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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 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

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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

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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

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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

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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 Chienand 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

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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.

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Parts of this thesis were published in the following places:

¾ 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.

¾ 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.

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Dedicated to My Beloved Parents

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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

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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

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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

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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

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4.6.2.2 XPS analysis of molded Al2O3 coated lens………117

V. SUMMARY……….120

VI. FUTURE WORK……….125

VII. REFERENCES……….126

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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

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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…… …....

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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 22

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`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...

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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………

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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………...

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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………...

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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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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

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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

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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

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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

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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

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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.

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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.

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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

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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):

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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

¾ 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

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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

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-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

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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 adhesion

SiC O O O ×

hBN O × × Δ

ZrO2 O Δ × ×

C O × × Δ

GC O O O O

Ceramic molds are more suitable for optical elements processing with low porosity and

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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

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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:

¾ Surface qualities--no scratch, no particles

¾ Surface roughness (Ra) < 5 nm

¾ Film thickness uniformity < 5%

¾ No influence on the form accuracy of molds

¾ Service life of molds > 3000 times

2.5 Necessities and Requirements of Protective Coatings for Glass Preforms

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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:

¾ Surface qualities--no scratch, no particles

¾ Surface roughness (Ra) < 5 nm

¾ Film thickness < 30 nm

¾ Film thickness uniformity < 20 %

¾ With limited influence on the molding parameters

¾ 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.

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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].

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Table: 2-2 Mechanical and thermal properties of different coating materials [38].

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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

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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

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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.

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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

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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

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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.

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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].

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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].

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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].

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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].

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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].

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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].

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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

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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].

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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.

¾ 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

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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.

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Figure: 2-10 Representation of dip coating process. [68]

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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

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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.

ƒ Better homogeneity and purity from raw materials

ƒ Lower temperature of preparation:

• Save energy;

• Minimize evaporation losses;

• Minimize air pollution;

• No reactions with containers, thus purity;

• Bypass phase separation;

• Bypass crystallization.

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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.

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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

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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.

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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;

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