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Composite sol-gel alumina ceramic/siloxane coatings Li, Gesheng 2004

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COMPOSITE SOL-GEL ALUMINA CERAMIC/SILOXANE COATINGS  B Y G E S H E N G LI  M. E N G . , I N S T I T U T E O F M E T A L R E S E A R C H , C H I N E S E A C A D E M Y O F S C I E N C E , 1994  B.ENG., D E P A R T M E N T O F M A T E R I A L S C I E N C E , S H A N G H A I J I A O T O N G U N I V E R S I T Y , 1991  A T H E S I S S U B M I T T E D IN P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R OF A P P L I E D S C I E N C E in  T H E FACULTY OF GRADUATE STUDIES  DEPARTMENT OF METALS AND MATERIALS ENGINEERING  W e accept this thesis as conforming T o the required standards  T H E U N I V E R S I T Y OF BRITISH C O L U M B I A June 2004 © G e s h e n g Li, 2004  THE UNIVERSITY OF BRITISH COLUMBIA  FACULTY OF GRADUATE STUDIES  Library Authorization  In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  Gesheng Li  26/07/2004  Name of Author (please print)  Date (dd/mm/yyyy)  Title of Thesis:  Composite Sol-Gel Alumina Ceramic/Siloxane Coatings  .Degree:  Master of Applied Science  Department of  Metals and Materials Engineering  Year: 2£>€>H  The University of British Columbia Vancouver, BC Canada  grad. ubc.ca/forms/?form I D=THS  page 1 of 1  last updated: 26-Jul-04  ABSTRACT  T h i s thesis relates to the n o v e l , r e l a t i v e l y l o w temperature process ( 1 6 0 - 3 0 0 ° C ) f o r preparation o f c h e m i c a l l y bonded composite sol-gel ( C B - C S G ) "warm"  temperature  process  (160-300°C)  overcomes  some  coatings.  drawbacks  This of  the  conventional c o m p o s i t e sol-gel process, i n particular m i n i m i z a t i o n o f stresses due to differential t h e r m a l expansion coefficient between the coating and the substrate. The biggest c o n t r i b u t i o n o f this research i n v o l v e s use o f siloxane b o n d coat between the C B C S G coating and the m i l d steel substrate, w h i c h relaxes residual stress i n the coating, as w e l l as protects the substrate f r o m damage (corrosion) d u r i n g the c h e m i c a l b o n d i n g process.  A  throughout  method the  o f multi-gun  coating  during  c o n t r i b u t i o n o f t h i s Thesis.  the  spraying to chemical  uniformly  distribute  b o n d i n g process  is  the also  phosphates an  original  T h e resulting coatings are free o f surface cracks, have  m e d i u m hardness ( 6 . 0 G P a ) , moderate adhesion ( 4 2 . 0 M P a ) and g o o d scratch resistance ( 1 7 . 0 k g f ) . E l e c t r o c h e m i c a l analysis shows that the m u l t i - l a y e r c o a t i n g c o m p o s e d o f the siloxane b o n d coat and the C B - C S G " t o p coat" f o r m s a p h y s i c a l barrier against wet corrosion.  ii  TABLE OF CONTENTS ABSTRACT TABLE OF CONTENTS LIST O F F I G U R E S LIST O F T A B L E S NOMENCLATURE ACKNOWLEDGEMENTS  CHAPTER 1  CHAPTER 2  , 1  INTRODUCTION 1.1 Introduction to Sol-Gel  2  1.2 Advantages o f Sol-Gel Technique  3  1.3 Limitations o f Sol-Gel Technique  4  1.4 Poly(methylphenylsiloxane)  6  1.5 Objectives  6  LITERATURE REVIEW  8  2.1 Composite Sol-Gel Ceramic Coatings  8  2.2 Aqueous Sol-Gel Ceramic Coatings on Steel  11  2.3 Functionally Graded Bond Coat  12  2.4 Chemically Bonded Phosphate Ceramics  14  2.5 Residual Stresses in Multi-layer Coatings  15  2.6 Ceramic/polymer Composite Coatings  18  2.7 Adhesion o f Coatings  20  2.8 Chloride Corrosion  23  iii  CHAPTER 3  CHAPTER 4  2.9 Corrosion Protection by M u l t i - L a y e r Coatings  25  EXPERIMENTAL PROCEDURE  27  3.1 Substrate Preparation  ^  3.2 Deposition o f Coatings  27  3.3 L o w Temperature Process  29  3.5 Scratch Tests  30  3.5 Hardness Measurements  31  3.5 Adhesion Tests  33  3.6 A i r Permeability Measurements  34  3.7 " W a f e r " Curvature Radius Measurements  34  3.8 Microscopy and X R D  3  3.9 Potentiodynamic Evaluations o f the Coatings  J  5  R E S U L T S A N D DISCUSSION  38  4.1 Coating Structures  38  4.1.1 Group A Structure  38  4.1.2 Group B Structure  40  4.1.3 Group C Structure  42  4.2 Cracks and Air Permeability  44  4.2.1 Cracks  43  4.2.2 A i r - P e r m e a b i l i t y  49  4.3 Contact Measurements  51  4.3.1 Hardness o f CB-CSG Ceramic  51  4.3.2 Interface Fracture Toughness  56  iv  4.3.3 Nano Indentation and Sonic Wave Modulus  60  62  4.4 Scratch Resistance  CHAPTER 5  CHAPTER 6  4.4.1 Critical Scratching Force for Coatings  62  4.4.2 Scratch Hardness o f Coatings  66  4.5 Thermal Residual Stress  69  4.6 Adhesion  76  4.6.1 Group A Adhesion  76  4.6.2 Group B Adhesion  79  4.6.3 Group C Adhesion  81  4.7 Potentiodynamic Evaluation of Coating  83  CONCLUSIONS  87  5.1 Summary on Coating Properties  87  5.2 Summary on Adhesion  88  5.3 Summary on Contact Measurements  89  5.4 Summary on Residual Stress and Potentiodynamic Evaluation  90  5.5 Conclusions  91  93  FUTURE WORK  REFERENCES  95  Appendix I  Determination o f Y o u n g ' s Modulus by Sonic Waves  110  Appendix II  Original Data o f the Radius o f Curvature vs. Thickness  114  v  LIST OF FIGURES PAGE Fig. 1.1.  M o l e c u l a r structure o f P M P S siloxane  5  Fig.3.1  Schematic representations o f three basic m u l t i - l a y e r structures  28  Fig.3.2  Setting o f the p u l l - o f f tests  33  Fig.4.1.1  S E M image o f dual-layer coating  38  Fig.4.1.2  C e r a m i c / p o l y m e r composite layer between bond-coat and top-coat  39  Fig.4.1.3.  C B - C S G / s i l o x a n e / C B - C S G tri-layer structural c o a t i n g ( G r o u p B )  40  Fig.4.1.4  T r i - l a y e r C o a t i n g structure  41  Fig.4.1.5  Cross section image o f m o n o layer C B - C S G c o a t i n g  42  Fig.4.1.6  T h e phosphorus E D X m a p p i n g o f Fig.4.1.5  42  Fig.4.2.1  D r y i n g - s h r i n k a g e cracks  44  Fig.4.2.2  W e b - l i k e phosphate cracks  45  Fig.4.2.3  The cracks due to the m i s m a t c h o f C T E  46  Fig.4.2.4  T h e surface crack-free coating by 160°C 4 days c u r i n g  47  Fig.4.2.5  The h i g h m a g n i f i c a t i o n o f surface crack-free c o a t i n g  48  Fig.4.3.1.  S E M i n d e n t a t i o n trace o f C B - C S G surface, V i c k e r s i n d e n t 3 0 0 g l o a d  52  Fig.4.3.2  S E M i n d e n t a t i o n trace o n C B - C S G surface, 3 0 0 g load  53  Fig.4.3.3.  The S E M i n d e n t a t i o n trace o f C B - C S G ceramic c o a t i n g , V i c k e r s indent  54  300g load, Group C Fig.4.3.4  The S E M i n d e n t a t i o n trace o f C B - C S G ceramic c o a t i n g , V i c k e r s indent  55  300g load, Group C  vi  F i g . 4.3.5  I n d e n t a t i o n trace o f substrate-polymer-coating cross section, indented at  55  300g load, Group A Fig.4.3.6  M o d e l f o r the i d e n t i f i c a t i o n o f interface fracture toughness  57  Fig.4.3.7.  O p t i c a l images o f t y p i c a l coating detachment after R o c k w e l l C  58  indentation Fig.4.3.8  D e t a c h m e n t area o f Fig.4.3.7 by image analysis  58  Fig.4.3.9.  A p l o t o f substrate surface roughness and interface fracture toughness  59  Fig.4.4.1.  (a) O p t i c a l m i c r o g r a p h o f a scratch g r o o v e ; (b) curves o f n o r m a l force,  63  acoustics, effective f r i c t i o n and distance; Fig.4.4.2.  T h e scattering o f scratch critical force and c o a t i n g thickness ( m o n o - l a y e r  64  G r o u p C C B - C S G coating) Fig.4.4.3.  O p t i c a l m i c r o - g r a p h o f scratching made o n C B - C S G c o a t i n g , 1 0 0 X ,  66  scratch m o v i n g d i r e c t i o n f r o m right to left Fig.4.4.4.  Schematic o f cohesive failure and interfacial f a i l u r e  67  Fig.4.5.1  The C B - C S G thickness V s . the radius o f curvature  70  Fig.4.5.2.  A t y p i c a l interferometric result f r o m a C B - C S G c o a t i n g ( r = l .35m)  71  Fig.4.5.3  A t y p i c a l interferometric result f r o m a bare substrate ( r = l 7 . 2 4 m )  72  Fig.4.5.4  30|j,m C B - C S G coating, containing 5-lOu.m siloxane b o n d coat ( r = 1 4 . 4 7 )  73  Fig.4.5.5  The thickness o f siloxane b o n d coat vs. curvature radius  74  Fig.4.6.1.  Schematics o f fracture mechanisms i n different c o a t i n g structures  76  Fig.4.6.2.  Fracture surface o f G r o u p A after an adhesion strength test  77  Fig.4.6.3.  Appearance o f the fracture surface generated i n adhesion test  78  vii  Fig.4.6.4.  A n appearance o f fracture surface after a p u l l - o f f adhesion test  78  Fig.4.6.5.  Schematic o f fracture path f o r sample s h o w n i n Fig.4.6.4  79  Fig.4.6.6.  S E M image o f G r o u p B fracture surface after an adhesion strength test  80  Fig. 4.6.7  A crack was constrained i n one layer b y the siloxane B C  80  Fig.4.6.8.  S E M image o f G r o u p C fracture surface after a n adhesion strength test  81  F i g . 4.7.1  P o l a r i z a t i o n curves o f the m u l t i - l a y e r C S G / s i l o x a n e c o a t i n g ( G r o u p A - C )  84  o n the m i l d steel substrates compared w i t h that o f the bare m i l d steel (pH=5, D I H 0 ) 2  viii  LIST OF TABLES PAGE Table 4.2.1  T h e air p e r m e a b i l i t y o f various structures  50  Table.4.3.1  T h e average values o f hardness and m o d u l u s measured b y nano-indenter  61  Table 4.4.1  Scratch resistance o f various structures  62  Table 4.7.1  C o r r o s i o n p r o t e c t i o n efficiency o f different structure coatings  86  Table.5.1  The s u m m a r y o f characteristics o f various coatings  93  Table I I . 1  C B - C S G thickness vs. curvature radius f o r G r o u p C  114  Table II.2  Siloxane thickness vs. curvature radius f o r G r o u p A  115  ix  Nomenclature Abbreviations  ASMC  a l u m i n a r e i n f o r c e d siloxane m a t r i x composite  BC  b o n d coat  CB  chemically bond  CB-CSG  c h e m i c a l l y b o n d e d composite sol-gel  CR  curvature radius o r radius o f curvature  CSG  c o m p o s i t e sol-gel  CTE  c o e f f i c i e n t o f t h e r m a l expansion  CPC  c e r a m i c / p o l y m e r composite  D I water  d e - i o n i z e d water  DSI  depth sensing i n d e n t a t i o n  FEA  f i n i t e element analysis  FGBC  f u n c t i o n a l l y graded b o n d coat  MAP  m o n o a l u m i n u m phosphate  MMT  montmorillonite  MPS  methylphenylsiloxane  MS  m i l d steel  PA  p h o s p h o r i c acid  PCSG  phosphated composite sol-gel  PMMA  poly (methylmethacrylate)  PMPS  poly(methylphenylsiloxane)  PVD  physical vapor deposition  Ref.  reference  SAMC  siloxane m o d i f i e d a l u m i n a m a t r i x composite  SEM  scanning electron microscope  SG  sol-gel  SS  stainless steel  TBC  t h e r m a l barrier coating  UTS  u l t i m a t e tensile strength  Vs.  versus  XRD  x-ray diffraction  L a t i n Symbols  a  contact radius ( u r n )  A, B  e m p i r i c a l c o r r e c t i o n factors i n sonic w a v e f o r m o d u l u s d e t e r m i n a t i o n  b  scratch w i d t h ( u m )  d  density o f sample ( g / c m ) 3  D  s  density o f substrate ( g / c m )  D  sc  average density o f coating and substrate ( g / c m )  E  Y o u n g ' s m o d u l u s (GPa)  Ei  elastic m o d u l u s o f C B - C S G coating (GPa)  3  3  £2  elastic m o d u l u s o f substrate (GPa)  £j  average elastic m o d u l u s (GPa)  En  Y o u n g ' s m o d u l u s o f f u l l y dense coating (GPa)  Ec  Y o u n g ' s m o d u l u s o f coating (GPa)  Eb  elastic m o d u l u s o f C B - C S G coating (GPa)  Ed  d y n a m i c Y o n g ' s m o d u l u s (GPa)  E  elastic m o d u l u s o f siloxane (GPa)  E  elastic m o d u l u s o f substrate (GPa)  /  effective f r i c t i o n coefficient  C  p  s  F  normal load (N)  F  transverse force ( N )  G  shear m o d u l u s (GPa)  G  average shear m o d u l u s o f coating (GPa)  Ga-c  average shear m o d u l u s o f coating and substrate (GPa)  hi, h2  thickness o f c o a t i n g and substrate respectively ( u r n )  h , hp  thickness o f C B - C S G layer and siloxane respectively ( u r n )  H  V i c k e r s hardness (GPa)  h  thickness o f C B - C S G layer ( u r n )  hp  thickness o f siloxane ( u r n )  i cor, icor  c o r r o s i o n current densities o f the bare and coated substrates respectively ( A / m )  K  interface toughness ( M P a . M  Kuc  Interface fracture toughness (shear) ( M P a . m  N  (  c  c  c  c  1 / 2  ) )  xiii  k  constant o f h y d r o l y s i s , m o l a r ratio o f h y d r o l y s i s  L  dimension (m)  m  mass (g)  n  n u m b e r o f samples  N  resonant frequency ( H z )  P  p r o t e c t i o n e f f i c i e n c y (%)  P  p o r o s i t y o f c o a t i n g (%)  r  the radius o f curvature ( u m )  ri  the radius o f indent ( u m )  Ri  radius o f annular crack ( u m )  R  the average surface roughness o f substrate ( u m )  c  a  R  the radius o f curvature w i t h coating ( u m )  Rs  the radius o f curvature w i t h o u t coating ( u m )  Sh  scratch hardness (GPa)  /  thickness ( u m )  c  T  thickness o f c o a t i n g ( u m )  T.  total thickness o f coating and substrate ( u m )  Ts  thickness o f substrate ( u m )  w  w i d t h (\ivri)  Y  y i e l d strength ( M P a )  c  s c  xiv  GREEK SYMBOLS a  constant o f p o r o s i t y  8  deflection (urn)  K  difference o f curvature w i t h and w i t h o u t coating ( u m )  cf  t h e r m a l expansion m i s m a t c h stress ( M P a )  27  elastic m o d u l u s ratio  v  average Poisson's r a t i o n o f coating and substrate  Vi  Poisson's ratio o f substrate  V2  Poisson's ratio o f C B - C S G coating  Vc  Poisson's ratio o f coating  vs  Poisson's ratio o f substrate  Acknowledgements  I w o u l d l i k e t o express m y sincere gratitude t o m y supervisor D r . T o m T r o c z y n s k i (Professor o f Ceramics, U B C ) , group leader D r . Q u a n z u Y a n g , a n d other colleagues. M a n y thanks t o D r . H o w a r d H a w t h o r n e f o r his suggestions o n m y thesis. I w o u l d l i k e t o acknowledge the e x p e r i m e n t a l help f r o m D r . Y o n g s o n g X i e and D r . Z h a o l i n T a n g .  I w o u l d also l i k e t o a c k n o w l e d g e the financial support o f the N a t i o n a l Science and Engineering Research C o u n c i l o f Canada, N C E - A u t o 2 1 , and Datec C o a t i n g C o r p o r a t i o n . The N a t i o n a l Research C o u n c i l o f Canada is a c k n o w l e d g e d f o r the experimental and equipment support.  xiii  C H A P T E R 1:  INTRODUCTION  The processing and characterization o f ceramic coatings are i m p o r t a n t issues i n this research. F o r e x a m p l e , the space-shuttle  Columbia disaster happened o n February 1,  2003. It is believed that the left w i n g o f the shuttle was scratched b y a piece o f f o a m that f e l l f r o m the external tank d u r i n g l a u n c h , w h i c h destroyed the heat-resistant ceramic coating o n the outer b o d y and a l l o w e d the surface temperature to exceed its c r i t i c a l value u p o n re-entry.  I f the c o a t i n g failure was an interfacial f a i l u r e , i t c o u l d be labeled as a  m a n u f a c t u r i n g p r o b l e m . I f the c o a t i n g failure was a cohesive f a i l u r e , h o w e v e r , it c o u l d be asserted to be a p r o b l e m related t o coating design or to the c o a t i n g m a t e r i a l itself.  I n this research, a n o v e l sol-gel processing m e t h o d was d e v e l o p e d t o produce a composite sol-gel c o a t i n g at " w a r m " temperatures ( 1 0 0 - 3 0 0 ° C ) . T h e average hardness o f the coating is - 6 . 0 G P a , the average adhesion strength is ~ 4 2 . 0 M P a , and the average scratch c r i t i c a l force is - 1 7 . 0 kgf. The " w a r m " temperature process successfully produces coatings free o f surface cracks. A l u m i n u m phosphate is used as the b i n d i n g phase i n the sol-gel coating. T o o b t a i n suitable strength, it is necessary to h o m o g e n e o u s l y distribute the phosphate t h r o u g h o u t the coating t h r o u g h a m e t h o d o f m u l t i - g u n spraying. A siloxane b o n d coat was d e v e l o p e d t o reduce the residual stresses due t o the d i f f e r e n t i a l t h e r m a l expansion o f the ceramic coating and steel substrate, w h i c h decreases the d e f o r m a t i o n o f the substrate.  A n o t h e r m e r i t o f this research is that i t provides a m e t h o d to produce a  m u l t i - l a y e r ceramic/siloxane coating o n m i l d steel ( M S ) w i t h o u t c o a t i n g b u c k l i n g .  1  1.1 Introduction to Sol-Gel  A  sol is a d i s p e r s i o n o f s o l i d particles ( - 0 . 1 - 1 u m ) i n a l i q u i d w h e r e  only  B r o w n i a n m o t i o n suspends the particles. A gel is a state i n w h i c h the l i q u i d phase is dispersed  i n the  solid  and vice-versa,  yielding  a solid  network  containing  liquid  components. T h e sol-gel c o a t i n g process usually consists o f f o u r steps: (1) T h e desired c o l l o i d a l particles are dispersed i n a l i q u i d to f o r m a sol. (2) T h e sol is deposited to produce coatings o n the substrates b y spraying, d i p p i n g or s p i n n i n g . (3) T h e particles i n the sol are p o l y m e r i z e d t h r o u g h the r e m o v a l  o f the s t a b i l i z i n g c o m p o n e n t s ,  which  produces a g e l . ( 4 ) T h e f i n a l heat treatments p y r o l y z e the r e m a i n i n g o r g a n i c o r i n o r g a n i c components and f o r m an a m o r p h o u s or crystalline coating [ 1 - 4 ] .  There are t w o distinct reactions i n the sol-gel process: h y d r o l y s i s o f a l c o h o l groups and condensation o f the resulting h y d r o x y l groups. For instance, the i s o m o r p h o u s y - A l O O H precursor exists as the u n - h y d r o l y z e d species [ A l ( O H 2 ) 6 ]  3+  b e l o w p H 3, and  can be h y d r o l y z e d e x t e n s i v e l y w i t h increasing p H [ 1 ] :  [Al(OH ) f  +kH 0^[Al(OH) {OH ) _ f-  +  2  k)+  6  2  kH 0 +kOH +  3  k  2 6  k  ~^2kH 0 2  +kH,0  +  (1.1.1) (1.1.2)  where k is d e f i n e d as the m o l a r ratio o f h y d r o l y s i s .  2  I t is generally agreed that the preferred condensation sites are those that m a x i m i z e interactions b e t w e e n lone p a i r electrons o n a b o u n d h y d r o x i d e l i g a n d o n one a l u m i n u m species w i t h a p r o t o n o n a w a t e r - m o l e c u l e b o u n d to another a l u m i n u m species [ 1 ] . F o r example, t w o s i n g l y - h y d r o l y z e d m o n o m e r s (k=\)  condense t o a d i m e r v i a an o l a t i o n  reaction [ 1 ] :  2[Al(H 0f 2  However,  + 6  -H  +  - > Al{H 0) OH ]-2H 0  - > Al {OH) (H 0)\  2+  2  the condensation  5  +  2  mechanism  2  o f aluminum  2  2  isopropoxide  (1.1.3)  is m o r e  c o m p l e x than the above description, and the h y d r o l y s i s o f the a l u m i n u m a l k o x i d e s is not very w e l l understood [ 1 ] .  1.2 Advantages of the Composite Sol-Gel Technique  Some n o n - h y d r a t e d f i l l e r s can be added into the sol-gel slurry t o decrease the shrinkage strain d u r i n g gelation, p r o d u c i n g w h a t is called a c o m p o s i t e sol-gel ( C S G ) . T h e composite sol-gel is a k i n d o f " c e r a m i c p a i n t " , and adheres easily t o v a r i o u s m e t a l l i c substrates l i k e m i l d steel, stainless steel, a l u m i n u m , n i c k e l , and copper. T h e m o n o a l u m i n u m phosphate can be used as the binder i n this paint. I t f o r m s a chemical b o n d w i t h almost e v e r y t h i n g that can stand the processing temperature  [ 3 ] . Because the  phosphate is used as a c h e m i c a l b o n d i n g phase, a " c h e m i c a l l y b o n d e d c o m p o s i t e s o l - g e l " ( C B - C S G ) c o a t i n g is p r o d u c e d .  3  The material i n the gel state can be easily shaped into c o m p l e x geometries, e.g. tube and w h e e l , b y s i m p l e a i r - s p r a y i n g , d i p p i n g or s p i n n i n g . A n o t h e r advantage is that h i g h p u r i t y products c a n be produced. Because the o r g a n o - m e t a l l i c precursor o f the desired ceramic oxides can be m i x e d , dissolved i n a specified solvent, and h y d r o l y z e d into a sol and subsequently a g e l , the c o m p o s i t i o n is h i g h l y c o n t r o l l a b l e . I t can be sintered at l o w temperatures, usually around 4 0 0 - 7 0 0 ° C . G e n e r a l l y , the c o m p o s i t e sol-gel technique p r o v i d e s a s i m p l e , inexpensive, and effective m e t h o d o f p r o d u c i n g h i g h q u a l i t y coatings [ 5 ] .  1.3 Limitations of Sol-Gel Techniques  There are some l i m i t a t i o n s to the sol-gel technique, e.g. w e a k b o n d i n g , porosity. I n particular, the c o a t i n g thickness is l i m i t e d since the m i s m a t c h o f the c o e f f i c i e n t o f thermal expansion ( C T E ) increases w i t h temperature. H i g h e r temperature processing or service often generates stresses that increase w i t h coating thickness. W h e n the residual stresses exceed the adhesion strength, the coating w i l l break u p . P a r t i c u l a r l y , the residual stresses i n c o m p l e x geometric locations, e.g. edges and corners, are m o r e severe than those i n a flat shape, w h i c h also l i m i t s w i d e a p p l i c a t i o n o f the sol-gel technique.  A q u e o u s sol-gel processing t y p i c a l l y proceeds i n an acidic e n v i r o n m e n t o f p H = 2-4, w h i c h can corrode m i l d steel. A conventional w a y to address this issue is to coat the substrate w i t h zinc or i r o n phosphate.  H o w e v e r , the r e s u l t i n g t h i n (less than a f e w  m i c r o n s ) and u s u a l l y m i c r o - p o r o u s phosphate f i l m s are unstable at the elevated process  4  temperature ( > 4 0 0 ° C ) needed f o r c h e m i c a l b o n d i n g . T h i s can lead to the creation o f additional interfacial p o r o s i t y , and w h i c h a l l o w s access o f c o r r o s i v e species to the steel surface.  Such a coating system does not p r o v i d e l o n g - t e r m c o r r o s i o n p r o t e c t i o n f o r the  m i l d steel.  Therefore, a n o n - p o r o u s and t h e r m a l l y stable " b o n d - c o a t " o f a l u m i n a reinforced siloxane m a t r i x composite ( A S M C ) f i l m has been developed i n this w o r k to coat the m i l d steel  surface.  This  should  protect  the  substrate  during  CB-CSG  processing  at  temperatures b e t w e e n 1 0 0 - 4 0 0 ° C as w e l l as p r o v i d i n g a siloxane m e m b r a n e f o r c o r r o s i o n protection i n storage and service [ 5 ] . I n other w o r d s , w h e n the siloxane is incorporated, the aqueous sol can be deposited o n the surface o f m i l d steel and heat treated at 100400°C.  I n order t o o v e r c o m e the l i m i t a t i o n s o f t r a d i t i o n a l C B - C S G coatings, e.g. h i g h residual stresses, l o w wear-resistance, and cracks, a " w a r m " temperature process has been developed. I n this system, a siloxane composite b o n d coat reduces the residual stresses b y g r a d i n g the t h e r m a l expansion.  CH  "I  3  Fig.l.l. Molecular structure of PMPS siloxane [6]  5  1.4 Poly(methylphenylsiloxane)  There are several reasons t o choose p o l y ( m e t h y l p h e n y l s i l o x a n e ) , P M P S , as a p o l y m e r component i n the m u l t i l a y e r coating. P M P S has a r e l a t i v e l y h i g h  service  temperature ( n o m i n a l : 4 8 0 ° C , Cotronics Inc. U S ) due t o its m o l e c u l a r structure. I t is an inorganic p o l y m e r w i t h no carbon atoms i n the backbone and h i g h l y cross-linked chains o f alternating s i l i c o n a n d o x y g e n atoms ( F i g . 1.1), b y means o f w h i c h i t c a n stand a relatively h i g h temperature [ 6 , 7 ] .  1.5 Objectives  The c o n v e n t i o n a l  c h e m i c a l l y b o n d e d sol-gel process includes an i n d i v i d u a l  deposition o f slurry and phosphate and f i r i n g at 4 0 0 - 9 0 0 ° C after each deposition. Therefore, i t was d i f f i c u l t  f o r such a process t o evenly  d i s t r i b u t e t h e phosphate  throughout the coating. M o r e o v e r , cracks were often observed i n t h e c o n v e n t i o n a l m e t h o d [ 5 , 8 ] . Therefore, the f i r s t objective w a s to develop a l o w - t e m p e r a t u r e process (160-300°C)  f o r the p r o d u c t i o n o f a crack-free coating w i t h phosphate  distributed  homogeneously t h r o u g h o u t .  I t was d i f f i c u l t t o deposit the aqueous slurry o n m i l d steel w i t h o u t b u c k l i n g o f the coating [ 9 ] . T h e second objective was thus to develop a siloxane b o n d coat f o r the m i l d  6  steel substrate to prevent this b u c k l i n g . The t h i r d objective was to characterize the C B C S G coating and its siloxane b o n d coat t h r o u g h i n d e n t a t i o n tests, scratch tests, adhesion tests, and electrochemical analysis.  Since the t h i n steel substrate o f t e n suffers d e f o r m a t i o n due t o residual stresses, it was believed that the siloxane b o n d coat c o u l d reduce these b y g r a d i n g the t h e r m a l expansion. Therefore, the f o u r t h objective was to gather evidence as t o w h e t h e r or not the siloxane b o n d coat reduces the residual stresses due to the d i f f e r e n t i a l t h e r m a l expansion o f coating and substrate.  7  CHAPTER 2: LITERATURE REVIEW  2.1 Composite Sol-Gel Ceramic Coatings  The sol-gel process has been successful i n the p r o d u c t i o n o f h a r d , erosion resistant and c o r r o s i o n resistant ceramic coatings and structural ceramics (e.g. AI2O3, Z r O i ) [ 1 - 5 ] . H o w e v e r , the sol-gel f i l m s have tended t o crack i f they are thicker than several m i c r o n s [ 3 ] . C a l c i n e d ceramic particulates dispersed i n a sol-gel m a t r i x produce a composite sol-gel ( C S G ) coating. T h i s C S G technique avoids the large shrinkage strain o f the sol-gel f i l m s f o l l o w i n g heat treatment and d e n s i f i c a t i o n [ 9 ] . T h e t h i c k e r coatings (up t o - 5 0 0 u.m) do n o t crack u p o n d r y i n g because the gel phase contains u p to about 8 0 v o l % o f ceramic f i l l e r . A n added advantage o f the c h e m i c a l l y b o n d e d composite s o l gel  (CB-CSG)  is the p o s s i b i l i t y  o f controlling  its strength  by  simple  chemical  reactions [ 5 , 8, 10, 1 1 ] .  I t was b e l i e v e d that the composite sol-gel ( C S G ) coatings still needed a r e l a t i v e l y h i g h temperature, t y p i c a l l y i n excess o f 6 0 0 ° C , t o gain e n o u g h strength and hardness, and to eliminate the p o r o s i t y due t o structural collapse [ 5 , 10]. F o r m o s t m e t a l l i c substrates o f interest, i n c l u d i n g steel, a l u m i n u m and m a g n e s i u m alloys, the m a x i m u m  curing  temperature m u s t be, h o w e v e r , b e l o w about 6 0 0 ° C . T h i s p r o b l e m c a n be avoided i f the d i f f u s i o n a l process (e.g. sintering) is replaced w i t h a c h e m i c a l b o n d i n g process [ 3 ] .  8  I n recent  research, a p a i n t - l i k e  aqueous  sol c o n t a i n i n g the  ceramic  oxide  precursors and inert f i l l e r s has been deposited using a pressurized air-spray g u n , f o l l o w e d by phosphating and c u r i n g at 4 0 0 - 9 0 0 ° C [ 8 ] . T h i s is the composite sol-gel ceramic technique.  T h i s technique includes dispersing fine ceramic p o w d e r s i n sol-gel s o l u t i o n , spraying the paint onto a substrate, and f i n a l l y f i r i n g at 400-900°C [ 1 2 ] . T h e technique has successfully p r o d u c e d 2 5 - 2 0 0 u m coatings [ 4 ] , i f phosphoric acid ( P A ) is used to initiate the f o r m a t i o n o f a phosphated a l u m i n a sol-gel coating b y subsequent heat treatment at 400-600°C.  It is generally agreed that the l i q u i d m o n o a l u m i n u m phosphate ( M A P ) is the best binder, w h i c h u p o n heat treatment is eventually converted to three phases, B e r l i n i t e and Cristobalite f o r m s o f A I P O 4  or Variscite, A K J H b P O ^ . The average hardness o f a C B -  C S G coating is about 4.0 G P a [ 1 3 ] , w h i l e the m a x i m u m hardness o f C B - C S G coating is reported to be ~ H V i o 6.47+1.44 G P a [ 6 ] . Sometimes, there were some h i g h hardness i n g  isolated areas (~8.0GPa) o n the C B - C S G coatings, usually w i t h i n smoother and shinier regions [ 4 0 ] . C h u r c h and K u n t s o n impregnated 8 5 w t % P A w i t h l o w concentrations o f some other inorganic chemicals, such as Z r C h , TiC>2, and Cr203, into the a l u m i n a m a t r i x to obtain 7.0-10.0  G P a hardness  [ 1 0 ] . The p u l l - o f f adhesion was reported to be  - 4 2 M P a [ 1 4 ] . I t is v e r y susceptible to substrate interactions [ 1 2 , 14], w h i c h included mechanical interlocks and c h e m i c a l bonds at the interface, i n d i c a t i n g an o p p o r t u n i t y to increase the adhesion further.  9  The c o n v e n t i o n a l composite sol-gel process is an i n d i v i d u a l d e p o s i t i o n m e t h o d that involves the p o l y m e r i z a t i o n o f a l u m i n u m i s o p r o p o x i d e t h r o u g h the r e m o v a l  of  stabilizing organic components. T h e coating is dried at 7 0 ° C to gel the layer and f i r e d at 4 0 0 - 9 0 0 ° C to p y r o l y z e the r e m a i n i n g organic components, w h i c h f o r m s an amorphous or crystalline layer [ 4 ] . T h e f o l l o w i n g is a well-established process f o r p r o d u c i n g composite sol-gel a l u m i n a coatings [ 3 ] :  1.  Heat at 5 5 0 ° C to f o r m a t h i n o x i d i z e d f i l m o n the sand-blasted M S substrate.  2.  Spray a 10-60 u m composite sol-gel a l u m i n a layer o n the substrate.  3.  Heat at 5 5 0 ° C f o r 20 minutes.  4.  Spray composite sol to the desired thickness.  5.  Heat at 5 5 0 ° C f o r 10 minutes.  6.  B r u s h w i t h 8 5 % p h o s p h o r i c acid.  7.  Heat at 2 5 0 ° C f o r 20 m i n u t e s and at 6 0 0 ° C f o r 20 minutes.  There are some inevitable drawbacks to this process. First, w h e n the phosphoric acid ( P A ) is o v e r - i m p r e g n a t e d o n the surface o f green C S G . I t f o r m s pseudoboehmite or gelatinous b o e h m i t e , w h i c h has a l o w strength [3, 4, 12], and produces s o l i d a l u m i n u m phosphate at r o o m temperature, w h i c h often induces c r a c k i n g . Second, the phosphate needs to reach the interface and f o r m a chemical b o n d b e t w e e n the coating and the substrate, but it is d i f f i c u l t f o r the i n d i v i d u a l l y deposited phosphate t o penetrate t h r o u g h a C S G layer m o r e than 20 u m t h i c k . T h i r d , the r e l a t i v e l y h i g h temperature, 5 5 0 - 6 0 0 ° C ,  10  sometimes affects the properties o f the substrate, e.g. A l o r M g . I t is probable that phase transformations occur i n the substrate. F i n a l l y , this process uses c h e m i c a l reactions at 550-600°C [ 3 ] :  Al 0 +2H PO, 2  3  -*  3  Al(H PO,) 2  4  3  2  3  2  (2.2)  <=> 2H PO, + AlPO,  3  3  Al(H P0 ) +Al 0, 2  (2.1)  2Al(H PO,) +H 0  2  (2.3)  <=>3AlPO, +3H 0 2  These reactions are related t o the c o m p l i c a t e d t r a n s f o r m a t i o n s o f various phases, w h i c h often generate large mismatches i n thermal expansion d u r i n g c o o l - d o w n f r o m 5506 0 0 ° C . Therefore, i f the c o n v e n t i o n a l m e t h o d is f o l l o w e d , cracks are inevitable.  A  viable  w a y t o produce  crack-free  coatings  is t o decrease  the process  temperature. U n f o r t u n a t e l y , there is no report o n a " w a r m " temperature ( < 2 5 0 ° C ) v e r s i o n o f the composite sol-gel a l u m i n a c o a t i n g process.  2.2 Aqueous Sol-Gel Ceramic Coatings on Steel  M i l d steel is v e r y d i f f i c u l t t o coat w i t h the acidic aqueous sol-gel slurry due t o its easy corrosion d u r i n g heat treatment i n air. T h e c o n v e n t i o n a l w a y t o t r y t o prevent this is as f o l l o w s :  ( 1 ) deposit the sol-gel d e r i v e d coating o n phosphatized  (zinc o r i r o n  11  phosphate) m i l d steel; ( 2 ) heat-treat i t i n an inert atmosphere; and ( 3 ) cover i t w i t h epoxy resin after heat treatment d u r i n g storage and service [ 1 5 ] .  T h i s m e t h o d has some disadvantages: ( 1 ) the treatment i n inert  atmosphere  requires c o m p l e x and expensive equipment [ 1 5 , 1 6 ] ; ( 2 ) the c o m m o n l y - u s e d phosphate layer cannot survive b e y o n d 3 0 0 ° C ; and (3) the e p o x y resin o n the surface cannot resist h i g h temperatures and possesses l o w scratch resistance.  Sometimes the sol-gel coating o n M S needs to be heat treated i n air, w h i c h requires an i n h i b i t o r - d o p e d s o l , b u t these i n h i b i t o r s often decrease the adhesion strength o f the coatings [ 1 7 ] . T h e r e f o r e , to deposit aqueous sol-gel c o a t i n g o n M S , the biggest challenge is t o o v e r c o m e the w e a k c o r r o s i o n resistance o f M S , a n d to produce a coating w i t h h i g h adhesion strength and g o o d resistance to b u c k l i n g .  2.3 Functionally Graded Bond Coat  T o address the p r o b l e m o f w e a k b o n d i n g , usually a b o n d coat ( B C ) is used to increase the adhesion strength o f a coating system. F o r e x a m p l e , the glue u t i l i z e d i n dental c r o w n s is a g o o d e x a m p l e o f a B C , i n this case a h i g h l y adhesive p o l y m e r connecting the t o p p o r c e l a i n layer and the t o o t h substrate. A n o t h e r e x a m p l e o f B C is the zinc phosphate used to b o n d an aluminosilicate top-layer o n an M S substrate [ 1 5 ] .  12  H o w e v e r , i n m o s t cases, the v a r i a t i o n i n c o m p o s i t i o n b e t w e e n the other layers and the B C is always the m a i n reason f o r a large m i s m a t c h i n t h e r m a l expansion. I n other w o r d s , the t r a d i t i o n a l  ceramic/metal  system c o m m o n l y  suffers  failures due to  the  excessive residual stresses generated d u r i n g heating and c o o l i n g . T h e residual stresses have a substantial effect  o n the coating properties, w h i c h can g i v e rise t o  either  d e f o r m a t i o n o f the substrate or b u c k l i n g o f the coating [ 1 8 - 2 1 ] .  T h u s , people have t r i e d t o use a f u n c t i o n a l l y graded b o n d coat ( F G B C ) , a B C w i t h a gradual v a r i a t i o n o f c o m p o s i t i o n , t o solve the p r o b l e m . T h e f i r s t concept o f F G B C was proposed to w i t h s t a n d the severe stress o f the C T E m i s m a t c h i n ceramic engines. T h e unique idea o f F G B C was to p r o v i d e a composite coating, w h e r e the c o a t i n g c o m p o s i t i o n varies gradually f r o m c o a t i n g to substrate [ 2 3 ] .  T h e literature [ 2 1 - 2 8 ] s h o w e d that F G B C s c o u l d grade the t h e r m a l expansion, and thus  reduce  the  residual  stresses  following  cool-down  from  a high  temperature.  Therefore, the F G B C has been an indispensable constituent f o r t h i c k coating systems to reduce the residual stresses [ 2 8 - 3 3 ] .  A p o l y m e r s h o u l d be an ideal candidate f o r the b o n d coat, since p o l y m e r s have a lot o f advantages, e.g. excellent resilience, g o o d adhesion, g o o d c o r r o s i o n resistance, and fluid-impermeability.  Unfortunately,  there has not been any  report  o n the  sol-gel  ceramic/siloxane F G B C , because most p o l y m e r s b e g i n to degrade above 150°C, w h i l e  13  the c o n v e n t i o n a l sol-gel process temperature is often > 4 0 0 ° C . T h e r e f o r e , there has not been very m u c h research o n the p o l y m e r / c e r a m i c sol-gel process.  2.4 Chemically Bonded Phosphate Ceramics  It  is  technologically  important  to  fabricate  alumina  ceramics  at  "warm"  temperatures ( < 3 0 0 ° C ) . T h e phosphate sol-gel a l u m i n a has a l o w f o r m a t i o n temperature o f ~ 1 5 0 ° C , at w h i c h the ceramic is l i k e l y to be stress-free. M o r e o v e r , the phosphate a l u m i n a ceramic ( B e r l i n i t e ) has a h i g h compressive strength o f ~ 1 1 0 M P a . T h e m o n o a l u m i n u m phosphate can c h e m i c a l l y b o n d to m a n y materials [ 3 4 ] , except some organic polymers.  K i n g e r y f i r s t used the phosphoric acid to b o n d a l u m i n a at 2 5 0 - 3 0 0 ° C [ 3 5 ] . Later, m o n o a l u m i n u m phosphate ( M A P ) hydrates were p r o d u c e d b y the reaction between a l u m i n a and o r t h o p h o s p h o r i c acid at 1 0 0 - 1 5 0 ° C . M A P can c o n v e r t to B e r l i n i t e above 150°C as a b i n d i n g phase [ 3 6 - 3 8 ] . A w e l l - c r y s t a l l i z e d AIPO4 ( B e r l i n i t e ) w a s even synthesized at 150°C b y r e a c t i n g boehmite hardness  values  of  Berlinite  and  and p h o s p h o r i c  Variscite  are  ~6.5  acid GPa  [37, 38-42]. The and  -4.0  GPa  respectively [ 4 3 - 4 6 ] . I t is generally agreed that the l i q u i d m o n o a l u m i n u m phosphate ( M A P ) is converted to three phases u p o n heat treatment: B e r l i n i t e , C r i s t o b a l i t e f o r m s o f AIPO4, or V a r i s c i t e , A1(H2P04)3 [ 3 6 ] . O n l y the B e r l i n i t e w a s f o r m e d w h e n the M A P was cured f o r 4 days at ~ 1 5 0 ° C  [ 3 4 - 3 6 ] . T h e B e r l i n i t e w a s f o r m e d b y the f o l l o w i n g  reaction [ 3 4 ] :  14  Al 0 2  3  + 2AIH (P0,) 3  2  • H 0 -> AAIPO, +4H 0 2  (2.4)  2  B e r l i n i t e is the phase that bonds i n d i v i d u a l particles a n d f o r m s the B e r l i n i t e bonded  alumina  ceramic  [34]. The  curing  time  decreases  with  increasing  temperature [ 4 0 - 4 3 ] . T h e m o n o l i t h i c a l u m i n a gels can be f o r m e d b y h y d r o l y s i s a n d condensation o f a l u m i n u m a l k o x i d e , a n d the phosphorus acts as the " b r i d g e " i n the n e t w o r k o f the phosphate ceramic [ 3 9 ] .  F r o m the earlier l i m i t e d research [ 4 4 - 4 9 ] , i t is seen t o be possible t o produce a w e l l - c r y s t a l l i z e d c o m p o s i t e sol-gel ceramic coating at ~ 1 6 0 ° C , w h i c h has h i g h adhesion strength, h i g h compressive strength, h i g h hardness, and even h i g h m o d u l u s .  2.5 Residual Stresses in Multi-layer Coatings  The m o s t suitable m e t h o d t o determine residual stresses is f i n i t e element analysis ( F E A ) , w h i c h c a n q u a n t i t a t i v e l y i d e n t i f y the stress d i s t r i b u t i o n i n c o m p l e x m u l t i - l a y e r structures. H o w e v e r , as a so-called " c l o s e d - f o r m analytical m e t h o d , " F E A i s l i m i t e d t o case-by-case studies [ 5 0 - 5 6 ] . Lattice structure measurement techniques, such as X - r a y d i f f r a c t i o n , are d i f f i c u l t t o i m p l e m e n t . A viable stress " i n d i c a t o r " is the curvature radius ( C R ) o f a coating system [ 1 3 ] using the so-called " w a f e r " m e t h o d . F o r e x a m p l e , a " w a f e r " curvature m e t h o d has been used to measure the residual stress ( 1 4 0 M P a ) i n an a l u m i n i d e b o n d coat (elastic m o d u l u s « 1 lOGPa) [ 1 5 ] .  15  The residual stresses i n the C B - C S G coating can be related to the f o l l o w i n g : (1) the stress due t o  c h e m i c a l reactions; ( 2 ) phase t r a n s f o r m a t i o n - i n d u c e d  stress;  (3)  geometrically i n d u c e d stress, e.g. at edges and corners; (4) t h e r m a l e x p a n s i o n coefficient ( C T E ) m i s m a t c h - i n d u c e d stress; ( 5 ) the external service stress. T h i s research is focused o n the C T E m i s m a t c h - i n d u c e d stress.  W h e n the C T E o f the substrate is smaller than that o f the c o a t i n g , the c o a t i n g is i n tension. W h e n the C T E o f substrate is larger than that o f c o a t i n g , the c o a t i n g is i n a state o f compression [ 8 , 32, 57, 5 8 ] . T h e b e n d i n g m o m e n t or the residual stress increases w i t h the increment o f c o a t i n g thickness, and correspondingly the curvature radius ( C R ) o f the coating decreases w i t h the increase o f bending force [23, 3 3 ] . F o r the same thickness, the residual stress o f a siloxane b o n d coating should be smaller t h a n that o f a 1 0 0 % C B - C S G coating, since the visco-elastic siloxane relaxes the stresses m o r e easily than brittle ceramics [ 1 2 ] .  The curvature radius ( C R ) can p r o v i d e a context f o r analysis o f the relationship between the stress and the c o a t i n g thickness. The salient characteristics are as f o l l o w s : the b e n d i n g m o m e n t increases w i t h the increment o f the c o a t i n g thickness, and tends to approach a f i x e d value, w h i l e the curvature radius o f the c o a t i n g decreases  with  m o n o t o n i c increase o f the c o a t i n g thickness [ 5 8 ] .  16  W h e n the thickness and elastic m o d u l u s o f the b o n d coat w e r e m u c h smaller than those o f the C B - C S G t o p layer, the C B - C S G / s i l o x a n e c o u l d be regarded as a single layer system i n order t o use t h e b i - m a t e r i a l m o d e l f o r calculating t h e residual stress. T h u s , the thermal m i s m a t c h stress f a ) i n the coating w a s calculated a c c o r d i n g t o the equation 7  b e l o w (2.5) [ 3 2 ] :  6£ (l  +  2  —  K =  £)(l-v,)o-  T  —  E h, [(zZ{ -l) +4zZZ(l 2  2  2  (2.5)  —  2  +  {) ] 2  Where K is the difference o f curvature w i t h and w i t h o u t c o a t i n g ( — - — ) respectively  hi and h2 are the thickness o f coating and substrate respectively, g=hi/h2 E2 and V2 are substrate elastic m o d u l u s and Poisson's ratio o f substrate respectively Ei and v\ are C B - C S G elastic m o d u l u s and Poisson's ratio o f c o a t i n g respectively  The d e f l e c t i o n 8 o f equation (2.5) is 8 =  KD / 8 [ 3 2 ] , w h e r e D is the substrate 2  diameter or length.  Elastic m o d u l u s ratio o f coating t o that o f substrate, 27 = - ^ ' ^  V  E /(l-v ) 2  ' ^  2  The average elastic m o d u l u s (Es) o f the dual layer c o a t i n g was estimated b y ,  17  (2.6)  [59-61]  h and h are the thickness o f the CB-CSG layer and the siloxane layer respectively c  p  E and E are the elastic m o d u l u s o f the CB-CSG layer and the siloxane layer respectively c  p  There have been m a n y debates over the accurate d e t e r m i n a t i o n o f the residual stress i n a b i l a y e r system. E q u a t i o n (2.5) was b u i l t u p o n a " f u l l y elastic" m o d e l [ 3 2 ] , w h i c h is also dependent o n the relative thickness h\/h2 and the relative m o d u l u s ratio Z. H o w e v e r , this equation c a n still help i n understanding the rationale o f the sol-gel process.  2.6 Ceramic/polymer Composite Coatings  H i s t o r i c a l l y , i t w a s b e l i e v e d that p o l y m e r s a n d ceramics w e r e irrelevant t o each other because p o l y m e r s  were m a i n l y used f o r l o w temperature  applications  while  ceramics were often used f o r h i g h temperature applications because o f their i n t r i n s i c a l l y strong  bonds.  ceramic/polymer polymer/ceramic  Consequently,  there  composite  system,  has n o t been except  some  very  much  studies  of  research low  o n the  temperature  composites, since the difference i n the processing temperatures o f  ceramics and p o l y m e r s is t o o large.  W i t h the d e v e l o p m e n t o f h i g h temperature p o l y m e r s and the l o w - t e m p e r a t u r e processing o f ceramics, e.g. the c h e m i c a l l y b o n d e d sol-gel technique, i t is n o w possible to heat-treat p o l y m e r s  a n d ceramics together and t o develop the c e r a m i c / p o l y m e r  18  composite system, w h i c h m a y p r o m i s e a c o m b i n a t i o n o f the advantages o f b o t h p o l y m e r s and ceramics, e.g. g o o d elasticity and h i g h hardness.  Recently,  ceramic/polymer  composite  (CPC)  coatings  have  evoked  intense  research interests due to their unique characteristics [ 6 0 - 8 3 ] , e.g. t h e r m a l barriers [ 6 6 ] , c o m p l i a n t layers [ 5 , 8 1 ] , mechanical strength [ 8 2 ] , stress tolerance [ 5 4 ] , m o l e c u l a r barriers [ 8 2 ] and f l a m e retardant properties [ 8 3 ] . The d e v e l o p m e n t o f C P C coatings can be tracked back to the research done b y T o y o t a i n 1990 [ 7 9 , 8 1 ] . CPCs were c o m m o n l y a sort o f l o w - l o a d ceramic r e i n f o r c e d p o l y m e r m a t r i x c o m p o s i t e , w h e r e the ceramic was usually m o n t m o r i l l o n i t e s ( M M T s ) , and the p o l y m e r m a t r i x c o u l d be n y l o n [ 7 9 ] , p o l y ( m e t h y l methacrylate) ( P M M A ) [ 8 4 ] , e p o x y resin [ 8 5 ] , p o l y s t y r e n e [ 8 6 ] , polyurethane [ 8 7 ] , p o l y a n i l i n e [ 8 8 ] , etc. Y e h and coworkers f o u n d that the i n c o r p o r a t i o n o f M M T into a p o l y a n i l i n e m a t r i x led t o an e f f e c t i v e l y enhanced c o r r o s i o n resistance o f p o l y a n i l i n e after a n u m b e r o f measurements o f electrochemical c o r r o s i o n [ 8 9 ] . Messaddeq et al. dispersed P M M A i n t o a z i r c o n i a sol and f i r e d at 2 0 0 ° C f o r 10 m i n u t e s , increasing the l i f e t i m e o f stainless steel [ 9 0 ] .  M a s a l s k i and colleagues [ 9 1 ] believed that the C P C c o a t i n g had to be f i r e d above 4 0 0 ° C to achieve h i g h adhesion strength, and that thus the m a i n d r a w b a c k o f the c e r a m i c / p o l y m e r c o m p o s i t e was the temperature l i m i t a t i o n . T h e i n d u s t r y a l w a y s demands high-temperature systems. U n f o r t u n a t e l y , there has not been v e r y m u c h research related to the high-temperature  (>450°C)  co-sintering process  o f ceramics  and  polymers.  Therefore, it is necessary to do m o r e intensive research e x p l o r i n g this u n k n o w n f i e l d .  19  2.7 A d h e s i o n o f C o a t i n g s  The  adhesion  is  one  of  the  most  important  issues  in  sol-gel  coating  characterization. U n f o r t u n a t e l y there is not v e r y m u c h i n f o r m a t i o n about the assessment o f coating adhesion because the correlation between failure modes and adherence is still p o o r l y understood despite the widespread use o f adherence tests. F o r e x a m p l e , D u et al. [70] reported that they s t i l l do not f u l l y understand the d i f f e r e n t adhesion performance a m o n g the sol-gel coatings after peel testing. S e x s m i t h and T r o c z y n s k i [ 9 2 ] proposed a m o d e l o f a peel test to calculate the e x p e r i m e n t a l l y d e t e r m i n e d stress-strain relationship. L i n and B e r n d t [ 9 3 ] s u m m a r i z e d the most w i d e l y - u s e d measurements o n adhesion: A S T M - C 6 3 3 or D I N - 5 0 1 6 0 . T h e y agreed that these tests are s i m p l e but do not p r o m o t e any deep understanding o f c o a t i n g performance. Scratch was a practical w a y to assess coating adhesion [ 4 8 ] . B a s i c a l l y , there are t w o types o f scratch f a i l u r e , cohesive failure and interfacial f a i l u r e [ 5 0 ] . Cohesive failure indicates g o o d interfacial adhesion, but interfacial failure is o f t e n generated by defects, e.g. pores and cracks, that are created d u r i n g the processing [ 5 1 ] .  X i e and H a w t h o r n e [ 1 3 , 94] summarized the advantages and disadvantages o f the different w a y s o f assessing c o a t i n g adhesion: p u l l - o f f tests, peel tests, f o u r - p o i n t b e n d i n g tests, indentation tests and scratching tests. T h e y attested that m i c r o - c r a c k i n g  could  release the t h e r m a l stresses, and that the C B - C S G coating interface was strengthened  20  w i t h increased processing temperature. T h e y also cited that they still d i d not c o m p l e t e l y understand the reasons o f the measured interfacial toughness and residual stress data trends i n their sol-gel c o a t i n g research [ 4 0 ] . B u t they b e l i e v e d that the interfacial fracture toughness was a reasonable w a y to characterize the adhesion o f sol-gel coatings.  I n p u l l - o f f tests, the c r i t i c a l n o r m a l load, at w h i c h the c o a t i n g  detachment  initiates, is often used to determine the adhesion strength o f a c o a t i n g . Generally, it is believed that the use o f c r i t i c a l n o r m a l load ( p u l l - o f f ) is adequate f o r a semi-quantitative routine m o n i t o r i n g o f the adhesion [ 5 0 ] . H o w e v e r , this is o n l y suitable f o r c o m p a r i s o n between coating systems w i t h close properties. For different coatings, the c o m p a r i s o n o f coating/substrate adhesion c o u l d not be achieved by s i m p l y c o m p a r i n g the c r i t i c a l n o r m a l load. For example, i n p u l l - o f f tests, the brittle sol-gel a l u m i n a c o a t i n g o f t e n b r o k e up at the interface, b u t the detachment o f the soft siloxane b o n d coat o f t e n occurred i n the m i d d l e o f the coating rather than at the interface. I n this case, i t was n o t suitable to use the p u l l - o f f adhesion test to compare the C B - C S G coating and the siloxane p o l y m e r coating.  A n o t h e r p r o b l e m i n the understanding o f coating adhesion was the lack o f reliable models, even t h o u g h m a n y researchers have proposed a n u m b e r o f m o d e l s f o r adhesion bonds,  such  as  chemical  bonding  [95,  96],  diffusion  [97-99],  mechanical  i n t e r l o c k i n g [ 1 0 0 ] , and residual stresses [ 8 1 ] . W h e n l o o k i n g at the literature related to the adhesion o f sol-gel coatings to m e t a l l i c substrates, clear relations c o u l d n o t be f o u n d , e.g.  21  substrate roughness, heat-treatment temperature and coating adhesion. T h i s was always due to incomplete i n f o r m a t i o n about processing conditions.  For e x a m p l e , M a s a l s k i and coauthors [ 1 0 2 ] reported that the sol-gel AI2O3 coating fired  at 5 0 0 ° C had a better failure resistance than that fired at 8 5 0 ° C , but they d i d not  give any reason w h y the higher temperature c u r i n g led to a better adhesion. B e r g m a n n [104, 105] believed that the surface roughness o f substrate increased the  adhesion  strength, and the adhesion increased w i t h the increment o f preheating temperature o f substrate f o r a g i v e n surface roughness. H e also observed that a l u m i n a and z i r c o n i a had poor contact w i t h the substrates w h e n they were sprayed o n a s m o o t h (roughness, R  a  ~  0.05 u m ) stainless steel substrate [ 1 0 6 ] . A c c o r d i n g to the w o r k o f R i c h a r d et al. [ 1 0 9 ] , the residual stresses near the interface decreased w i t h decreasing surface roughness. T h e adhesion increased l i n e a r l y w i t h the increment o f c o l d ( < 1 0 0 ° C ) substrate roughness, and the adhesion increased w i t h the increment o f substrate temperature, where the best adhesion c o u l d be obtained b e t w e e n 300 and 5 0 0 ° C f o r stainless steel [ 9 6 ] . H o w e v e r , a clear q u a n t i f i c a t i o n o f c o a t i n g adhesion vs. substrate roughness was n o t g i v e n .  O n the other h a n d , the substrate surface conditions were also i m p o r t a n t factors c o n t r i b u t i n g to the adhesion strength. The metallic-substrate surfaces i n air w e r e covered w i t h a layer o f metal o x i d e . The density and thickness o f this m e t a l o x i d e layer v a r i e d depending o n the metal-substrate, and o n h o w the surface was treated. For example, phosphoric acid a n o d i z i n g o f steel generated a hard porous i r o n o x i d e layer w i t h a thickness o f about 50 n m [ 1 1 0 , 111]. The i r o n oxide layer had a s i g n i f i c a n t p o p u l a t i o n o f  22  h y d r o x y l groups i n a h u m i d e n v i r o n m e n t . These surface h y d r o x y l s c o u l d participate i n the sol-gel condensation reaction to f o r m a chemical l i n k a g e , M - O - F e . T h i s chemical b o n d f o r m a t i o n p r o d u c e d a strong interaction o f the sol-gel layer w i t h the M S surface i n the i n i t i a l stage [ 7 0 ] . T h i s research can give an explanation as to the rationale o f the i n d i v i d u a l d e p o s i t i o n o f c o n v e n t i o n a l sol-gel process. It is possible to f o r m w e a k bonds between the non-phosphate sol-gel coating and the steel substrate, b u t these w e a k bonds can degrade  at elevated temperatures  and thus y i e l d  a decrease  i n the  adhesion  strength [ 7 0 ] . T h e r e f o r e , the question o f h o w to c o n t r o l the substrate-surface conditions was another i m p o r t a n t issue i n sol-gel coating research.  O n the w h o l e , it was b e l i e v e d that there was not e n o u g h i n f o r m a t i o n to p r o m o t e a deep  understanding  of  the  relationship  between  coating  adhesion  and  sol-gel  processing [ 1 1 2 - 1 2 0 ] . The standard o f adhesion is still p o o r l y u n d e r s t o o d [ 1 2 1 - 1 2 8 ] . Therefore, i n order t o understand the abovementioned p r o b l e m s t o the necessary extent, further i n v e s t i g a t i o n o f adhesion is required. The interfacial fracture toughness is a basic parameter o f the interface. Therefore, it is a potential candidate f o r characterization o f the adhesion o f the sol-gel coating.  2.8 C h l o r i d e C o r r o s i o n  The f o l l o w i n g electrochemical equations g o v e r n the c o r r o s i o n o f i r o n i n a s o d i u m chloride s o l u t i o n [ 1 1 3 ] :  23  Fe^>Fe  3+  0  +3e'  (2.7)  + 2H 0 + 4e~ -+40H~  2  2H  2  +  +2e~ - > #  2  t  (neutral/alkaline)  (acid)  (2.8)  (2.9)  The contact o f i r o n w i t h water and o x y g e n or h y d r o g e n f o r m s a galvanic cell leading to the o x i d a t i o n o f i r o n . Since i r o n possesses anodic and cathodic sites, e.g. Carbon i n the constant electric contact, the o n l y w a y to i n h i b i t c o r r o s i o n is to eliminate the contact b e t w e e n the electrolyte and the reactants [ 1 1 3 ] . T h e siloxane b o n d coating suppresses the cathodic reaction b y l i m i t i n g the d i f f u s i o n o f the electrolyte, i.e. o x y g e n and water, to the substrate. It a l s o . b l o c k s  the transport  o f electrons  to the  iron  surface.  I n a s o d i u m c h l o r i d e s o l u t i o n , the chloride is one o f the m o s t deleterious factors i n terms o f steel c o r r o s i o n . The antagonistic nature o f the Cl" ions is due to their a b i l i t y to absorb on the steel surface, w h e r e h i g h current densities are generated at the Cl" adsorption site. H y d r o l y s i s o f i r o n ions f r o m the anodic reaction causes a decrease i n p H , w h i c h discourages o x i d e film repair and accelerates attacks. E v e n i n h i g h - p u r i t y water, i n w h i c h the level o f Cl" is as l o w as a f e w m i l l i g r a m s per liter, the attack o f C l " ions o n m i l d steel results i n u n i f o r m rather than localized c o r r o s i o n [ 1 1 3 ] .  24  2.9 Corrosion Protection by Multi-Layer Coatings  A ceramic c o a t i n g c a n reduce corrosion o f a metal substrate, since the ceramic material is usually nobler than the metal. C o m p o s i t i o n s , interfaces, defects, thickness, and structures are the i m p o r t a n t parameters affecting the c o r r o s i o n resistance o f a coating.  The c h e m i c a l c o m p o s i t i o n o f the coating and the m i c r o s t r u c t u r e o f the substrate exhibits a direct i n f l u e n c e o n the electrochemical behavior o f the system [ 1 2 9 ] . T h e corrosion rate is n o r m a l l y p r o p o r t i o n a l t o the corrosion-current density  [113]. The  corrosion resistance o f the m u l t i - l a y e r coatings is s i g n i f i c a n t l y d i f f e r e n t f r o m that o f the single-layer coatings [ 1 3 0 ] . T h e m u l t i - l a y e r coatings have a m u c h smaller c o r r o s i o n current density than the m o n o - l a y e r coatings and the bare M S because i n the m u l t i - l a y e r system, the pinholes can be f u l l y b l o c k e d b y c o r r o s i o n products, e.g. i r o n oxides, w h i c h prevents further transport o f o x y g e n t o the steel substrate [ 1 3 1 ] .  The p r o t e c t i o n e f f i c i e n c y , P can be represented b y :  P(%) = 100(l-i /i° ) cor  where i°  cor  and i  cor  cor  (2.10)  denote c o r r o s i o n current densities o f the bare a n d coated substrates  respectively [ 1 2 6 ] .  25  C o a t i n g defects act as pathways that a l l o w the corrosive species to reach the interface [ 1 3 2 ] . T h e pores a n d m i c r o - c r a c k s p r o v i d e a direct path f o r c o r r o s i v e species and lead to r a p i d l o c a l i z e d galvanic c o r r o s i o n o f the metal substrate [ 1 3 2 , 133]. The corrosion current o f the sol-gel coatings decreases to ~ 1 0 " t i m e s that o f the m i l d steel 4  substrates w i t h the presence o f homogeneous macro-pores [ 1 3 4 ] .  The ceramic particles incorporated into the m e t h y l p h e n y l s i l o x a n e ( M P S ) h y b r i d coating enhance the c o r r o s i o n resistance o f steel: the c o r r o s i o n resistance increases w i t h increasing coating thickness, and acts as a physical barrier, w h i c h e f f e c t i v e l y separates the anode f r o m the cathode [5, 126]. The corrosion-resistance o f the interface is increased w i t h the i n c o r p o r a t i o n o f the p o l y m e r component, a fact attributed t o the f o r m a t i o n o f chemical b o n d i n g at the interface [ 1 2 6 ] .  The p o l y m e r b o n d coat not o n l y i m p r o v e s the adhesion, b u t also increases the corrosion resistance o f the system [ 1 3 6 ] . The c o r r o s i o n p r o t e c t i o n o f the b o n d coat increased w i t h increasing c o a t i n g thickness [ 1 3 7 ] .  I n s u m m a r y , it is believed that the m u l t i - l a y e r c o a t i n g f o r m s a better barrier against wet c o r r o s i o n t h a n the single-layer coating. The p o l y m e r b o n d coat can increase the corrosion resistance o f the c o a t i n g system, i n a d d i t i o n to i m p r o v i n g b o n d strength and decreasing residual stresses i n the system, this supports the o r i g i n a l objective o f this thesis.  26  CHAPTER 3: EXPERIMENTAL PROCEDURE  3.1 Substrate Preparation  A I S I 1010 m i l d steel w a s used as a substrate. T h e substrates ( 3 . 8 x 3 . 8 c m ) were sandblasted b y 7 0 - 8 0 psi air pressure u s i n g 2 2 0 g r i t b r o w n f u s e d - a l u m i n a particles, resulting i n a surface roughness o f 0.5-5 u m . T h e average surface roughness (R ) was a  measured b y a W y k o ® optical interference surface p r o f i l e r ( V e e c o Instruments Inc., USA).  3.2 Deposition of Coatings  The o r i g i n a l a l u m i n a sol w a s prepared b y a d d i n g 306 g (1.5 m o i ) a l u m i n u m isopropoxide ( A l ( i O C H ( C H ) ) , 9 8 % , A l d r i c h ) to 3 L h o t ( 8 5 ° C ) d i s t i l l e d water. 3  2  3  IM  nitric acid was used t o adjust the p H o f the sol to 3. T h e m i x t u r e w a s stirred v i g o r o u s l y f o r 16 hours at 8 5 ° C .  T h e excess solvent w a s s l o w l y evaporated f r o m the sol u n t i l its  m o l a r i t y w a s 1.5 M . 4 2 g calcined ct-alumina (0.3-0.5 u m , A 1 6 S G A l c o a I n d u s t r i a l Chemicals, U S A ) w a s added t o 100 m L o f the o r i g i n a l sol as the f i l l e r . T h e sol was b a l l m i l l e d f o r 2 4 hours to break apart particle aggregates a n d t o ensure u n i f o r m i t y o f the solution before the s p r a y i n g o f the coating.  27  The sol was sprayed o n the m i l d steel substrate, f o l l o w e d b y the spraying o f m o n o aluminum  phosphate  ( M A P ) and m e t h y l p h e n y l s i l o x a n e  ( M P S , Dureaseal  1529H  Cotronics C o r p . , U S A ) . T h e M A P and M P S were deposited b y u s i n g t w o spray-guns i n a back-draft  booth.  Subsequently,  the sol was deposited  again  followed  b y the  i m p r e g n a t i o n o f 2 5 w t % m o n o a l u m i n u m phosphate ( M A P ) , c u r i n g at 121 ° C f o r 4 hours, and f i n a l l y b a k i n g at 3 0 0 ° C f o r 3 0 minutes.  A t w o - g u n m e t h o d is used to distribute the M A P t h r o u g h o u t the coating. I t consists o f one g u n spraying the sol and another g u n l i g h t l y spraying 2 5 w t % M A P occasionally. I t is believed that this is the o n l y w a y t o distribute phosphate t h r o u g h o u t the C B - C S G coating w h e n the coating is thicker than 2 0 u m .  C B - C S G 30-40 urn Siloxane 5-10  um  Group A  Substrate  C B - C S G 30-40 u m Siloxane 5-10 u m  Group B  C B - C S G 10-20 u m Substrate  C B - C S G 30-40 u m  Group C Substrate  Fig.3.1. Schematic representations of three basic multi-layer structures  28  Three variants o f this basic process were used t o produce c o a t i n g types A t o C, as s h o w n i n F i g . 3 . 1 . G r o u p A w a s the dual-layer system c o n t a i n i n g the siloxane b o n d coat and the C B - C S G t o p coat o n the M S substrate. G r o u p B w a s a t r i - l a y e r  system,  containing a C B - C S G p r i m e r , a siloxane b o n d coat and a C B - C S G t o p coat. G r o u p C was a m o n o - l a y e r C B - C S G c o a t i n g o n the M S substrate. T h e M S substrates used i n the m o n o layer G r o u p C w e r e heated t o 5 0 0 ° C t o o x i d i z e their surfaces so that they c o u l d be treated w i t h aqueous s l u r r y a f t e r w a r d . T h e samples were kept i n a glass container w i t h a desiccant t o prevent c o n t a m i n a t i o n .  3.3 Low Temperature Process  T h i s process uses l o w - t e m p e r a t u r e c u r i n g t o produce a C B - C S G a l u m i n a coating free o f surface cracks, i n c l u d i n g 1 day o f c u r i n g at 121°C and 5 days o f c u r i n g at 160°C. A f t e r the 1 day o f c u r i n g at 121 ° C , the M A P reacts w i t h h y d r a t e d a l u m i n a and f o r m s AIPO4. C h e m i c a l b o n d i n g occurs w h e n the AIPO4 is converted t o B e r l i n i t e as the b o n d i n g agent after 5 days o f c u r i n g at 160°C. T h e residual phosphate w a s washed a w a y w i t h w a r m ( 4 5 ° C ) water. T h e f o l l o w i n g is the detailed procedure f o r this l o w - t e m p e r a t u r e process:  1.  Heat at 5 5 0 ° C t o f o r m a t h i n o x i d i z e d f i l m o n the sand-blasted m i l d steel.  2.  Spray sol and 2 5 % M A P w i t h t w o guns t o the desired thickness.  3.  Cure at 1 6 0 ° C f o r 1 day.  29  4.  Spray 5 0 % M A P onto the coating surface.  5.  Cure at 1 6 0 ° C f o r 5 days.  6.  W a s h a w a y the residual phosphate w i t h water at 4 5 ° C .  This  low  temperature  process  includes  a multi-gun  spraying  technique  to  distribute the phosphate t h r o u g h o u t the coating and prevent cracks. It can produce the same coatings as the c o n v e n t i o n a l m e t h o d (referred to p p . 1 0 ) . I n this l o w temperature process, m o n o  aluminum  phosphate  reacts p r e f e r e n t i a l l y  with  the  small,  reactive  particles, e.g. y-hydrated a l u m i n a , d u r i n g the 160°C c u r i n g , and p r o v i d e s a b o n d i n g phase f o r the coarser f i l l e r particles, e.g. ct-alumina. T h i s coating is g r a d u a l l y consolidated w i t h o u t generation o f strains d u r i n g the 5 days o f c u r i n g at 160°C.  3.4 Scratch Tests  The scratching c r i t i c a l force at w h i c h a coating fails was used to represent its scratch resistance. A l l measurements are p e r f o r m e d o n a R o m u l u s I V apparatus (Quad G r o u p , U S A ) at a c h a n g i n g vertical load o f 0-45 k g and a 2.0 c m distance. The coating m o v e d at a constant rate o f travel (0.04 m m / s ) and the force increased at a constant rate o f l o a d i n g (0.1 kg/s) u n t i l the coating failed. A sensor near the scratch t i p detected the acoustic signals f r o m the coating fracture and c o n f i r m e d the c r i t i c a l force values.  30  I n this research, a h e m i s p h e r i c a l d i a m o n d t i p w i t h a radius o f 533 u m was used. The scratch tests w e r e conducted o n the f i n e l y p o l i s h e d c o a t i n g surfaces. T h e surface o f the coating was p o l i s h e d t o a roughness o f less than 1 u m w i t h 2 4 0 0 g r i t sandpaper.  Scratch hardness (S/,) is defined b y the f o l l o w i n g equation:  S =^t h  (3-D  where F^is the n o r m a l load and b is the scratch w i d t h [ 4 0 ] , w h i c h is the distance between  the scratch ridges.  is the projected load bearing area based o n t h e assumption that 8  the coating behaves p e r f e c t l y p l a s t i c a l l y and t h e load is carried o n l y b y t h e f r o n t h a l f o f the spherical t i p . F o u r samples per group were used t o determine the average c r i t i c a l force and the average scratch hardness.  3.5 Indentation Hardness Measurements  T h e average hardness o f 3 0 samples w a s measured b y a micro-hardness tester ( M i c r o m e t 3, T e c h - m e t , O N , Canada) using a V i c k e r s indenter at a load o f 300 g and a duration o f 15 seconds.  A n a n o - i n d e n t a t i o n system (Fisherscope H I 0 0 , G e r m a n y ) w a s used t o measure the elastic m o d u l u s o f the coating. N o r m a l load o n the B e r k o v i c h indenter w a s ranged  31  f r o m 0.4 to 10 m N i n 25 equal increments at a rate o f one per second, and then held at the peak load f o r another 50 seconds.  The 5 m m t h i c k substrates were used to prevent the indenter f r o m a f f e c t i n g the d e f o r m a t i o n o f the c o a t i n g system, because there was a l w a y s a plastic zone under the indent f i e l d . The depth o f this plastic zone was - 1 . 5 t i m e s the indent diagonal-length. Each successive indent was isolated at a distance o f - 1 m m to a v o i d o v e r l a p p i n g o f plastic zone onto n e i g h b o r i n g indents.  A R o c k w e l l C hardness tester (Buehler Inc., U S A ) was used to determine the coating's interfacial fracture toughness. Each sample ( G r o u p C ) was cut into three coupons f o r measurement o f the interfacial fracture toughness.  T h e R o c k w e l l C indenter was a spherical d i a m o n d w i t h a 0.2 m m t i p radius. T h e test procedure was conducted according to A S T M standard E l 8 - 0 2 . A s the h i g h - l o a d indenter (150 k g f ) was a p p l i e d o n the coating and caused the plastic d e f o r m a t i o n o f the u n d e r l y i n g substrate, the severe d e f o r m a t i o n o f the substrate f o r c e d the c o a t i n g to be displaced r a d i a l l y and i n d u c e d a compressive radial stress i n the c o a t i n g w h i c h decreased w i t h the increase i n distance f r o m the indenter. T h e c r i t i c a l crack extension force was determined b y m e a s u r i n g the size o f the detachment area [ 1 3 , 4 0 , 4 8 ] . T h e measured crack size was the average o f three indentations. The c a l c u l a t i o n o f interfacial fracture toughness was p e r f o r m e d according to the m e t h o d o f Ref. [ 1 3 ] .  32  3.5 A d h e s i o n Tests  The adhesion strength o f the coatings to the steel substrate was tested b y a p u l l - o f f test, A S T M standard C - 6 3 3 - 7 9 . F i v e samples per group were tested to determine the average tensile strength. T h e coatings were glued to t w o identical rods b y a 3 M 2214 regular e p o x y adhesive.  T h e adhesion measurements  were  made u s i n g an I n s t r o n universal  testing  machine ( I n s t r o n C o r p . U S A ) w i t h 4,450 k g load cell at a cross-head speed o f 1 m m / m i n u t e . The m a x i m u m force at w h i c h the t w o rods were separated was determined (see Fig.3.2). A f t e r the test, the cross-sections o f the fractures were observed b y S E M .  F i g . 3 . 2 . S e t t i n g o f t h e p u l l - o f f tests  33  3.6 Air Permeability Measurements  The average air p e r m e a b i l i t y was measured b y a " V a c u p e r m " p e r m e a b i l i t y tester ( U n i v e r s i t y o f M i s s o u r i - R o l l a , U S A ) . T h e coatings were deposited onto the porous tiles, w i t h a p e r m e a b i l i t y o f ~ 5 . 0 m i l l i d a r c y s . T h e average value o f f i v e samples w a s taken as the air p e r m e a b i l i t y o f the coating.  I n the p e r m e a b i l i t y tester, i t p u m p s the air into the chamber and then forces the air to permeate the coating. A sensor simultaneously detects the pressure i n the chamber and sends i t t o a computer. T h e c o m p u t e r calculates the p e r m e a b i l i t y o f the samples b y a m o d e l between the pressure and the t i m e .  3.7 "Wafer" Curvature Radius Measurements  200-300 u m t h i c k 316 stainless steel w a s used as the substrate. A l l the samples were cooled d o w n f r o m the 3 0 0 ° C f i r i n g before the radius o f the surface curvature was measured.  The f i r s t g r o u p o f samples ( G r o u p C ) i n c l u d e d bare substrate, l O u m , 2 0 u m , 3 0 u m , 4 0 u m , 6 0 u m , 80 u m , and 100 u m t h i c k C B - C S G coatings respectively. A n o t h e r six G r o u p A coatings contained a 30 u m C B - C S G t o p coat and d i f f e r e n t thickness values for the siloxane p r i m e r , e.g. 0 u m , 5 u m , 8 u m , 10 u m , 15 u m , a n d 2 0 u m respectively,  34  and each sample was measured 4 times at different locations. There was a p p r o x i m a t e l y ± 2 u m d e v i a t i o n o f the thickness measurement f o r each sample.  A n i n t e r f e r o m e t r i c surface i m a g i n g system ( W Y K O N T - 2 0 0 0 , V e e c o Instruments Inc. U S A ) was used t o measure the radius o f curvature o f the bare substrates and the coatings. Each measurement was focused o n a 3.7x4.8 m m area. T h e f i n a l curvature radius R„ o f each area was the average o f absolute R and R \ x  y  therefore, the radius o f  curvature o f each sample was calculated b y ,  r = -I.\R \  (3.1)  n  "„=y  The radius o f curvature  o f each sample was taken as the average value o f  four  measurements. T h e highest and lowest values were not i n c l u d e d .  3.8 Microscopy and XRD  The preparation o f the samples used f o r S E M observations c o m p r i s e d sectioning with  a  diamond  saw,  vacuum  mounting  with  a  low-viscosity  resin  (Industrial  Formulators, Inc. Canada), f o l l o w e d b y p o l i s h i n g w i t h 5 0 - 1 2 0 0 g r i t sandpaper, 5, 1, and 0.5 u m d i a m o n d slurries, and silk c l o t h on the g r o o v e d metal platens. The samples were polished to a flatness < 0 . 1 u m w i t h nano-size c o l l o i d a l silica.  35  For the fracture observation samples, the coupons w e r e cut v e r t i c a l l y f r o m the substrate towards the c o a t i n g w i t h a high-speed saw to o b t a i n a V - g r o o v e close to the coating.. The sample was placed i n l i q u i d n i t r o g e n f o r a p p r o x i m a t e l y 1 m i n u t e , and then b r o k e n i m m e d i a t e l y after b e i n g taken out. 600 and 1,200 g r i t sandpaper and 1 urn d i a m o n d paste w e r e used t o p o l i s h the faces n o r m a l to the fracture plane o f samples. The m o v i n g d i r e c t i o n o f the p o l i s h i n g w h e e l was f r o m coating to substrate, to prevent cracks f r o m propagating due t o the external load w h i l e p o l i s h i n g . T h e e p o x y m o u n t i n g also c o u l d have protected the cracks f r o m d e f o r m a t i o n to some extent. T h e samples f o r the crack observations w e r e prepared i n the same w a y .  The specimens w e r e carbon coated ( - 2 0 electron)  detection o f the  angstrom) f o r B S E  cross-sectional m o r p h o l o g y  coupled w i t h  (backscattering EDX  (Energy  Dispersed X - r a y ) . T h e cross-sections o f the siloxane layer sputtered b y g o l d were detected at a h i g h m a g n i f i c a t i o n b y SE (secondary electron) w i t h a H i t a c h i S-2300 S E M instrument at a w o r k i n g voltage o f - 2 0 . 0 k V .  A coating thickness gauge (Positector 6000, D e F e l s k o C o r p . , N Y , U S A ) was used to measure the thickness o f the sol-gel coatings. A l l the c o a t i n g thicknesses  were  c o n f i r m e d b y the measurements f r o m the S E M images o f their cross-sections.  X - r a y d i f f r a c t i o n ( X R D ) was p e r f o r m e d o n an X - r a y generator p l a t f o r m (Philips P W 1830). The surface m o r p h o l o g y o f the coatings was observed b y a N i k o n E P I P H O T 300 optical m i c r o s c o p e . The a l u m i n a v o l u m e percentages o f f i v e samples per group were  36  assessed b y an image analyzer ( C l e m e x version P E  3.5, C l e m e x Technologies I n c . ,  Canada).  3.9 Potentiodynamic Evaluations of the Coatings  A l l P o t e n t i o d y n a m i c evaluations were carried o u t b y a S o l a r t r o n 1286 potentiostat (Solartron G r o u p  C o m p a n i e s , U K ) u s i n g the W i n d o w s ™  X P o p e r a t i n g system i n  c o n j u n c t i o n w i t h a D e l l ™ P H I P C . T h e P o t e n t i o d y n a m i c evaluations w e r e conducted as follows:  • A piece o f copper w i r e was w e l d e d onto the back surface o f the substrate. • T h e exposed surface o f the copper w i r e and the edges o f t h e samples were stamped w i t h epoxy. • T h e cut edges and e p o x y crevices were sealed w i t h a w a t e r p r o o f T E F L O N ® tape and an acetone d i l u t e d s t o p - o f f lacquer ( 5 0 % M i c r o s t o p , P y r a m i d Plastics, I n c ) , w h i c h was a insulating and w a t e r p r o o f paint, i n order t o prevent galvanic actions a l o n g the edges and defects between the e p o x y and the coatings. • T h e sample w a s f i x e d i n a quartz glass tube whose end w a s sealed b y w a t e r p r o o f tape. A 0.7x0.7 m m section w i t h i n the center o f each sample w a s exposed t o s o l u t i o n d u r i n g the testing and faced w i t h the l u g g i n capillary as close as possible. • N i t r o g e n ( + 1 0 p s i ) w a s connected t o d r i v e o f f the o x y g e n i n the water, and the electrolyte was m a g n e t i c a l l y stirred w h i l e the experiment was r u n n i n g .  37  C H A P T E R 4: R E S U L T S A N D D I S C U S S I O N  4.1 Coating Structures  4.1.1 G r o u p A S t r u c t u r e  Figure 4.1.1 shows a typical dual-layer microstructure of the Group A coating. The coating is comprised of a siloxane bond coat and a C B - C S G top coat, where the C B CSG top-layer is well anchored to the siloxane bond coat.  Fig.4.1.1 S E M image o f d u a l - l a y e r coating  The  OC-AI2O3  particles or the CSG aggregates penetrated into the siloxane primer  and acted as reinforcing phases. These reinforcing phases form mechanical interlocks between the siloxane and the CSG. In Fig.4.1.2, there was a siloxane/A^Os composite area in the siloxane bond coat near the top coat. Some of the C B - C S G aggregates even 38  penetrated into the b o t t o m o f the siloxane b o n d coat, f o r m i n g a C B - C S G / s i l o x a n e composite p r i m e r . A c t u a l l y , the siloxane f i l m is i n a discontinuous  state at some  locations. T h i s m i g h t be w h y the G r o u p A coating h a d a higher adhesion strength ( - 1 0 . 0 M P a ) than that o f the 1 0 0 % siloxane (4-7.0 M P a ) .  x£.Qk  Q088  28kV  28j-'m  Fig.4.1.2 Ceramic/polymer composite layer between bond-coat and top-coat.  Since the p o l y ( m e t h y l p h e n y l s i l o x a n e ) ( P M P S ) does n o t adhere w e l l to ceramics, it was necessary t o simultaneously spray the m o n o m e r M P S a n d the C S G sol before c u r i n g t h e m together. T h i s process a l l o w s the a l u m i n a particles t o penetrate into the siloxane m a t r i x a n d t o f o r m g o o d anchoring points.  39  Therefore, the siloxane composite layer c o u l d be considered as a f u n c t i o n a l l y graded b o n d coat w i t h a graded c o m p o s i t i o n f r o m t o p coat t o b o n d coat, reducing the thermal expansion m i s m a t c h a m o n g the different coating layers.  4.1.2 Group B Structure Figure 4.1.3 is a S E M picture o f the tri-layer m i c r o s t r u c t u r e o f the G r o u p B coating, consisting o f a ~5 u m siloxane layer interleaved w i t h a 3 0 - 4 0 p m C B - C S G t o p coat and a 10-20 p m C B - C S G p r i m e r , resulting i n a s a n d w i c h structure coating. Figure 4.1.4 is a higher m a g n i f i c a t i o n o f Fig.4.1.3; i n some places, the siloxane b o n d coat presents a discontinuous siloxane film. T h e purpose o f d e v e l o p i n g the t r i - l a y e r G r o u p B coating was t o increase the strength o f the b o n d coat.  Substrate  x 8 8 G  0 0 0 8  2 8 HV  50.urn  Fig.4.1.3. CB-CSG/siloxane /CB-CSG tri-layer structural coating (Group B)  40  1  Fig.4.1.4 Tri-layer Coating structure  The s t i f f C B - C S G layer is to absorb external loads a n d protect the soft siloxane under-layer i n service. T h e siloxane under-layer i n t u r n prevents the spread o f cracks into other layers and relaxes the stress to the substrate. Some cracks originate at the coating /substrate interface a n d develop t h r o u g h the coating, h u r t i n g its adhesion properties. Therefore, the siloxane bond-coat can also act as a crack i n h i b i t o r , thereby possibly increasing the fracture resistance o f the coating.  41  Substrate  xl.Qk  00  50JLim  0kV  Fig.4.1.5 Cross section image of mono layer C B - C S G coating  • •  1  .p^ - .  •\  . • •*  i  L*« •  »  j "  " i J '"••*«" ^i  Fig.4.1.6 T h e phosphorus E D X mapping of Fig.4.1.5  4.1.3 G r o u p C Structures  Figure 4.1.5. shows a mono-layer CB-CSG coating on an oxidized mild steel substrate. There is no crack observed through the cross-section. Figure 4.1.6 is a 42  phosphorus m a p p i n g b y E D X , and shows evidence o f the u n i f o r m phosphate d i s t r i b u t i o n throughout the c o a t i n g achieved v i a the t w o - g u n preparation m e t h o d .  I n most cases, such phosphorus mappings b y E D X  s h o w that the  phosphorus  concentration g r a d u a l l y increases f r o m the interface to the surface. T h e surface has the highest c o n c e n t r a t i o n o f phosphate, and the interface has the l o w e s t concentration. T h e f i n a l step o f the process has t r a d i t i o n a l l y been to deposit a large v o l u m e o f phosphate o n the coating surface, a n d this is the reason w h y the surface u s u a l l y has the highest concentration o f phosphorus. T h i s p h e n o m e n o n also shows that the phosphate cannot penetrate t h r o u g h the t h i c k C B - C S G layer, and thus that o n l y t w o - g u n spraying can distribute phosphate e v e n l y t h r o u g h o u t the coating.  43  4.2 Cracks and A i r Permeability  4.2.1 Cracks in Coatings  The conventional composite sol-gel method involves multi-step spraying and individually firing at 550-600°C, which often generates cracks, e.g. shrinkage cracks, phosphate cracks and thermal expansion-mismatch cracks.  Fig.4.2.1 Drying-shrinkage cracks  44  D r y i n g shrinkage o f t e n induces cracks w h e n the water evaporation occurs too q u i c k l y . The shrinkage cracks are often o f small size and i n v i s i b l e t o the eye ( F i g . 4.2.1), and can be easily obscured b y p o l i s h i n g w i t h 1200 grit sandpaper. Freeze-drying is also a viable w a y to decrease the shrinkage strain and a v o i d shrinkage cracks.  Fig.4.2.2 W e b - l i k e p h o s p h a t e c r a c k s  T o increase the hardness and adhesion o f the sol-gel c o a t i n g , it is necessary to impregnate e n o u g h phosphate. H o w e v e r , brushing o f 8 5 % phosphoric acid onto the green-body o f composite sol-gel a l u m i n a often generates the phosphate cracks even at r o o m temperature. Figure 4.2.2 is an example o f "phosphate cracks". The phosphate cracks often appear i n a w e b - l i k e shape on the coating surface.  45  Essentially, phosphate cracks are generated b y the c h e m i c a l reactions o f the phosphoric acid and the hydrate a l u m i n a . W h e n the phosphoric acid ( P A ) is overimpregnated o n the surface o f green b o d y , it f o r m s pseudoboehmite or gelatinous boehmite, w h i c h has a l o w strength (referred to pp.10). T h e gelatinous boehmite reacts w i t h the residual phosphoric acid ( P A ) , as s h o w n i n equation (4.2.1).  H PO, 3  + AlOOH => AlPO, i +2H 0 2  (4.2.1)  Fig.4.2.3 T h e c r a c k s d u e t o t h e m i s m a t c h o f C T E  46  T h i s reaction produces solid a l u m i n u m phosphate at r o o m temperature, w h i c h often induces strains i n the green n e t w o r k . W h e n the stresses a c c o m p a n y i n g the strains exceed the strength o f the green n e t w o r k , they w i l l induce phosphate cracks. Usage o f m o n o a l u m i n u m phosphate ( M A P ) c a n decrease the p r o b a b i l i t y o f p r o d u c i n g phosphate cracks b y means o f retarding the chemical reactions at r o o m temperature, since M A P is less c h e m i c a l l y active than P A .  W h e n the green sol-gel is deposited w i t h 8 5 % p h o s p h o r i c a c i d , the phosphoric acid w i l l react w i t h a l u m i n a to f o r m M A P . W h e n fired at 5 5 0 - 6 0 0 ° C , the M A P w i l l q u i c k l y convert t o B e r l i n i t e , Cristobalite or other phases, w h i c h causes v o l u m e changes over a short t i m e . W h e n the c o a t i n g is cooled d o w n , the d i f f e r e n t i a l t h e r m a l expansion o f the different phases w i l l generate m i s m a t c h cracks.  Fig.4.2.4 The surface crack-free coating after 4 days of curing at 160°C  47  O n the other h a n d , the differential thermal expansion o f the sol-gel coating and the metallic substrate also induces strains i n the conventional sol-gel process, w h i c h lead to large cracks v i s i b l e t o the eye. Figure 4.2.3 is a h e a v i l y phosphatized coating. T h e coating surface induces cracks due to the brushing o f 8 5 % P A and C T E m i s m a t c h . T h i s k i n d o f cracking is the biggest challenge i n the traditional c h e m i c a l l y b o n d e d sol-gel process. The C T E m i s m a t c h cracks are very d i f f i c u l t to prevent, since the t h e r m a l expansion differences are inevitable w i t h c h a n g i n g temperature.  Fig.4.2.5 The high magnification of surface crack-free coating  48  One v i a b l e w a y t o prevent the phosphate cracks and the C T E - m i s m a t c h cracks is to decrease the processing temperature. C u r i n g at 160°C f o r 5 days a l l o w s some stress relaxation and produces crack-free coatings. T h i s l o w temperature c u r i n g a l l o w s the M A P to gradually t r a n s f o r m t o B e r l i n i t e w i t h o u t i n d u c i n g large strains. A f t e r the phase t r a n s f o r m a t i o n is c o m p l e t e d , the coating is almost stress-free. T h i s w a s evidenced b y " W a f e r " experiment, w h i c h w i l l be discussed i n chapter 4.5. A f t e r l o n g - t e r m c u r i n g , the coating obtains a strength that a l l o w s i t to be f i r e d again at a h i g h e r temperature ( > 3 0 0 ° C ) and c o o l e d d o w n w i t h o u t generation o f cracks. Fig.4.2.4 shows a surface crack-free coating after 5 days o f c u r i n g at 160°C. There are n o cracks even at the corners and the edges. Fig.4.2.5 is a higher m a g n i f i c a t i o n o f the surface crack-free c o a t i n g b y the l o w temperature process. I t is reasonable that l o w temperature ( 1 6 0 ° C ) c u r i n g reduces the p o s s i b i l i t y o f c r a c k i n g due t o d i f f e r e n t i a l thermal expansion.  4.2.2 Air-Permeability  Table.4.2.1 shows that the C B - C S G coating is air permeable at a thickness o f - 4 0 urn. T h e air p e r m e a b i l i t y o f traditional C B - C S G coatings is h i g h e r than that o f l o w temperature C B - C S G coatings, i m p l y i n g that the l o w temperature c u r i n g produces f e w e r defects, e.g. pores, cracks and fissures, than the t r a d i t i o n a l sol-gel m e t h o d . T h e air p e r m e a b i l i t y after c u r i n g at 160°C is - 0 . 2 3 m i l l i d a r c y , i n d i c a t i n g that the sol-gel coating is relatively dense.  49  Table 4.2.1 the air permeability of various structures Traditional  New  300-500°C firing  160°C c u r i n g  A i r Permeability (milidarcy)  Dual-layer Group A  NM  Non-detectable  Tri-layer Group B  NM  Non-detectable  Mono-layer Group C (40 p m )  0.45 (0.08)  0.23 (0.03)  *Values in parentheses show the standard deviation * N M : not measured  Generally,  the air p e r m e a b i l i t y  of CB-CSG  coatings  decreases  with  increasing  thickness. T h e a i r - p e r m e a b i l i t y o f the siloxane incorporated C B - C S G m u l t i - l a y e r coating is non-detectable (Table 4.2.1). Plausibly, the hermetic m o n o m e r sealed the v o i d s o f the g e l - n e t w o r k d u r i n g spraying a n d prevented the substrate f r o m c o n t a c t i n g  corrosive  species such as o x y g e n , and the p o l y m e r islands decreased the n u m b e r o f C B - C S G channels after the p o l y m e r i z a t i o n o f m e t h y l p h e n y l s i l o x a n e ( M P S ) . W h e n these islands connect together and f o r m a continuous layer, i t is d i f f i c u l t f o r the o x y g e n t o contact the substrate i n the m u l t i - l a y e r c o a t i n g system. T h e c o r r o s i o n p r o t e c t i o n o f the m u l t i - l a y e r coating w i l l be discussed i n chapter 4.7.  50  4.3 Contact Measurements  4.3.1 H a r d n e s s o f C B - C S G C e r a m i c  T h e average hardness o f the C B - C S G surface w a s measured as 6.24 G P a , close t o the hardness o f B e r l i n i t e . T h i s c o u l d be explained b y the processing, w h e r e , after the f i n a l coating step ( s p r a y i n g o f 5 0 w t % M A P onto the C B - C S G surface), the m o n o a l u m i n u m phosphate ( M A P ) f o r m e d a c h e m i c a l b o n d w i t h the C S G a n d g r a d u a l l y t r a n s f o r m e d t o B e r l i n i t e or Cristobalite d u r i n g c u r i n g at 160°C f o r 5 days (referred t o p p . 1 4 ) .  F i g u r e 4.3.1 is an i n d e n t a t i o n trace o n the t o p surface o f a G r o u p A coating, the hardness o f w h i c h w a s measured as 6.05 GPa. S i m i l a r l y , the hardness o f the t o p surface o f the G r o u p C c o a t i n g (Fig.4.3.2) w a s 6.26 GPa. T h e C B - C S G c o a t i n g ( G r o u p C ) is believed t o be a B e r l i n i t e - b o n d e d a l u m i n a n e t w o r k ; h o w e v e r , there are some m i c r o - p o r e s , channels f o r the e v a p o r a t i o n o f water, o n the surface o f the C B - C S G c o a t i n g , causing the measured hardness t o be l o w e r than that o f B e r l i n i t e .  The hardness o f B e r l i n i t e is - 6 . 5 GPa, and the average hardness o f the G r o u p C coating is - 6 . 2 4 GPa, h o m o g e n e o u s l y distributed across the c o a t i n g surface. Therefore, i t is believed that the B e r l i n i t e AIPO4 is distributed h o m o g e n e o u s l y across the surface o f the C B - C S G coating. I t w a s accepted that o n l y the B e r l i n i t e w a s f o r m e d after the M A P  51  was cured at ~ 1 5 0 ° C f o r 4 days, where the m o n o a l u m i n u m phosphate hydrate reacted w i t h a l u m i n a at ~ 1 5 0 ° C and f o r m e d B e r l i n i t e b y the f o l l o w i n g reaction [ 3 4 ] :  Al 0, + 2AlH (PO ) 2  i  A 2  • H 0 -> AAIPO, + 4H 0 2  2  (4.3.1)  Fig.4.3.1. SEM indentation trace of CB-CSG surface, Vickers indent 300g load  B e r l i n i t e is the phase that bonds i n d i v i d u a l a l u m i n a particles and f o r m s the b e r l i n i te -bonded a l u m i n a ceramic.  H o w e v e r , i n this research, a w e l l - c r y s t a l l i z e d AIPO4  n e t w o r k was n o t f o u n d b y X R D . I t is believed that o n l y the vitreous B e r l i n i t e (hardness 6.5 GPa) was f o r m e d o n the surface and inside the coating [ 6 5 ] .  T h e cross-sectional hardness measurements w e r e c o n d u c t e d o n samples m o u n t e d w i t h i n a 2 5 . 0 m m diameter cast e p o x y - r e s i n b l o c k , w h i c h c o u l d prevent the error caused  52  b y the coating thickness. A d d i t i o n a l l y , the 4 0 - 1 0 0 urn C B - C S G coating decreased the influence o f c o a t i n g thickness to a tolerable extent, since, i n t h i n f i l m s , the cross-sectional hardness was affected b y the substrate. Figure 4.3.3 shows an image o f the mono-layer G r o u p C coating. A hardness o f 6.0 G P a was measured at the interface, p r o v i d i n g further evidence o f phosphate d i s t r i b u t i o n to the interface b y the m u l t i - g u n spray technique. F r o m observation o f Fig.4.3.3, the B e r l i n i t e phase f o r m e d at the interface  without  generating a crack along the interface. I n Fig.4.3.4, the indentation was conducted right on the interface b e t w e e n coating and substrate, but crack onset d i d n o t occur at the apexes o f the indent contour, demonstrating the h i g h adhesion strength o f the C B - C S G coating. H o w e v e r , as seen i n Fig.4.3.5, there m a y have been an interfacial crack induced along the interface, w h i c h w a s obscured b y the elastic siloxane. T h e hardness o f siloxane was measured at 0.12 GPa.  Fig.4.3.2 S E M i n d e n t a t i o n t r a c e o n C B - C S G s u r f a c e , 3 0 0 g l o a d  53  For some samples, the cross-sectional hardness o f the C B - C S G coating was between 3.0-6.0GPa, i.e. between that o f Variscite and B e r l i n i t e , i n d i c a t i n g that the b o n d i n g agent c o u l d be a m i x t u r e o f the t w o . Occasionally, h o w e v e r , some o f the 3.0GPa values c o u l d have been related to the heterogeneity o f m i c r o - s t r u c t u r e , i.e. porosity or agglomeration o f y - a l u m i n a or V a r i s c i t e , even t h o u g h the 4 5 ° C water washes away most o f the Variscite a n d hydrated y - a l u m i n a o n the surface. I t is b e l i e v e d that l o n g - t e r m curing w o u l d eventually convert the rest o f the V a r i s c i t e t o B e r l i n i t e .  Fig.4.3.3. The SEM indentation trace of CB-CSG ceramic coating, Vickers indent 300 g load, Group C  I n some extreme results, a surface hardness o f - 1 0 . 0 G P a w a s observed, w h i c h m i g h t have been caused b y the hard aggregation o f alpha a l u m i n a a n d strong ligands o f chemical bonds, or p o s s i b l y b y w e l l - c r y s t a l l i z e d B e r l i n i t e o r Cristobalite (referred t o  54  pp.14). I t is possible t o f o r m a n e t w o r k w i t h the c o m b i n a t i o n o f B e r l i n i t e binders and cca l u m i n a f i l l e r s , w h i c h have an average hardness over 10 GPa. Therefore, the relationship between the c o n t r o l o f i m p r e g n a t i n g chemicals and the hardness o f the coating needs further research.  4.3.4 The SEM indentation trace of CB-CSG ceramic coating, Vickers indent 300 g load, Group C  4.3.5 Indentation trace of substrate-polymer-coating cross section, indented at 300 g load, Group A  55  4.3.2 Interfacial Fracture Toughness  The interfacial fracture toughness is a basic material parameter o f the interface. The higher the i n t e r f a c i a l fracture toughness, the higher the adhesion strength o f the coating. Therefore, i t is believed that the interfacial fracture toughness is a reasonable indicator o f c o a t i n g adhesion and that its measurement is a g o o d w a y t o q u a n t i t a t i v e l y assess the interface.  W h e n a h i g h - l o a d R o c k w e l l C indenter is applied t o a c o a t i n g surface, the coating experiences interfacial cracks between coating and substrate r=a. T h e interfacial cracks spread r a d i a l l y t o w a r d a c r i t i c a l p o i n t where the energy release rate w a s unable t o propagate the cracks. T h e value f o r interfacial fracture toughness w a s i d e n t i f i e d w i t h the value o f energy release at w h i c h the cracks arrested, as schematically depicted i n Fig.4.3.6, s h o w i n g a m o d e l o f the detachment b y a h i g h load a x i s y m m e t r i c indenter. T h e detachment occurs as f o l l o w s : (1) the interfacial crack f r o n t is f o r c e d t o advance and to break u p the c o a t i n g b y displacement o f the substrate; ( 2 ) the detachment occurs w i t h an u n b u c k l e d annular plate o f coating, w h i c h remains intact; ( 3 ) the annular plate o f coating buckles.  56  Fig.4.3.6 Model for the identification of interface fracture toughness  The R o c k w e l l C i n d e n t a t i o n c o u l d induce some radial cracks i n the coating that were n o t caused b y displacement o f the substrate. O p t i c a l i m a g e analysis avoids the obscuring o f m i c r o - c r a c k s due to g o l d sputtering or carbon c o a t i n g f o r S E M sample preparation, and therefore an optical microscope was used instead o f S E M . F i g u r e 4.3.7  57  depicts the trait o f the interface between C B - C S G coating and substrate where the radial cracks resulted f r o m the tensile hoop stresses w h e n the coating w a s displaced radially. The detached coating adjacent t o the indentation was c a r e f u l l y discerned t h r o u g h the use o f image analysis software ( C l e m e x version P E ™ 3.5, C l e m e x Technologies Inc., Canada), w i t h w h i c h the contact radius (a) and the radius o f the indentation-induced annular cracks (R,) were measured, as s h o w n i n Fig.4.3.7 and Fig.4.3.8.  Fig.4.3.7. Optical images of typical coating detachment after Rockwell C indentation  Fig.4.3.8 Detachment area of Fig.4.3.7 by image analysis  58  Since the d e l a m i n a t i o n is due t o mode I I (shear) interface c r a c k i n g o n l y , the interfacial fracture toughness is presented b y K n c . T h e m o d e l s and c a l c u l a t i o n o f interfacial fracture toughness are taken f r o m the literature [ 1 3 ] .  4.5  E  re Q_  4 3.5  H  3 JLZ O) 2.5 3 O 12 <D k. 3 O 1.5 -  1-  terf  re  LL Q) O  0.5 -  re  0 2  3  4  5  Substrate Roughness (micron) Fig.4.3.9. A plot of substrate surface roughness and interfacial fracture toughness  Figure 4.3.9 shows a p l o t o f interfacial surface roughness a n d interfacial fracture toughness, where the i n t e r f a c i a l fracture toughness increased w i t h t h e surface roughness o f the substrate; the rougher the substrate surface, the higher the adhesion strength. H o w e v e r , after the surface roughness reached a value higher than 4 p m , the interfacial fracture toughness tended t o become stable at 3.5-4.0 M P a . m  1 / 2  . T h e r e f o r e , the o p t i m u m  surface roughness o f the substrate s h o u l d be - 4 . 0 - 5 . 0 p m , at w h i c h the c o a t i n g possessed a good capability t o w i t h s t a n d shatter or to absorb i m p a c t energy. T h e sand-blasted steel substrate resulted i n a higher interfacial fracture toughness t h a n the g r i n d e d  steel  59  substrate [ 1 3 ] . H o w e v e r , the m o d e l o f the interfacial fracture toughness w a s based u p o n the " f u l l y - e l a s t i c m o d e l " [ 1 3 ] . T h e d r a w b a c k o f u s i n g o p t i c a l image analysis t o measure the radius o f annular cracks is that i t depends o n the experience and s k i l l s o f the operator. A s seen i n Fig.4.3.9, t y p i c a l data scatter f o r the K n is 0 . 5 - 0 . 7 M P a . m C  1/2  or u p t o 3 0 % o f  the average.  4.3.3 Nano Indentation and Sonic Wave Modulus  Table 4.3.1 lists the average hardness and elastic m o d u l u s o f the C B - C S G t o p coat and the siloxane b o n d coat, measured b y a depth-sensing i n d e n t a t i o n . T h e hardness and elastic m o d u l u s o f siloxane were m u c h l o w e r than those o f the C B - C S G ceramic. These values w i l l be used t o determine residual stresses i n a later chapter. T h e Poisson's ratio o f C B - C S G was 0.25 as measured b y sonic waves (see A p p e n d i x I ) .  F r o m Table 4 . 3 . 1 , the d y n a m i c Y o u n g ' s m o d u l u s o f the c o a t i n g as measured b y acoustic m e t h o d (sonic w a v e s ) w a s - 2 2 5 GPa, w h i c h seemed r e m a r k a b l y d i f f e r e n t f r o m that measured b y the nano-indenter. I t is probable that the acoustic m e t h o d neglected the presence o f defects, e.g. cracks and pores, and was averaged v i a the v i b r a t i o n a l frequency o f the coatings c o n t a i n i n g 8 0 w t % a - a l u m i n a (elastic m o d u l u s : - 3 7 0 GPa). O n the other hand, the surface defects, e.g. open-pores and surface fissures, affected the values o f the indentation m e t h o d , so the values measured b y acoustic methods w e r e o f t e n higher than those measured b y i n d e n t a t i o n .  60  Table.4.3.1. The average values of hardness and elastic modulus measured by nanoindenter Hardness (GPa)  Elastic M o d u l u s - ^ 7  1-V  (GPa)  Elastic M o d u l u s b y Sonic W a v e (GPa)  Siloxane B o n d Coat  0.12(0.05)  5.0 (0.4)  NM  C B - C S G T o p Coat  7.65 (2.7)  122.0 (30.0)  225.45 (40.0)  *Values in parentheses show the standard deviation * N M : not measured  61  4.4 Scratch Resistance  4.4.1 Critical Scratching Force for Coatings  A s measured b y parallel scratching, the average c r i t i c a l f o r c e values f o r Groups A , B , and C were 4.7 k g , 10.3 k g and 17.0 k g respectively (Table 4.4.1). depend  o n the v o l u m e  percentage  o f reinforcement  i n the siloxane,  These values referred t o  chapter 4.6.  Table 4.4.1 Scratch resistance of various coating structures Groups Group A  ~~  Structure  Critical Force (kg)  Dual-layer  4.7 ( 0 . 5 )  Group B  Tri-layer  10.3(1.0)  Group C  Mono-layer  17.0(1.4)  * Values in parentheses show the standard deviation  The image analysis s h o w e d that the percentage o f r e i n f o r c e m e n t i n the siloxane was ~ 1 0 v o l % and ~ 2 5 v o l % i n dual-layer G r o u p A and t r i - l a y e r G r o u p B respectively. The critical force increased w i t h the increment o f c o a t i n g thickness. T r i - l a y e r G r o u p B had a C B - C S G p r i m e r - 5 - 1 0 p m t h i c k e r than that o f dual-layer G r o u p A . Therefore, t r i layer G r o u p B h a d a h i g h e r c r i t i c a l force than dual-layer G r o u p A . A l t h o u g h m o n o - l a y e r G r o u p C d i d n o t have the p o l y m e r constituent referred t o Fig.4.4.1 (a), i t possessed the highest scratch resistance.  62  As shown in Fig.4.4.1 (b), the distance traveled by the stylus under an increasing load (0-45 kg) at the onset of coating failure was -0.82 cm. This result also was verified by the assessment of effective friction, which was defined as the transverse force (F , t  diamond drag force) divided by the normal force (F ). n  r  f =  Transverse  F x 100 = — x 100 (no unit) Normal F. t  n  n  (4.4.1)  (a)  Hormal  (b)  10  12  -10 Distance (mm)  Fig.4.4.1. (a) Optical micrograph of a scratch groove ( at 100X); (b) curves of normal force, acoustics, effective friction and distance;  63  A t a m a g n i f i c a t i o n o f 1 0 0 X , the scratching artifacts o f the c o a t i n g w e r e clearly discerned. A n abrupt change i n the effective f r i c t i o n , i n Fig.4.4.1 ( b ) , signaled the c o a t i n g failure, w h i c h is t h o u g h t t o be caused b y one o f the f o l l o w i n g c r i t i c a l events:  ( 1 ) the  C B - C S G is t o r n f r o m t h e substrate; ( 2 ) the substrate fails and t h e d i a m o n d dislodges substrate m a t e r i a l ; ( 3 ) t h e adhesive strength o f the c o a t i n g i s exceeded.  I n the case s h o w n i n Fig.4.4.1 (b), the acoustic signal r e c o r d i n g indicates that the surface o f the C B - C S G c o a t i n g cracked i n i t i a l l y at 0.18 c m a n d t h e c r i t i c a l f o r c e at the onset o f C B - C S G surface spallation was - 2 . 5 k g .  20  40  60  80  100  120  Coating Thickness (micron) Fig.4.4.2. The scratch critical force vs. coating thickness (for mono-layer Group C CB-CSG coating)  64  The scratch c r i t i c a l force at w h i c h the coating fails increases w i t h an increase i n coating thickness: the t h i c k e r the coating, the higher the c r i t i c a l force. F i g u r e 4.4.2 shows the critical force vs. the c o a t i n g thickness. W h e n coating thickness was b e l o w 60 u m , the critical force tended to increase w i t h the coating thickness. H o w e v e r , the average residual stresses i n the C B - C S G c o a t i n g also increased w i t h increasing c o a t i n g thickness (e.g. compare Fig.4.5.1 i n Chapter 4.5). Therefore, after a certain thickness, the c r i t i c a l force begins to decrease w i t h increasing coating thickness.  W h e n the c o a t i n g is b e i n g scratched, the stresses i n c l u d e the residual stress f r o m processing and the b e n d i n g stress f r o m scratching. F r o m F i g . 4.4.2, i t is apparent that: (1) w h e n the thickness o f the G r o u p C coatings was b e l o w 20 p m , the stresses c o u l d be considered t o be n e g l i g i b l e . The c r i t i c a l force increased almost l i n e a r l y w i t h increasing coating thickness; (2) i n the 2 0 - 6 0 p m thickness range, the effects o f the stresses c o u l d not be i g n o r e d , the rate o f change o f the critical force w i t h thickness decreased due to the stresses; (3) after 60 p m , the c r i t i c a l force decreased w i t h f u r t h e r increase i n c o a t i n g thickness because o f the residual stresses. H o w e v e r , i f the residual stress were relaxed, the critical force w o u l d l i k e l y continue to increase w i t h increasing o f the  coating  thickness. Stress r e l a x a t i o n w i l l be discussed i n chapter 4.5.  Generally, the C B - C S G m o n o - l a y e r coatings h a d g o o d scratch resistance w h e n their thickness was b e l o w 100 p m .  65  4.4.2 Scratch Hardness of Coatings  There are t w o types o f scratch failure: cohesive failure and interfacial failure. T h e f o r m o f cohesive failure is usually partial cone cracking o r c o n f o r m a l c r a c k i n g . Partial cone c r a c k i n g often occurs around the perimeter o f the t r a i l i n g edge o f the scratch t i p . C o n f o r m a l c r a c k i n g f o r m s i n f r o n t o f the m o v i n g indenter at the l e a d i n g edge rather than at the t r a i l i n g edge. F i g u r e 4.4.1 (a) shows the onset o f some c o n f o r m a l c r a c k i n g at the leading edge o f the scratch. T h i s k i n d o f c r a c k i n g is o f t e n due t o the cohesive properties o f the coating n e t w o r k .  Fig.4.4.3. Optical micro-graph of scratching made on CB-CSG coating, 100X, scratch moving direction from right to left  66  Interfacial f a i l u r e o f t e n presents as a c h i p p i n g o f the c o a t i n g i n f r o n t o f the scratch t i p , w h i c h is related t o failure at the interface.  T h i s type o f f a i l u r e develops  with  increasing scratch load. N o t a l l o f the c h i p p i n g is d i r e c t l y correlated w i t h the interface; some failures related t o the pores or m i c r o - c r a c k s can also be considered t o be interfacial.  Interfacial failures o f t e n manifest as a r i n g crack i n the c o a t i n g a n d lead t o c h i p p i n g , w h i c h is essentially the k i n k i n g o f a d e l a m i n a t i o n crack back i n t o and t h r o u g h the coating, as schematically s h o w n i n Fig.4.4.4. Generally, i n t e r f a c i a l f a i l u r e is the l i k e l y cause w h e n a c h i p p i n g event is observed.  Partial Cone Cracking  —^  Cohesive Failure  J  Conformal Cracking  Chipping  Interfacial failure  Fig.4.4.4. schematic of cohesive failure and interfacial failure  For e x a m p l e , i n - s i t u observation o f Fig.4.4.3, w h i c h w a s a C B - C S G produced b y t h e c o n v e n t i o n a l  m e t h o d , indicated that the o b v i o u s  chipping  coating event  dominated the scratching procedure, and no cone cracks o r c o n f o r m a l cracks appeared.  67  I n consequence o f this observation, the defects at the interface, e.g. fissures and f l a w s , c o u l d not be considered negligible.  Interfacial f a i l u r e d o m i n a t e d the  whole  scratching procedure, even under a l o w load. I n other w o r d s , no cone cracks were f o u n d at the t r a i l i n g edge or at the scratching head. Therefore, interfacial f a i l u r e is the d o m i n a n t mode.  Interfacial failures are due to the defects, e.g. pores and cracks, w h i c h are often generated d u r i n g h i g h temperature processing. For e x a m p l e , the h i g h e r the process temperature, the faster the water evaporation and the greater the p o r o s i t y . Therefore, l o w temperature c u r i n g is an effective w a y to prevent interfacial failure.  The scratch hardness was independent o f the surface roughness o f the coating. Literature [13] shows the linear trend o f the scratch-hardness values across the specimen obtained f r o m the e x p e r i m e n t a l data. The scratch hardness is b e t w e e n 5.5-6.0 GPa, and it is a little l o w e r than that measured b y V i c k e r s indentation because the scratch hardness measurement includes the effects o f porosity.  E v e n t h o u g h there was some hardness v a r i a t i o n across the c o a t i n g , it  still  reflected the homogeneous d i s t r i b u t i o n o f hardness over the c o a t i n g surface and thus the homogeneous d i s t r i b u t i o n o f the c h e m i c a l bonds.  68  4.5 Thermal Residual Stresses  The steel substrate is always subject to residual c o m p r e s s i o n i n a ceramic coating/metal system f o l l o w i n g c o o l - d o w n f r o m a h i g h temperature, and as a result suffers d e f o r m a t i o n . One purpose o f u s i n g the siloxane b o n d coat ( B C ) is to decrease the effect o f the residual stress to the substrate. I n this chapter, the residual stress was estimated by a " w a f e r " curvature m e t h o d . T h i s approach revealed w h e t h e r or n o t the siloxane b o n d coat c o u l d relax the residual stresses due to visco-elastic d e f o r m a t i o n o f the B C .  I t is h y p o t h e s i z e d that the i n c o r p o r a t i o n o f siloxane r e l a x e d the  interfacial  differential t h e r m a l c o n t r a c t i o n stress d u r i n g c o o l i n g f r o m process temperature t h r o u g h the visco-elastic d e f o r m a t i o n o f the siloxane. W h e n a coating/substrate c o m p o s i t e is at an elevated temperature and c o o l e d d o w n to r o o m temperature, the m i s m a t c h o f thermal expansion coefficient b e t w e e n the coating and the substrate results i n the residual stress. The d i r e c t i o n o f w a r p i n g is always the same: the c o a t i n g warps t o w a r d s the substrate, w h i c h indicates that the t h e r m a l expansion c o e f f i c i e n t o f the C B - C S G c o a t i n g is l o w e r than that o f the steel substrate.  The plots i n F i g . 4.5.1 s h o w that the bending m o m e n t increased w i t h increasing coating thickness. T h e o r i g i n a l data are listed i n the A p p e n d i x I I . I n this result, the  69  curvature radius decreased a s . c o a t i n g thickness increased, i n d i c a t i n g that the average stress increased w i t h increasing coating thickness.  20  40  60  80  120  100  CB-CSG Thickness (micron) Fig.4.5.1 The CB-CSG thickness vs. the radius of curvature  Figure 4.5.2 is a t y p i c a l interferometric result f o r the 6 0 p m t h i c k G r o u p C coating w i t h o u t siloxane b o n d coat, where the average curvature radius is 1.35 m . Figure 4.5.3 is the result f o r the bare substrate, where the measured curvature radius is 17.24 m . Figure 4.5.4 is the result f o r G r o u p A , i.e. f o r the 60 p m thickness t o p coat and the 5-10 p m siloxane B C . T h e average curvature radius i n Fig.4.5.4 is 14.47 m , m u c h larger than that o f a C B - C S G c o a t i n g o f the same thickness.  70  a o a.  X (mm)  X (mm)  Fig.4.5.2. A typical interferometric result from a Group C coating (r=1.35m)  71  Fig.4.5.3 A typical interferometric result from a bare substrate (r=17.24m)  C o n s i d e r i n g that the C B - C S G Poisson's ratio is 0.25 and that the Y o u n g ' s m o d u l u s o f siloxane as measured b y a depth-sensing i n d e n t a t i o n w a s 5.0 G P a , the calculated average residual stress f o r the G r o u p C coating i n Fig.4.5.2 is calculated u s i n g  72  equation (2.5) to be -8.0 MPa, and that for the Group A coating in Fig.4.5.4 is -0.30 MPa.  a o  E >>  -10  X (mm)  w  3.59  X (mm)  Fig.4.5.4 30pm CB-CSG coating, containing 5-10pm siloxane bond coat (r=14.47m) The residual stress for the Group A coating system was much lower than that for the Group C system, indicating that the incorporation of siloxane substantially decreased the thermally induced stress. The siloxane bond coat significantly decreases the thermal  73  stress between the CB-CSG and the substrate. The siloxane BC acts as a compliant layer, which relaxes the stress caused by the thermal expansion coefficient mismatch. The siloxane bond coat thus significantly decreases deformation of the substrate.  16 -  4T  2 -I  1  0  5  1  1  1  10  15  20  25  Siloxane Thickness (micron) Fig.4.5.5 The thickness of siloxane bond coat vs. the curvature radius  The trend of the experimental results in Fig.4.5.5 showed that, for the Group A coating, the curvature radius rapidly increased when the siloxane thickness was below -10 um. At a thickness higher than 10 um, the CR stabilized at 14-15 m, which appeared to indicate that the 10 um siloxane BC was enough to relax the residual stresses. It thus seems unnecessary to increase the siloxane thickness beyond 10 um.  74  It is not easy to c o n t r o l the thickness o f the bond-coat b y h a n d - s p r a y i n g w h e n its thickness  is b e l o w  10 p m , i n particular b e l o w  5 p m . T h e b o n d coat was  often  discontinuous w h e n it was thinner than 10 p m , since the surface roughness o f the substrate was around 5 p m . A t a thickness greater than 10 p m , h o w e v e r , the b o n d coat became a continuous layer, and the residual stress and the curvature radius tended to be stable.  Therefore, the c o m p l i a n t b o n d coat can release the energy o f t h e r m a l stress, accommodate  the  bending  severity  o f the  coating  or  substrate, and decrease  the  d e f o r m a t i o n o f the substrate. A plausible analysis shows that the natural w a r p o f the substrate d u r i n g f i r i n g does not greatly affect the f i n a l curvature radius o f the coating. Since the steel substrates are isotropic, the heat treatment and a m b i e n t c o o l i n g can also be made homogeneous, s h o w i n g i m p l i c i t l y that the w a r p i n g and d e f o r m a t i o n o f the substrate can be neglected d u r i n g the heat treatment.  I t can therefore be c o n c l u d e d that the siloxane p r i m e r decreases the t h e r m a l stress to w h i c h the substrate is subjected.  The siloxane BC acts as a c o m p l i a n t layer, grading  the thermal expansion c o e f f i c i e n t m i s m a t c h b y releasing the energy a n d r e l a x i n g the stresses.  75  4.6 Adhesion  4.6.1 Group A Adhesion  Figure 4.6.1 gives a schematic of the fracture mechanisms of the coatings Groups A to C. CB-eSG Siloxane  (a)  Substrate  CB-CSG  Siloxane  (b)  eB-C$G Substrate;  Group B -CB-CSG  (c) -Substrate  Fig.4.6.1. Schematics of fracture mechanisms in different coating structures  I n dual-layer G r o u p A , the C B - C S G ceramic layer generally provides the wear resistance, a n d the siloxane b o n d coat is able t o undergo visco-elastic d e f o r m a t i o n i n service. T h e average b o n d i n g strength o f G r o u p A is - 1 0 . 0 M P a ( 1 0 % a l u m i n a ) , a higher adhesion strength than that o f 1 0 0 % siloxane.  Figure 4.6.2 is an S E M image o f a G r o u p A coating after a n adhesion test, as schematically depicted i n Fig.4.6.1 (a). It shows that break-up seemed t o occur at the interface between the b o n d coat a n d the substrate. It appears, f r o m Fig.4.6.3, that the siloxane adhesion t o the substrate was stronger than that o f the C B - C S G t o the siloxane; but under the o p t i c a l m i c r o s c o p e , i n Fig.4.6.4, i t w a s f o u n d that the fracture occurred inside the siloxane b o n d coat and n o t at the interface between the C B - C S G and the siloxane. Figure 4.6.5 indicates that the ultimate tensile strength o f siloxane is l o w e r than its adhesion strength, because the adhesion o f siloxane is c o m p r i s e d o f t w o components, mechanical interlocks and p o l y m e r i c adhesion.  SE  20KV  50um  Fig.4.6.2. Fracture surface of Group A after an adhesion strength test  77  Fig.4.6.3. Appearance of the fracture surface generated in adhesion test  Stud GSG Coating  s o li xane: Substrate ; . I Stud  ;  I  1 Before adherence testing  After adherence testing  Fig.4.6.5. Schematic of fracture path for sample shown in Fig.4.6.4  4.6.2 Group B Adhesion The tri-layer G r o u p B ( 2 5 % alumina) h a d a n average u l t i m a t e tensile strength f o r the siloxane b o n d coat o f - 1 3 . 0 M P a . T h e b o n d i n g o f the C B - C S G p r i m e r to the sandblasted m i l d steel consisted o f mechanical interlocks a n d c h e m i c a l bonds p r o v i d e d by the a l u m i n u m phosphates.  Figure 4.6.6 shows the fracture o f a tri-layer G r o u p B c o a t i n g , w h i c h started at the weakest p o i n t o f the b o n d coat and ended at the outer surface o f the t o p coat, where the fracture was contained w i t h i n the top coat b y the b o n d coat, referred t o Fig.4.6.1 (b). T h e weakest p o i n t o f B C strength is usually the area o f lowest a l u m i n a concentration. Spurious cracks, generated b y the p u l l i n g force, were sometimes f o u n d t o occur i n the C B - C S G t o p coat, b u t they were u n l i k e l y t o penetrate into t h e sub-layer unless they occurred i n those areas where the siloxane was thinnest. F i g u r e 4.6.7 shows a crack that was prevented f r o m d e v e l o p i n g into the p r i m e r b y the b o n d coat.  79  Fig.4.6.6. SEM image of Group B fracture surface after an adhesion strength test  Fig. 4.6.7 A crack was constrained within one layer by the siloxane BC  80  The t r i - l a y e r G r o u p B coatings h a d a higher y i e l d strength than that o f the duallayer G r o u p A coatings because the strength o f the b o n d coat appears t o be dependent o n the v o l u m e percentage o f a l u m i n a , increasing w i t h v o l u m e percentage o f alumina. Compared w i t h G r o u p A , G r o u p B h a d a higher percentage o f a l u m i n a , g i v i n g i t a higher strength. H o w e v e r , t h e adhesion o f G r o u p A was as l o w as 10.0 M P a , i n d i c a t i n g that the strength o f 1 0 0 % siloxane w a s l o w e r than 10.0 M P a .  Fig.4.6.8. SEM image of Group C fracture surface after an adhesion strength test  4.6.3 Group C Adhesion Figure 4.6.8 shows a b r i t t l e fracture i n adhesion test f o r a m o n o - l a y e r C B - C S G coating ( G r o u p C ) , as schematically depicted i n Fig.4.6.1 ( c ) . T h e fracture occurred at the interface and developed a l o n g the coating, i n d i c a t i n g that the tensile strength o f C B - C S G is higher than the strength o f its adhesion to the substrate. T h e average adhesion strength o f G r o u p C w a s measured as 4 2 . 0 M P a . T h e adhesion strength increased w i t h increasing v o l u m e percentage o f siloxane [ 5 ] .  81  I n some cases, an adhesion strength as h i g h as ~ 7 0 . 0 M P a f o r G r o u p C was observed. There are several factors that affect the measurement o f p u l l - o f f adhesion strength: (1) the adhesion is l i m i t e d b y the strength o f the e p o x y glue used ( - 7 0 . 0 M P a ) ; (2) the effect o f pores i n the b r i t t l e C B - C S G coatings is sensitive to the a l i g n m e n t o f the applied force; (3) the prevalence o f crack propagation i n b r i t t l e C B - C S G coatings reduces the critical force o f detachment; ( 4 ) the adhesion strength is dependent o n the surface roughness o f the substrate. H e n c e , the actual adhesion m i g h t be h i g h e r t h a n the p u l l - o f f results f o r a certain coating/substrate c o m b i n a t i o n , since the adhesion strength c o u l d be detrimentally affected b y the above defects.  82  4.7 Potentiodynamic Evaluations of Sol-Gel Alumina Coatings  It was anticipated that the composite m u l t i - l a y e r e d c o a t i n g w o u l d i m p r o v e the corrosion resistance o f the m e t a l l i c substrate due t o : ( i ) the statistical p o s s i b i l i t y o f the number o f t h r o u g h - c o a t i n g defects (e.g. pores, cracks) decreasing w i t h increasing coating thickness; ( i i ) the decrease i n the o p p o r t u n i t y f o r t h r o u g h - p o r e f o r m a t i o n , due t o the mechanical penetration o r anchorage o f different layers; ( i i i ) the p a r t i a l f i l l i n g o f ceramic pores b y the p o l y m e r d u r i n g c u r i n g .  The results o f the electrochemical evaluation, F i g . 4 . 7 . 1 , revealed that the m u l t i layer coatings f o r m e d an effective p h y s i c a l barrier against w e t c o r r o s i o n .  There was a  siloxane layer i n d u a l - l a y e r G r o u p A and i n t r i - l a y e r G r o u p B , w h i c h increased the o h m i c resistance o f the w h o l e system, and decreased the c o r r o s i o n rate s i g n i f i c a n t l y . T r i - l a y e r G r o u p B h a d a l o w e r c o r r o s i o n current density ( 2 - 3 x 1 0 " A / m ) t h a n dual-layer G r o u p A 7  6  2  2  ( 5 - 7 x 1 0 " A / m ) , because the G r o u p B coating w a s t h i c k e r ( ~ 1 0 p m ) than the G r o u p A coating f o r the same thickness o f siloxane b o n d coat a n d C B - C S G t o p layers. T h e corrosion currents o f the coated m i l d steel were around one order o f m a g n i t u d e smaller than those o f the bare m i l d steel. M o n o - l a y e r G r o u p C h a d a higher c o r r o s i o n current ( - 1 0 " A / m ) and a smaller c o r r o s i o n resistance than Groups A a n d B because there were more  permeable  channels  i n the m o n o - l a y e r  Group  C  coating,  associated  with  p h y s i c o c h e m i c a l changes i n b o t h the siloxane and C B - C S G m a t e r i a l - e.g. shrinkage,  83  water vapor, and organics-burnout d u r i n g the c u r i n g i n b o t h the siloxane and the C B C S G material.  r -  "  3.50--!  i  ^ .  r  5  0  _ j  Current Density log l (A/cm ) 2  10  Fig.4.7.1. Polarization curves of the CSG/siloxane coating (Group A-C) on the mild steel substrates compared with that of the bare mild steel (pH=5, DI H2O)  Fig.4.7.1 shows that the dual-layer G r o u p A coating f o r m e d an e f f e c t i v e p h y s i c a l barrier against w e t c o r r o s i o n . T h e shape o f the p o l a r i z a t i o n c u r v e o f the coated substrates was n o t v e r y d i f f e r e n t f r o m that o f bare substrate: there w e r e n o t passivation regions present, w h i c h i m p l i e s that the coating indeed p r o v i d e d a p h y s i c a l barrier f o r b l o c k i n g the electrochemical c o r r o s i o n process, b u t that the electrochemical b e h a v i o r o f the substrate also affected the trend o f p o l a r i z a t i o n at this thickness.  B y p l o t t i n g the p o t e n t i a l versus the l o g a r i t h m o f the current f o r the various thicknesses o f C S G / s i l o x a n e b o n d coatings, the relationship b e t w e e n current density and coating thickness was d e t e r m i n e d f o r a G r o u p A coating system i n c l u d i n g a 5-10 u.m  84  siloxane b o n d coat, and a 10-100 u r n C B - C S G coating [ 5 ] . T h e c o r r o s i o n current density 9  2  •  decreased w i t h increasing thickness, and was constant at - 1 0 " A / c m f o r the coatings thicker than ~ 5 0 p m [ 5 ] . T h i s m i g h t be because the l i m i t e d n u m b e r o f through-pores h a d been f u l l y b l o c k e d b y c o r r o s i o n products, p r e v e n t i n g f u r t h e r transport o f o x y g e n t o the steel substrate w h e n the thickness o f C B - C S G was over 50 p m .  Table 4.7.1 Corrosion protection efficiency of different structure coatings  Structure  Group A  Group B  Dual-layer  Tri-layer  Group C Mono-layer C B - C S G  DIH 0 2  97%  99%  92%  lwt%NaCl  94%  96%  84%  The c o r r o s i o n current densities were obtained f r o m the intersection o f the anodic and cathodic T a f e l lines f o r D I water and l w t % N a C l s o l u t i o n . T h e c o r r o s i o n p r o t e c t i o n e f f i c i e n c y o f the G r o u p A c o a t i n g i n D I H2O w a s calculated t o be - 9 7 % according t o equation (2.10). T h e c o r r o s i o n e f f i c i e n c y was 9 4 % i n l w t % N a C l s o l u t i o n ( p H = 5 ) . Table 4.7.1 shows the c o r r o s i o n p r o t e c t i o n e f f i c i e n c y o f the other groups i n b o t h the D I H2O and the l w t % N a C l s o l u t i o n .  The C B - C S G / s i l o x a n e b o n d coating had a better c o r r o s i o n p r o t e c t i o n e f f i c i e n c y than the traditional n o n - s i l o x a n e C B - C S G coating i n D I H2O because the insulating siloxane decreased the p e r m e a b i l i t y o f the coating t o l i q u i d corrosive m e d i u m .  85  In NaCl solution, the siloxane bond coat offers better corrosion resistance, since the PMPS has two advantages in terms of corrosion resistance to cathode ions, e.g. OH" or Cl": (1) The hydrophobic nature of the PMPS repels water molecules being absorbed onto its surface. (2) The siloxane component separates the metallic substrate from the corrosive solution, i.e. because of the polymer barrier there is no anodic reaction caused by hydrolysis of iron ions and changing of localized pH values.  86  C H A P T E R 5: C O N C L U S I O N S  5.1 S u m m a r y o f C o a t i n g P r o p e r t i e s  A " w a r m " temperature ( 1 6 0 - 3 0 0 ° C ) process has been d e v e l o p e d to produce C B C S G coatings free o f surface cracks. It overcomes the d r a w b a c k s o f the t r a d i t i o n a l process f o r p r o d u c i n g c h e m i c a l l y bonded composite sol-gel coatings, e.g. cracks, l o w thickness, l o w hardness, and p o o r phosphate d i s t r i b u t i o n . T h e use o f m o n o a l u m i n u m phosphate leads to f e w e r phosphate cracks than the use o f p h o s p h o r i c acid. The l o w temperature ( 1 6 0 ° C ) c u r i n g alleviates the p r o b l e m s associated w i t h the t h e r m a l expansion coefficient m i s m a t c h . A  m u l t i - g u n spray technique was d e v e l o p e d to distribute the  phosphate h o m o g e n e o u s l y t h r o u g h o u t the coating. The technique results i n a crack-free c h e m i c a l l y b o n d e d c o m p o s i t e sol-gel ( C B - C S G ) a l u m i n a c o a t i n g w i t h m e d i u m hardness (6.0 GPa), moderate adhesion (42.0 M P a ) and g o o d scratch-resistance (17.0 k g f ) .  The n o v e l siloxane/ceramic f u n c t i o n a l l y gradient m u l t i l a y e r structural coatings were successfully f a b r i c a t e d u s i n g m u l t i - g u n deposition methods. I t is b e l i e v e d that m u l t i - g u n spraying is c r i t i c a l f o r the process, as it a l l o w s the d e p o s i t i o n o f an aqueous sol onto m i l d steel w i t h o u t r i s k o f coating b u c k l i n g and interfacial c o r r o s i o n . The m u l t i - g u n spraying m e t h o d disperses the C S G particles t h r o u g h o u t the siloxane a n d the phosphate throughout the C S G , w h i c h s i g n i f i c a n t l y increases the m e c h a n i c a l p e r f o r m a n c e o f the coating and distributes u n i f o r m l y the phosphate bonds t h r o u g h o u t the ceramic coating.  87  The resulting s i l o x a n e / C B - C S G m u l t i l a y e r coatings were f o u n d to be u n i f o r m , adhesive, and relatively dense.  Three groups o f coatings ( A , B, C ) were processed. F o r the G r o u p A and B coatings, the t h e r m a l l y stable siloxane-based " b o n d coat" f i l m was deposited onto the m i l d steel to protect the metal surface d u r i n g the C B - C S G processing. T h e C B - C S G " t o p coat" protected the siloxane-based b o n d coat (and the m e t a l l i c substrate) f r o m wear damage. The  siloxane-based  b o n d coat p r o v i d e d c o r r o s i o n resistance  and  damage  tolerance t h r o u g h its l o w p e r m e a b i l i t y , its g o o d adhesion, and its elasticity.  5.2 S u m m a r y o f A d h e s i o n  The adhesion o f the coatings was dependent o n the s i l o x a n e / a l u m i n a composite strength i n the m u l t i - l a y e r structure.  D u r i n g processing o f the coatings, the alpha  a l u m i n a particles w e r e m i x e d into the siloxane, w h i c h increased the strength o f the w h o l e coating system.  T h e t r a d i t i o n a l m o n o - l a y e r G r o u p C ( 4 0 p m ) h a d a 42.0 M P a adhesion  strength, a 17.0 k g scratch c r i t i c a l force, and a - 0 . 2 m i l i d a r c y p e r m e a b i l i t y to air.  The adhesion strength o f the coatings increased w i t h the surface roughness o f the substrate: the rougher the substrate surface, the higher the adhesion strength. H o w e v e r , after the surface roughness reached a value higher than 4 p m , the rate o f increase o f interfacial fracture toughness tended to become stable at 3 . 5 - 4 . 0 M P a . m  . Therefore, the  88  o p t i m u m surface roughness o f the substrate is - 4 . 0 u r n . T h e sand-blasted stainless steel substrate resulted i n a h i g h e r fracture toughness than the g r i n d e d stainless steel substrate.  5.3 Summary of Contact Measurements  The average hardness o f the m o n o - l a y e r C B - C S G ceramic coatings w a s 6.0 GPa. The m o n o a l u m i n u m phosphate ( M A P ) f o r m e d chemical bonds w i t h the C S G and converted t o b e r l i n i t e or cristobalite after 5 days o f c u r i n g at 1 6 0 ° C .  The average scratch c r i t i c a l force values f o r Groups A , B , and C were 4.7 k g , 10.3 k g and 17.0 k g respectively, and these values were dependent o n the v o l u m e percentage o f siloxane. T h e scratch critical force increased w i t h increasing  coating  thickness: the t h i c k e r the coating, the higher the c r i t i c a l force. W h e n the thickness w a s b e l o w 20 urn, the stresses w e r e considered t o be n e g l i g i b l e . T h e c r i t i c a l force increased almost linearly w i t h increasing coating thickness. F o r thickness values f r o m 2 0 - 6 0 u m , the effects o f the residual stresses c o u l d n o t be i g n o r e d , as the rate o f increase o f scratch critical force vs. thickness decreased due to the c o n t r i b u t i o n o f the residual stresses. A f t e r 60 u m , the c r i t i c a l force decreased w i t h increasing c o a t i n g thickness due t o the effect o f these stresses.  The interfacial scratch failures were due to defects, e.g. pores a n d cracks, w h i c h are often generated b y the h i g h temperature processing. T h e r e f o r e , l o w temperature  89  c u r i n g is an effective w a y t o prevent interfacial failure. T h e C B - C S G m o n o - l a y e r G r o u p C coating generally h a d g o o d scratch resistance w h e n its thickness was less than 100 p m .  5.4 Summary of Residual Stress and Potentiodynamic Evaluations  A n i m p o r t a n t m e r i t o f this w o r k is the use o f the i n o r g a n i c (siloxane) p o l y m e r b o n d coat ( p r i m e r ) at the interface between the m e t a l l i c substrate a n d the ceramic coating. I t is believed that such a coating system has been proposed and e x p l o r e d f o r the first t i m e i n this w o r k .  The " W a f e r " experiments t o determine the curvature radius s h o w that the siloxane p r i m e r can s i g n i f i c a n t l y decrease the t h e r m a l l y i n d u c e d stress i n t h e C B - C S G coating. The siloxane p r i m e r can be used as a c o m p l i a n t layer, w h i c h relaxes the stress caused b y the thermal expansion c o e f f i c i e n t m i s m a t c h .  T h e siloxane b o n d coat c a n also increase  the b o n d strength o f the c o a t i n g b y decreasing the n u m b e r o f c o a t i n g defects.  The p o t e n t i o d y n a m i c and p e r m e a b i l i t y experiments s h o w e d that the C B - C S G ceramic coatings w i t h siloxane s i g n i f i c a n t l y decreased ceramic p o r o s i t y and increased corrosion resistance o f the m i l d steel substrates. T h e c o r r o s i o n current density o f the C B C S G coating decreased w i t h increasing overall coating thickness ( > 1 0 0 p m ) , a n d f i n a l l y tended to stabilize at ~10~ A / c m . T h e influence o f the substrate decreased 9  2  with  increasing coating thickness. T h e corrosion p r o t e c t i o n e f f i c i e n c y o f t h e t r a d i t i o n a l C B C S G is - 9 2 % . M u l t i l a y e r coatings f o r m effective p h y s i c a l barriers t o enhance c o r r o s i o n  90  protection o f the m i l d steel substrate. I n particular, the i n c o r p o r a t i o n o f siloxane increases the corrosion resistance o f coated m i l d steel i n l w t % N a C l s o l u t i o n . A s u m m a r y o f the coating characteristics f o r G r o u p s A to C is g i v e n i n Table 5 . 1 .  5.5 C o n c l u s i o n s  A " w a r m " temperature process ( 1 6 0 - 3 0 0 ° C ) has been developed to m o d i f y the traditional sol-gel c o a t i n g process. It overcomes the drawbacks o f the t r a d i t i o n a l process f o r p r o d u c i n g composite sol-gel coatings, e.g. cracks, l o w thickness, l o w hardness, and poor phosphate d i s t r i b u t i o n . A m u l t i - g u n spraying technique was d e v e l o p e d to m o r e effectively distribute the phosphate t h r o u g h o u t the coating. The n e w process e m p l o y s l o w temperature c u r i n g ( 1 6 0 ° C ) , w h i c h results i n a sol-gel a l u m i n a c o a t i n g free o f surface cracks w i t h a m e d i u m hardness (6.0 GPa), a moderate adhesion (42.0 M P a ) and g o o d scratch resistance (17.0 k g f ) . T h i s process can be used to produce r e l a t i v e l y t h i c k (40-300 u m ) coatings. T h e air p e r m e a b i l i t y o f the C B - C S G c o a t i n g is - 0 . 5  m i l i d a r c y at a  thickness o f 40.0 u m . T h i s research also explains w h y C B - C S G coatings have m e d i u m hardness and g o o d scratch resistance.  A n o t h e r m e r i t o f this research is the use o f p o l y m e r i c siloxane as the b o n d coat f o r C B - C S G coatings, w h i c h acts to relax the residual stress b e t w e e n the  CB-CSG  coating and the steel substrate, and therefore decreases substrate d e f o r m a t i o n .  The  siloxane and the aqueous sol were deposited onto the m i l d steel t h r o u g h a m u l t i - g u n  91  spraying technique, w h i c h successfully prevented the coating f r o m b u c k l i n g o n the n o n o x i d i z e d m i l d steel substrate d u r i n g the heat treatment at temperatures u p t o 3 0 0 ° C .  Table.5.1. The summary of characteristics of various coatings Properties  Group A  Group B  Group C  Siloxane  Dual-layer  Tri-layer  Mono-layer  NM  Adhesion strength (MPa)  13.0  18.0  42.0  <10MPa  Scratch Critical Force (kg)  4.7  10.3  17.0  NM  Hardness (GPa)  N/A  N/A  6.24  0.12  Poisson's Ratio  N/A  N/A  0.25  0.35  Non-detectable  Non-detectable  -0.2  impermeable  Residual Stress at 40pm (MPa)  <1.0  N/A  -8.0  0  Interface Fracture Toughness (MPa.m )  N/A  N/A  3.5  NM  N/A  N/A  225.5  5.0  Protection Efficiency (DI H 0)  97%  99%  92%  NM  Protection Efficiency (1% NaCl)  94%  96%  84%  NM  Coating Structure  Air Permeability (milidarcys)  1/2  Elastic Modulus (Sonic, GPa) 2  * N M : n o t measured  92  C H A P T E R 6: F U T U R E  WORK  One m e r i t o f this w o r k was the development o f a " w a r m " temperature process f o r p r o d u c i n g a crack-free composite sol-gel a l u m i n a c o a t i n g , w h i c h includes a m u l t i - g u n spraying technique t o distribute phosphate t h r o u g h o u t the coating. Future w o r k should therefore focus o n d e v e l o p i n g a further understanding o f the issues related to the " w a r m " temperature process, to increase the adhesion, hardness and scratch resistance o f the coatings. The f o l l o w i n g subjects should be addressed:  1.  A d j u s t i n g the " w a r m " temperature process to h o m o g e n e o u s l y  distribute  the  phosphate t h r o u g h the coating. 2.  M o d i f y i n g the " w a r m " temperature process to f o r m a w e l l - c r y s t a l l i z e d phosphate a l u m i n a coating.  3.  Increasing the adhesion and the hardness o f the coatings u p t o 70 M P a and 10 G P a respectively.  4.  M o d e l i n g the b e h a v i o r o f the C B - C S G coating at h i g h temperatures.  A n o t h e r c o n t r i b u t i o n o f this w o r k is the development o f i n o r g a n i c p o l y m e r b o n d coat f o r residual stress r e l a x a t i o n and corrosion p r o t e c t i o n . H o w e v e r , there are some uncertanties i n the p r o d u c t i o n o f the siloxane b o n d coat. Since the adhesion o f the siloxane b o n d coat is dependent o n mechanical i n t e r l o c k i n g , the m e c h a n i c a l a n c h o r i n g is  93  stronger and thus the adhesion strength o f the b o n d coat is higher at points where there is greater penetration o f a l u m i n a particles.  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T e c h n o l , 124  53-60 (2000)  [137] J. S. C h e n , J. G. D u h , F. B. W u , " M i c r o h a r d n e s s and C o r r o s i o n B e h a v i o r i n CrN/Electroless N i / M i l d Steel C o m p l e x C o a t i n g , " Surf. Coat. T e c h n o l , 150  239-245  (2002)  109  Appendix I Determination of Young's modulus by Sonic Waves  An  ultrasonic  wave  tester  (GrindoSonic  MK5  "Industrial"  Instrument,  J . W . L e m m e n s , I n c . , M O , U S A ) is used t o measure the elastic m o d u l u s and Poisson's ratio according t o A S T M C - 1 2 5 9 - 0 4 .  The d y n a m i c Y o u n g ' s m o d u l u s can be obtained f r o m [ 1 1 7 - 1 1 9 ] : 10~  ,  7  E= — d  "  *AL N d 2  981  where £ ^ = m o d u l u s o f sample  2  ,  (1-1)  (GPa)  Z = d i m e n s i o n o f sample (cm) N= resonant f r e q u e n c y (Hz) ^ d e n s i t y o f sample  (g/cm ) 3  For rectangular shape substrate, shear m o d u l u s can be calculated f r o m [ 1 2 0 , 1 2 1 ] :  ALmN  2  G=  [B 1(1 +Ay]  wt  (1-2)  where I = d i m e n s i o n o f sample, /=thickness, w = w i d t h , w = m a s s , 7V=resonant frequency, A, B = e m p i r i c a l c o r r e c t i o n factors dependent o n w and /.  Poisson's ratio can be calculated b y :  ^ = T ^ - 1 2G  (1-3)  C  where:  110  vc=Poisson's ratio o f c o a t i n g . E c ^ Y o u n g ' s m o d u l u s o f coating Gc=shear m o d u l u s o f c o a t i n g  C o n s i d e r i n g that there are some pores i n the coating, the corroborated equation can be used as [ 1 2 2 ] :  (1-4)  E = E (\-aP ) 0  c  where E is the Y o u n g ' s m o d u l u s o f porous coating, E  0  f u l l y dense coating, P  c  represents the m o d u l u s o f  is the v o l u m e f r a c t i o n porosity. F o r p e r f e c t l y spherical holes the  constant (a) is the f o l l o w i n g f u n c t i o n o f coating Poisson's ratio v [ 1 2 2 ] : c  3(9 + 5v )(l-v ) a =— — — 2(7-5v ) c  c  (1-5) 1  V  c  The f o l l o w i n g m e t h o d includes testing o f uncoated substrates as a reference f r o m w h i c h the effect o f the c o a t i n g m a y be obtained and the c o a t i n g ' s m o d u l u s can be calculated [123, 124]:  1.  Measure the bare substrate's density D and thickness T  2.  Measure the coated substrate's density D .  s  s  s c  and thickness T . . s c  Coating's  density (D) can be calculated b y :  111  T  T S  rri  S—C  1  rr\  s-c  *• S-C  3.  I n p u t substrate's Poisson's ratio .  4.  Measure the bare substrate's Y o u n g ' s M o d u l u s Ed. b y 9 0 ° f l e x u r a l mode  5.  I n p u t i n i t i a l coated substrate's Poisson's ratio v/  6.  Measure the coated substrate's Y o u n g ' s M o d u l u s Ed- b y 9 0 ° f l e x u r a l m o d e  7.  Measure the coated substrate's shear M o d u l u s Gd.  8.  Calculate the coated substrate's Poisson's ratio v.  s  C  c  v =  2G  If  9.  1i  E  > 0 . 0 2 % then v,=v go to step 7  v  10. Calculate the c o a t i n g ' s m o d u l u s Ec b y : 77  11. E  c  =(-*-) E  S  T  )  —T  E  d  12. Calculate the c o a t i n g ' s shear m o d u l u s Gc b y :  T  13.  T -T G =-i-G _,+-zz -LG c  s-c  tl  r  d  *• s-c  14. Calculate the c o a t i n g ' s Poisson's ratio vc'.  y =-^~i c  2G  C  15. C h e c k e d b y the equation:  112  N' 11  , where N: l + a «( c  resonant f r e q u e n c y o f coated substrate,  1)  T„-T, No: resonant f r e q u e n c y o f bare substrate,  ~  1 s  c  , Ec'. c o a t i n g ' s elastic  m o d u l u s , Es: substrate's elastic m o d u l u s , Dc'. c o a t i n g ' s density, Ds: substrate's density. 16. F i n a l vc, Ec and Gc obtained.  N o t e : this m e t h o d shows the satisfactory results o n l y w h e n the thickness o f substrate is b e l o w 4 0 0 u m and c o a t i n g thickness is above 4 0 u m [ 1 2 5 ] .  11  Appendix II Original Data of the Radius of Curvature vs. Thickness  Table II.l CB-CSG thickness vs. curvature radius for Group C Group C  X(m)  Y(m)  (|X|+ |Y|)/2 ( m )  Average(m)  Bare substrate,  18.62  15.86  17.24  16.5 (0.63)  18.32  15.14  16.73  16.23  15.76  15.82  15.29 16.72  10±2um C B -  9.82  11.44  14.63  CSG  10.89  16.25  13.57  14.11  10.57  12.34  16.27  14.93  12.79  13.86  20±2um CB-  12.16  9.82  8.99  CSG  3.99  -19.48  7.74  2.69 12.62  18.20 11.04  8.45 8.82  5.69  0.66  3.18  3.26  3.02  3.14  2.29  4.84  3.81  -3.26  3.30  3.28  -2.07  3.95  3.01  2.09  4.03  2.06  -0.75  3.53  2.14  1.48  1.79  1.64  60±2um C B -  2.15  1.61  1.88  CSG  3.01  1.22  1.22  30±2 u m C B CSG  40±2 u r n C B CSG  1.74  0.50  1.12  2.43  1.17  1.80  80±2um C B -  1.76  2.12  1.94  CSG  0.57  1.50  1.04  2.12  1.69  1.91  1.10  1.02  1.06  100+5 u m C B -  0.73  2.64  1.69  CSG  1.48  1.45  1.47  0.67  1.27  0.97  2.09 1.50 *Values in parentheses show the standard deviation  13.6(0.95)  8.5 (0.55)  3.35 (0.31)  2.21 (0.58)  1.51 (0.39)  1.49 (0.51)  1.47 (0.35)  1.73  114  Table II.2 Siloxane Thickness vs. Curvature Radius for Group A Group A  X(m)  Y(m)  0 p m siloxane  6.51 -8.26 -3.82  (|X|+ Y | ) / 2 ( m )  Average(m)  -0.45  3.48  3.5 (0.92)  -0.63 -0.73  4.45 2.28  -3.29  4.48  3.89  9.46 21.25  11.63 6.21  12.38 13.73  13.06 8.72  .. 13.40  • -13.23  13.80  11.60  11.46 12.70  20.30  7.10  13.70  15.40  11.40  13.40  -14.13  14.27  10+2 p m  11.76  13.20  14.20 13.48  siloxane  14.00  15.22  14.61  21.20  0.75  15.98  17.06  -13.20  13.13  15±2pm  -11.11  -18.81  14.96  siloxane  -11.61  -19.99  15.80  -12.87  16.21  14.54  -10.18  14.42  12.30  20±2 p m  21.98  7.78  14.88  siloxane  16.25  9.57  12.91  20.81  8.41  14.61  5±2 p m siloxane  8±2 p m siloxane  -14.20  17.26 17.14 *Values in parentheses show the standard deviation  12.7 (0.99)  13.5 (0.63)  14.3 (1.29)  14.4(1.49)  14.9(1.76)  17.20  115  

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