is no Ni or Cr peak appears in the slumped glass ball surface
from SEM/EDX analysis as shown in Figure 4-20. It means that there is no chemical interaction
occurred at the interfaces between the Al2O3 coated substrate and glass. The sol-gel coated Al2O3
film acted like as an excellent diffusion barrier which effectively inhibit the element diffusion
and reaction between the substrate and glass. This is the reason why the final contact angle of
Al2O3 coated substrate has minor change from initials at 800°C. The glass ball still remains fully
transparent after contacting with the Al2O3 coated substrate during the wetting test.
105
Figure: 4-20 SEM/EDX results of tested glass surface after wetting test when contacted with
Al2O3 coated substrate.
4.5.3 Glass on SiO2 Coated Mold
In the case of molten glass on SiO2 coated steel substrate; it was observed that the contact
angle decreased slowly from initial 100 seconds and again rapid change in contact angle
occurred in the until 200 seconds because of the fluctuations at the low frequency limit due to
van der wall forces between the contact surfaces. Heat treatment is qualitatively different in its
effects because it is a time-dependent process. The contact radius increases with the time
duration, as shown in Figure 4-21. The final contact angle is 77° and final contact radius is 3.9
mm. The appearance of the substrate contacted by glass ball looks clean from a low
magnification SEM picture. The EDX results show Fe, Ni, Cr, C, Al, and O are the main
elements observed from the surface of SiO2 coated substrate. It worth to note that there are a
106
few of Sb-rich particles can be observed on the glass ball contacted area from high magnification
SEM/EDAX analysis (Figure 4-22). The glass contacted surface of SiO2 coated steel substrate
becomes very rough after the test. The weak Ni peak was detected on the slumped glass surface
which is eliminated from the substrate (Figure 4-23). This indicates the mass diffusion and
redox reaction occurred at interfaces, which resulted in the obvious decrease of the contact angle.
0 50 100 150 200 250 30070
80
90
100
110
120
130
140
150
160
Con
tact
Rad
ius(
mm
)
Holding time ( Sec)
Con
tact
ang
le (
degr
ees)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
(b)
(a)
Figure: 4-21 The variation of (a) contact angle and (b) contact area radius as a function of
holding time for molten glass on SiO2 coated substrate at 800°C.
107
Figure: 4-22 SEM /EDX results of SiO2 coated substrate after wetting test.
Figure: 4-23 SEM/EDX results of tested glass surface after wetting test when contacted with
SiO2 coated substrate.
108
The results show that the adhesion and contact angle can be attributed to two possible
mechanisms. The interfacial chemical reaction dominates wetting behavior due to the
occurrence of oxydo-reduction and complex basic-acid reactions. Second possible mechanism
may be from the surface roughness of the substrates and other physical forces which need to be
further studied.
4.5.4 Al2O3 Coated Glass on Mold
From our wetting experimental results, Al2O3 coated glass preform on stainless steel
substrate (shown in Figure 4-24); the glass ball spreads from initial contact angle to reach a final
value. It spreads from θ0 to θf value; the spreading rate is determined by viscous force or
resistance. Means it is control by viscous resistance only, because there is limited reactive
product formation at the interface.
From previous studies, coated mold materials performed better anti-sticking behavior
than uncoated ones [101].However, exact reasons and mechanism behind the glass-to-mold
sticking is not discussed yet. It’s important to understand the interaction of atoms and molecules
at interface (for example: chemical reactions, adsorption/desorption or mass transformation
etc,).This requires careful analysis to judge the wetting phenomenon. In general, wetting can be
divided to partial wetting or non-wetting. If the probability of reaction is less; then there is very
less probability of sticking at triple line. The variation of (a) Contact angle and (b) contact area
radius as a function of time for sol-gel Al2O3 coated glass preform on the stainless steel substrate
109
at 800°C was shown in Figure 4-25 and the final contact angle values is 136° as shown in
Figure 4-26.
Figure: 4-24 The variation of contact angle as a function of holding time for sol-gel Al2O3 -
coated glass preform on the stainless steel substrate with respect to temperature profile.
110
Figure: 4-25 Variation of (a) Contact angle and (b) contact area radius as a function of time for
sol-gel Al2O3 coated glass preform on the stainless steel substrate at 800°C for 5 minutes holding
period.
Figure: 4-26 Images of the final contact angle of sol-gel Al2O3 coated glass preform on the
stainless steel at 800°C.
111
The variation of the contact angle with respect to the temperature profile from 550 to
800°C and processing time is shown in Figure 4-27. According to temperature profile, the
contact angle curve can be classified into 3 stages, such as heating, holding and cooling stages.
In the first stage, the contact angle of molten glass slowly decreases while heating from
temperature 550 to 800°C followed by a very slow spreading kinetics for uncoated glass balls.
In the second stage the temperature was kept at 800°C for five minutes. The contact angle was
rapidly decreasing with respect to holding time. In cooling stage; the contact angle changes are
negligible, small variation of contact angle due to expansion or contraction of glass material
during the cooling process. The total cycle time is twenty five minutes
Figure 4-27 The variation of contact angle as a function of holding time and temperature for
uncoated and sol-gel Al2O3 coated glass ball on stainless steel.
The Al2O3 coated glass ball still remains fully transparent after wetting test when
contacted with stainless steel substrate (Figure 4-28(a)). The surface area of stainless steel when
contacted with Al2O3 coated glass ball remains smooth and free of reactants (Figure 4-28(b)).
112
Although stainless steel was oxidized, there is no observable glass sticking products, such as Zn,
Ba, Al and Si oxides to be detected on the glass contacted surface (Figure 4-28(c)). It means that
there is no detectable chemical interaction occurred at the interfaces between the Al2O3 coated
glass ball and stainless steel substrate. The sol-gel coated Al2O3 film acted as an excellent
diffusion barrier which effectively inhibit the element diffusion and reaction between the
substrate and glass. This is the reason why the final contact angle of Al2O3 coated substrate has
little change at 800°C.
Figure: 4-28 (a) Appearance of Al2O3 coated glass ball (b) SEM image of stainless steel and (c)
EDX results after wetting test.
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4.5.5 Al2O3 Coated Glass on Al2O3 Coated Mold
The wetting behaviors between the substrate and glass preforms, which are coated by
Al2O3, were determined by IR heating source at high temperature. The pair of this system is
called as a “similar metals interaction”. The interaction of similar metals (Al-Al) at high
temperature was investigated (Figure 4-29). It is observed that the final contact angle of Al-Al
interaction is 134°, and this value is smaller than the glass on Al2O3 coated substrate (136°) and
Al2O3 coated glass preform on substrate cases. The deviation of final contact angles in all these
cases are all most negligible values. In case of Al-Al interaction, after 15 minutes holding time
final contact angle is observed. The constant final contact angle is achieved dues to repulsive
force of inter atoms or molecules of contacted aluminum and it depends on interatomic distance
of interacted or contact materials. Either similar or dissimilar metal interaction at evaluated
temperature plays an important role in wetting or non wetting systems. No interdiffused reactive
elements from either substrate or glass preforms are observed. Measured values of wetting will
differ from intrinsic work of adhesion (WA) values because of contributions of chemical
interactions, inter-diffusion effects, internal film stresses, interfacial impurities, imperfect contact,
etc at interface.
114
2 4 6 8 10 12 14 16 18 20 22115
120
125
130
135
140
145
150
155
160
165C
onta
ct A
ngle
( ?)
H old ing T im e (m in )
Figure: 4-29 Variation of Contact angle as a function of time for the Al2O3 coated substrate and
Al2O3 coated glass preform at 800°C for 5- minute holding time.
4.6 Molded Lens Analysis
The sets of samples have been prepared the first one having thin coating obtained by
single dipping and the second corresponding to thicker layers produced by multidipping process.
4.6.1 Al2O3 Coated L-BAL 42 Molded Lens
Figure 4-30 shows the elements depth profile of Al2O3 coated lens after glass molding
test at 580°C. It shows that the Al2O3 film has thermodynamic stable phase which can
effectively hinder out diffusion of elements such as Si, Zn, B and Ba at 580℃. Carbon based
contaminants still exist on the Al2O3 coated glass surface which need to be avoided.
115
Figure : 4-30 Elements depth profile of Al2O3 coated glass lens produced by molding process at
580°C.
A desired optical lens is obtained by press molding a glass preform using stainless steel
material as the mold (Figure 4-31(a)). When press molding of a glass materials having an Al2O3
film on the surface, deformation of the glass materials is accompanied by extension of Al2O3
film on the surface. When extension of the Al2O3 film cannot keep up with deformation of glass
material, breach will occur. Thus, the glass material is exposed at the breached portions,
resulting in the risk of fusion to the molding surface. Our results showing the surface of molded
lens appear defectless within the designed aspherical aperture when the thickness of Al2O3 film
is 34 nm (Figure 4-31 (b)). The breaches only appear near the edge of the molded lens due to a
116
greater glass deformation on this portion (Figure 4-31 (c)). The thickness of Al2O3 film less
than 15 nm is suggested to completely avoid breaches on the whole molded lenses.
117
Figure: 4-31 (a) Appearance of molded lens (b) SEM surface image of molded lens and (c) high
magnification SEM image near the edge of the molded lens.
118
4.6.2 Al2O3 Coated Chalcogenide Molded Lens
4.6.2.1 Surface Morphology of Molded Al2O3 Coated Lens
Increasing the molding temperature to over 304°C, a few small bubbles and precipitates
are appeared at edge of molded lens (Figure 4-32), this can be attributed to the occurrence of
evaporation of some volatile elements from chalcogenide glass. Further increasing the molding
temperature to 308°C, severe sticking occurred between the glass and mold. The sticking
products appearing on the surface of mold are mainly Se and Te compounds identified from
EDX results.
Figure: 4-32 SEM surface images of (a) molded lens near the edge of the molded lens and (b) at
magnification image of near the edge of the molded lens at the molding temperature of 304℃.
In the case of molding glass lens using uncoated chalcogenide glass ball as the preform,
there was no reaction products and cracks to be detected on the surfaces of molded lenses after
molding test at the molding temperature from 302 to 308 ℃(Figure 4-33(a)). The relatively less
119
bubbles were observed from SEM analysis even under a molding temperature of 308 ℃. The
surface quality and yield was significantly improved. When press molding of a glass preform
with a thin Al2O3 film on the surface, deformation of the glass materials is accompanied by the
extension of Al2O3 film on the surface. When the extension of the Al2O3 film cannot keep up
with the deformation of glass material, micro-cracks and tiny voids will nucleate. Thus, the glass
material is exposed at the breached portions, resulting in the risk of fusion to the molding surface.
Our results showing the surface of molded lens appear defectless within the designed the
aspherical aperture when the thickness of Al2O3 film is 34 nm (Figure 4-33 (b)). Very limited
fine cracks and tiny voids only appear near the edge of the molded lens due to a greater glass
deformation on this portion (Figure 4-33 (c)). The thickness of Al2O3 film less than 15 nm is
suggested to completely avoid cracks on the whole molded lenses.
120
Figure: 4-33 Represents (a) Appearance of molded lens with protective Al2O3 film on the
surface (b) SEM surface image of molded lens and (c) high magnification SEM image near the
edge of the molded lens.
121
4.6.2.2 XPS Analysis of Molded Al2O3 Coated Lens
Figure 4-34 shows the elements depth profile of Al2O3 coated lens after glass molding
test at 305℃. It shows that the Al2O3 film is thermodynamic stable phase which can effectively
hinder out diffusion of elements such as Ge, As and Se at 305℃. Carbon based contaminants
still exist on the Al2O3 coated glass surface which need to be avoided.
Figure: 4-34 XPS –elements depth profile of molded glass lenses (SCHOTT - Ge28Sb12Se60)
after molding test at molding temperature of 305℃ and applied load of 800 N.
122
Table: 4-2 calculated results from the wetting test.
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4 SUMMARY
Well known as, glass molding process (GMP) has a potential tool to fabricate high
precision discrete optical components as smaller and smaller with high efficient optical
performance rather than conventional grinding/polishing routine over past decades. GMP has no
limitations to develop high precision optics. However, GMP has problems of its own. From past
few decades onwards, lots of research groups and industries are design and suggested various
protective coatings (Example: Pt-Ir, Mo-Ru, Ru-Ir, TiAlN, BN, CrN, DLC etc.) with suitable
glass material combinations. From previous reports, whatever the materials selected for
protective coating on mold, method suggested by researchers, how thick it is, how many multi
layers are there and even how best the selected coating material is, still the chemical reactions
are take’s placed by glass and service life of mold decreased. It is believed that, sticking is
caused by high reactivity of glass itself. So that, the present work focused on deposition of anti-
sticking coating on glass preforms through economical way to reduce the interaction of reactant
from the glass with mold surface. It is strongly believed that, protective coating on glass act as
barrier layer in between the glass and mold from final results of this study. The major
contribution of this work is deposition of protective thin film on glass preforms and produced
efficient mold lens through GMP. The best unique method for optical coating is sol-gel method.
A newly developed water based Al2O3 solution is selected as a protective layer for both mold
and glass preforms. Sol-gel method promotes the production of less expensive optical lens
rather than other methods.
This work presents the current understanding of wetting behavior of water based Al2O3
coatings that has been gained in laboratory testing, in-plant trials, and modeling in an economic
124
way. In-house developed water based sol-gel Al2O3 protective coating having good resistance to
chemical attack of reactant elements from molten glass. Single-coated and several superimposed
sol–gel Al2O3 coated mold and glass preform samples have been prepared and tested. The
morphology and surface roughness of the coated mold under controlled and satisfy the criteria of
the protective coating for mold.
First of all, wetting behavior of normal mold and glass was investigated at 800°C, the results
shows:
1. Holding at 800°C for 5 minutes holding period, the final contact angle and final contact
radius of the uncoated mold with molten glasses are 29° and 5.5mm respectively. Glass adheres
to mold permanently by external perturbations.
2. From EDX and mapping results, high affinity reactive elements, Cr and Fe enhanced
chemical reactions, resulting in the formation of Cr-O and Fe-O based compound layer at
interface. It has also observed that some of the reactive elements Si and Ba diffused out from the
molten glass into the substrate.
3. Gibbs free energy of each diffuse element redox reactions was calculated. It is observed
that barium oxide (BaO) contributed the adhesion or strong hermetic bonding through diffusion
at interface from glass side. The constituents of glass (i.e. network formers, network modifiers,
intermediates) are plays key role in sticking. High reactivity and unstable network modifiers
(BaO) enhanced chemical reactions at evaluated temperature. It concluded that by reduce or
avoid the BaO percentage in main glass composition, which discriminate the less tendency of
glass sticking with mold material.
4. The value of surface energy for uncoated mold has been obtained from the sessile drop
method is about 0.128 Nm-1 and the calculated work of adhesion is about 0.249 Nm-1. Spreading
125
coefficient has been obtained as -0.00856 Nm-1. The ratio of work of adhesion and cohesion is
much higher (98.73%), which is good evidence for effective chemical bonding through transfer
of the mass or forces.
5. The influence of ridge formation and oxidation of substrate on spreading kinetics were
discussed. The spreading kinetics depends on size of ridge at interface. The net weight of
oxidation increases with the increase of isothermal holding time and net weight of oxidation
(with oxide thickness 18 µm at 800°C, 1hr holding time) is linearly proportional to holding time.
It is concluded that, protective coating is essential for either mold or glass preform in order to
overcome the existed problems in GMP.
Wettability of derived protective coatings has been investigated by different approaches.
The results from each approach are:
Approach 1: Sol-gel SiO2 and Al2O3 coated mold with glass preform
1. The final contact angle and contact radius of Al2O3 coated mold with glass preform are
136° and 2.1mm and for SiO2 coated mold are 77° and 3.3mm respectively. The Al2O3 coated
mold demonstrated an excellent anti-sticking behavior as compared to SiO2 coated mold.
2. From EDX and mapping results, there are no active elements such as Zn and Ba, which
diffuses from glass, can be found on the glass contacted area after wetting test and glass ball still
remains fully transparent after contacting with the Al2O3 coated mold during the wetting test. In
case of SiO2 coated mold, there are a few Sb-rich particles can be observed on the glass ball
contacted area and weak Ni peak was detected from the slumped glass surface.
126
3. The values of surface energy, and ratios of work of adhesion and work of cohesion are
0.03 Nm-1 and 14.03 for Al2O3 coated mold. In case of SiO2 coated mold, the values are 0.077
Nm-1 and 61.24. Al2O3 coating is applicable for commercial GMP as a good anti-sticking coating.
4. Thus, it is possible for optical mold to replace physical vapor deposition (PVD) based
oxide coatings by water based sol-gel derived oxide coatings to extend its service life.
Approach 2: Mold with Al2O3 coated glass preform
1. The final contact angle and contact radius are as same as of Al2O3 coated mold with glass
preform. It is noticed that spreading kinetics of coated preform is well controlled and quickly
come to stable contact angle in short period of holding time.
2. From EDX and mapping results, no detectable chemical interaction occurred at the
interfaces between the Al2O3 coated glass ball and mold. Al2O3 film acted as an excellent
diffusion barrier which effectively inhibits the element diffusion and reaction between the mold
and glass.
3. Transparency of coated preform remains good enough after the wetting test.
Approach 3: Al2O3 Coated Glass on Al2O3 Coated Mold
The final contact angle of Al-Al interaction is 134° which is smaller than previous
approaches and is obtained quite faster and remains stable value during wetting test. No
fluctuations in dropped contact angles and a smooth stepped graph are achieved due to more
relaxation of similar materials with different surface geometrical contacts. It noticed that no need
to deposite protective coating on both mold and glass at same time from economical point of
view.
Glass Lens Fabrication from Coated Preform:
127
Successful desired optical lens are fabricated with Al2O3 coated glass preforms. Two type
of glass materials (L-BAL 42 and Chalcogenide glasses) are used in mold pressing by Toshiba
machine at 580°C. Al2O3 film is thermodynamic stable phase which can effectively hinder out
diffusion of elements such as Si, Zn, B and Ba at 580℃. Lens appears defectless within the
designed aspherical aperture when the thickness of Al2O3 film is 34 nm. From EDX analysis of
both molded lens and mold, no reactants from glass observed on mold, except carbon based
contaminants still exist on the Al2O3 coated glass surface which needs to be avoided.
From scratch test results, Al2O3 film composed by aggregated nanoparticles, which are able to
flow to accommodate a large amount of ploughing and associated shear stress through
densification and shear deformation.
Finally, Al2O3 coated mold and glass preforms demonstrated an excellent anti-sticking
behavior. The SEM/EDX analysis clearly showing a weak interaction or inter-diffusion between
the glass and Al2O3 coated mold. Thus, it is possible for optical mold to replace physical vapor
deposition (PVD) based oxide coatings by water based sol-gel derived oxide coatings to extend
its service life.
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5 FUTURE WORK
Rapid cooling process helps to regulate the sticking/adhesion problem of glass with molds. It is
necessary to design accurate the rapid cooling system for precision glass molding. As we know, glass is a
true super-cooled liquid. Deep molecular dynamic simulation needs to carry to know the effects of
interdiffusive elements and surrounding other polluted elements influence on sticking tendency and also
crack and rupture simulation of coating at edge of molded lens will be next step of our future work.
129
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