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Paper V (iii) Assessing the durability of organic coatings
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Paper V (iii)
Assessing the durability of organic coatings M.J. Adams, B.J. Briscoe, A.L. Carter and P.J. Tweedale
T h i s p a p e r d e s c r i b e s an e x p e r i m e n t a l and a n a l y t i c a l s t u d y of t h e m e c h a n i c a l p r o p e r t i e s of a group of o r g a n i c c o a t i n g s . The s t u d y h a s sought t o s i m u l a t e t h e m e c h a n i c a l damage e n c o u n t e r e d i n p r a c t i c e by t h e u s e of model p o i n t c o n t a c t s . The p a p e r r e p o r t s measurements on t h e f r i c t i o n and damage c h a r a c t e r i s t i c s g e n e r a t e d by a r a n g e o f m a i n l y c o n i c a l i n d e n t o r s o r s l i d e r s of v a r y i n g i n c l u d e d a n g l e . The damage p r o d u c e d , where a p p a r e n t , h a s been s u b j e c t i v e l y a s s e s s e d . The f r i c t i o n a l d a t a have been compared w i t h t h e p r e d i c t i o n s p r o v i d e d by v a r i o u s models i n o r d e r t o i d e n t i f y t h e l i k e l y damage I n a d d i t i o n , t h e dynamic c h a r a c t e r of t h e f r i c t i o n f o r c e p r o c e s s e s and t h e i r e x t e n t . h a s been s t u d i e d i n e a r l i e r c a s e s t o i d e n t i f y t h e c o n t r i b u t i o n from f r a c t u r e and tearing. The e x p e r i m e n t a l methods d e s c r i b e d combined w i t h t h e v a r i o u s a n a l y s e s p r o v i d e a u s e f u l f i r s t o r d e r e s t i m a t e o f t h e m e c h a n i c a l d u r a b i l i t y of o r g a n i c c o a t i n g s .
1.INTRODUCTION.
Organic c o a t i n g s a r e used i n a v a r i e t y of applications i n order t o preserve t h e m e c h a n i c a l and c h e m i c a l i n t e g r i t y o f t h e s u b s t r a t e b e n e a t h . Such c o a t i n g s may b e commonly s u b j e c t e d t o a r a n g e o f m e c h a n i c a l a b u s e which i n v a r i a b l y r e d u c e s their efficiency. D e t a i l e d m e c h a n i c a l s t u d i e s of c o a t i n g s have b e e n l a r g e l y c o n f i n e d t o t h o s e f i l m s t h a t a r e r e l a t i v e l y hard and t o u g h I n g e n e r a l , t h e m a j o r a r e a of i n t e r e s t has been t h e i r r e s i s t e n c e t o d e t a c h m e n t . The most common method u s e d i n v o l v e s t h e s l i d i n g of a s t y l u s a c r o s s t h e c o a t i n g t o determine t h e c r i t i c a l detachment l o a d ; t h i s method was p o p u l a r i s e d by Weaver (1). I n c o n t r a s t , more c o m p l i a n t c o a t i n g s s u c h a s o r g a n i c s y s t e m s have r e c e i v e d c o n s i d e r a b l y l e s s a t t e n t i o n . For such s y s t e m s , t h e adhesion is o f t e n l e s s o f a problem b e c a u s e t h e thermodynamic f o r c e s a c t i n g a t t h e i n t e r f a c e a r e s i g n i f i c a n t l y augmented by the associated viscoelastic or plastic detachment work. F u r t h e r m o r e , t h e i r g r e a t e r c o m p l i a n c e means t h a t r e s i d u a l e l a s t i c s t r a i n s developed d u r i n g t h e coating process a r e generally q u i t e small compared w i t h t h e i r i n t e r f a c i a l o r i n d e e d b u l k r u p t u r e s t r a i n s . A much more s e r i o u s a r e a of c o n c e r n i s t h e i r t o l e r a n c e t o o t h e r damage, p a r t i c u l a r l y c o h e s i v e , p r o c e s s e s r e c e i v e d d u r i n g mechanical a b u s e , which would c l e a r l y a f f e c t t h e chemical o r p h y s i c a l p r o p e r t i e s . The aim o f t h e p r e s e n t s t u d y i s t o d e s c r i b e a p r e l i m i n a r y e v a l u a t i o n on t h e d u r a b i l i t y of compliant o r g a n i c c o a t i n g s u s i n g simple m e c h a n i c a l s i m u l a t i o n s . The technique used c o n s i s t e d of s c r a t c h i n g t h e c o a t i n g s w i t h c o n i c a l i n d e n t o r s of
known i n c l u d e d a n g l e w h i l s t m e a s u r i n g t h e f r i c t i o n f o r c e and a s s e s s i n g t h e damage c a u s e d . The f r i c t i o n a l work done i n t h e o r g a n i c f i l m p r o d u c e s damage and u l t i m a t e l y a t t r i t i o n o r wear o f t h e c o a t i n g . A m a j o r a d v a n t a g e of t h i s t y p e of e n d e a v o u r i s t h a t t h e r e s p o n s e t o s l i d i n g of such i s o l a t e d stress i n d e n t o r s h a s been s t u d i e d i n d e t a i l f o r b u l k s u b s t r a t e s of s i m i l a r p r o p e r t i e s . I n p a r t i c u l a r , c o n s i d e r a b l e a d v a n c e s have b e e n made r e c e n t l y w i t h r e s p e c t t o t h e b e h a v i o u r of o r g a n i c p o l y m e r s which a r e c a p a b l e o f e x h i b i t i n g a wide s p e c t r u m o f response c h a r a c t e r i s t i c s ( 2 , 3 ) . The d a t a and a n a l y s i s c o n t a i r e d in t h e p a p e r i s p r e s e n t e d and d i s c u s s e d i n t h r e e b r o a d s e c t i o n s . The a p p r o a c h developed i n each s e c t i o n r e l i e s h e a vi l y upon t h e p r e c e d e n t s d e v e l o p e d i n t h e s t u d y o f b u l k polymer f r i c t i o n and w e a r . One s e c t i o n d e a l s s p e c i f i c a l l y w i t h t h e s u b j e c t i v e e v a l u a t i o n o f t h e damage observed using i n d e n t o r s of varying i n d e n t e d cone a n g l e . The two r e m a i n i n g s e c t i o n s d e a l w i t h t h e i n t e r p r e t a t i o n of friction data. A f i r s t o r d e r a n a l y s i s o f t h e mean f r i c t i o n a l forces provides a useful b a s i s f o r e s t i m a t i n g t h e damage p r o d u c e d by t h e s l i d i n g work. A more d e t a i l e d s t u d y o f the fluctuations i n the frictional forces ( s t i c k - s l i p ) o f f e r s a means of e s t i m a t i n g t h e e x t e n t of t e a r i n g o r b r i t t l e f r a c t u r e w i t h t h e f i l m s . The c r a c k i n g o r t e a r i n g of a f i l m i s r e g a r d e d a s a p o t e n t i a l l y s e r i o u s form o f damage and h e n c e i s worthy o f s p e c i a l a t t e n t i o n . I n g e n e r a l t h e r e w i l l n o t be a c o r r e l a t i o n between t h e m a g n i t u d e of t h e f r i c t i o n and t h e e x t e n t o f damage. However w i t h i n a g i v e n c l a s s o f m a t e r i a l s of t h e s o r t s t u d i e d , a p o s i t i v e
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c o r r e l a t i o n w i l l be e x p e c t e d . The q u a l i t y of t h i s c o r r e l a t i o n i s examined f o r t h e p r e s e n t systems i n t h e concluding s e c t i o n s of t h e p a p e r . The c o a t i n g s s t u d i e d h e r e were a range of p r o p r i e t o r y co-polymers comprised of rubbery and g l a s s y b l o c k s . The a s s o c i a t i o n of t h e g l a s s y blocks l e a d s t o a continuous rubbery phase, which i s e f f e c t i v e l y c r o s s - l i n k e d . The g r o s s mechanical p r o p e r t i e s t h e r e f o r e a r e c h a r a c t e r i s t i c of e l a s t o m e r s w i t h a hardness t h a t i s depenedent on t h e r e l a t i v e p r o p o r t i o n s of t h e two block types. I t i s however important t o emphasise a t t h i s s t a g e t h a t t h e organic materials used i n t h i s s t u d y , while producing t h i n coherent f i l m s , cannot be prepared i n t h e form of m o n o l i t h i c specimens. The d e s o l u t i o n p r o c e s s e s induce very s u b s t a n t i a l shrinkage s t r a i n s which r u p t u r e t h i c k samples. I n a d d i t i o n i t i s b e l i e v e d t h a t t h e p r o c e s s i b i l i t y of t h e i n t e r f a c e has a marked i n f l u e n c e on t h e morphology of t h e m a t e r i a l and hence conveys p e c u l i a r p r o p e r t i e s t o t h e f i l m . 1 . 1 F r i c t i o n and Damaae
i n P o i n t Contacts
This s t u d y has r e l i e d h e a v i l y upon t h e p r e c e d e n t s developed t o i n t e r p r e t t h e f r i c t i o n and damage produced i n polymers during s l i d i n g , p a r t i c u l a r l y f o r t h e c a s e s i n v o l v i n g t h e use of i s o l a t e d p o i n t c o n t a c t s . A b r i e f review of t h i s s u b j e c t i n t r o d u c e s t h e approach adopted i n t h i s s t u d y . Two s o r t s of experiment a r e u s u a l l y performed t o e s t i m a t e t h e adhesion and deformation components of the f r i c t i o n a l work(2,3). I n the l a t t e r c a s e , r e l a t i v e l y t h i c k polymeric f i l m s o r bulk polymers a r e used and an attempt i s made t o d i s t i n g u i s h t h e n a t u r e of t h e d i s s i p a t i o n processes involved. I n v a r i a b l y , v i s c o e l a s t i c work i s done but t h e c o n t r i b u t i o n from t e a r i n g and p l a s t i c flow may a l s o be e s t i m a t e d . 1 . 1 . 1 U i c a t e d Contacts The understanding of t h e response of viscoelastomers t o s l i d i n g i s o l a t e d s t r e s s i n d e n t o r s i s w e l l advanced. Greenwood and T a b o r ( 4 ) have shown t h a t i n t h e c a s e of s p h e r e s o r b l u n t cones s l i d i n g a c r o s s t h e s u r f a c e of l u b r i c a t e d specimens, t h e n t h e e l a s t o m e r i s compressed a t t h e f r o n t of t h e c o n t a c t and r e l a x e s a t t h e r e a r . The a s s o c i a t e d sub-surface c y c l i c compressive and s h e a r deformations l e a d t o v i s c o e l a s t i c h y s t e r e s i s l o s s e s , which c r e a t e a f r i c t i o n a l r e s i s t a n c e t o s l i d i n g but r e s u l t s i n no permanent damage i f t h e y e i l d s t r e s s i s not exceeded. These f r i c t i o n a l work c o n t r i b u t i o n s may be computed w i t h some confidence f o r bulk samples a t l e a s t .
1 . 1 . 2 U u b r i c a t e d Contacts Schallamach ( 5 ) i n h i s c l a s s i c study i n v e s t i g a t e d u n l u b r i c a t e d s l i d i n g where t h e f r i c t i o n a l t r a c t i o n r e s u l t i n g i s much g r e a t e r . When t h e t e n s i l e s t r e s s e s developed a t t h e r e a r of t h e p o i n t c o n t a c t may r i s e s u f f i c i e n t l y t o then cause r u p t u r e . The
corresponding t e a r s observed were i n a d i r e c t i o n p e r p e n d i c u l a r t o t h e motion, o f t e n t h e r e were r a i s e d l i p s a t t h e f r o n t of t h e t e a r . Much g r e a t e r damage occured i f s h a r p cones were used; t h e e x a c t n a t u r e of t h e damage depended on t h e h a r d n e s s of t h e e l a s t o m e r and a p p l i e d normal l o a d . The t e a r i n g work c o n t r i b u t i o n s t o t h e damage were found t o be r e l a t e d t o t h e s t i c k - s l i p motion. For s o f t e l a s t o m e r s , t h e s u r f a c e s appeared a s a s e r i e s of r e l a t i v e l y widely spaced p i t s w i t h p r o t u b e r a n c e s of rubber a t t h e e x i t s of t h e cone c o n t a c t p a t c h e s . The damage was more e x t e n s i v e f o r e l a s t o m e r s w i t h an i n t e r m e d i a t e h a r d n e s s . I n t h i s c a s e , a s e r i e s of e l o n g a t e d t e a r s were formed w i t h much g r e a t e r volumes of dislodged m a t e r i a l a t t h e e x i t . For hard e l a s t o m e r s even more e x t e n s i v e damage occured i n which t h e t e a r s had c h a r a c t e r i s t i c arrow-head geometries p o i n t i n g i n t h e reverse d i r e c t i o n t o t h a t of motion. I n c r e a s i n g t h e normal l o a d l e d t o a h i g h e r volume deformation and hence g r e a t e r s p a c i n g between t h e damage d i s c o n t i n u i t i e s . A t t h e normal l o a d s used i n Schallamach's studies, the f r i c t i o n trace exhibited g r o s s s t i c k s l i p behaviour which i n c r e a s e d w i t h t h e magnitude of t h e normal l o a d . While t h i s p r o v i d e s u s e f u l d a t a , a s w i l l be d i s c u s s e d l a t e r , n o t h i n g i s known about t h e c h a r a c t e r i s t i c response a t low normal l o a d s where t h e f r i c t i o n a l f o r c e might be expected t o appear v i r t u a l l y c o n t i n u o u s . The p r e s e n t work has sought t o a d d r e s s both of t h e s e response a r e a s by t a k i n g measurements a t both h i g h and low normal l o a d s . The f r i c t i o n a l 1.1.3 c h a r a c t e r i s t i c s of t h i n o r g a n i c f i l m s s h e a r e d between hard e l a s t i c s u b s t r a t e s has been e x t e n s i v e l y s t u d i e d ( 6 ) . The experimental method u s u a l l y c o n f i n e s t h e f i l m between a s p h e r i c a l i n d e n t o r and a p l a t e . The t e c h n i q u e p r o v i d e s a measure of t h e adhesion component of t h e f r i c t i o n . I f simple a d d i v i t y of t h e two normal f r i c t i o n p r o c e s s e s , adhesion and ploughing components, i s assumed then an e s t i m a t e of t h e r e l a t i v e c o n t r i b u t i o n s of t h e two may be roughly e s t i m a t e d f o r c e r t a i n c o n t a c t s ( 7 , 8 ) . This e x e r c i s e i s undertaken i n t h i s p r e s e n t work. ,
I
1 . 1 . 4 T h e General I n t e r w r e t a t i o n of The adhesion component of f r i c t i o n when s u b t r a c t e d from t h e t o t a l f r i c t i o n p r o v i d e s a f i r s t o r d e r measure of t h e deformation components. The r e s u l t i n g deformation components may then b e i n t e r p r e t e d a s f o l l o w s . A more d e t a i l e d d e s c r i p t i o n of t h i s t y p e of a n a l y s i s i s given elsewhere ( 2 ,3 ) . The way i n which t h e c o e f f i c i e n t of f r i c t i o n v a r i e s with a t t a c k a n g l e ( 0 ' ) i s c h a r a c t e r i s t i c of t h e m a t e r i a l s behaviour and does not always l e a d t o permanent damage of t h e s u b s t r a t e . T h i s can be t h e c a s e f o r v i s c o e l a s t i c materials, a s discussed i n t h e i n t r o d u c t i o n . The c o e f f i c i e n t of f r i c t i o n ,
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141
a) NATURAL RUBBER
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OELASTIC HYSTERERSIS VISCOELASTIC TIME
'pb) LEAD
E F i g u r e 1. Schematic r e p r e s e n t a t i o n of a cone s c r a t c h i n g a v i s c o e l a s t i c body of included a n g l e 2 0 and a t t a c k a n g l e 0 ' .
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Fvep = 3 W tans tan@'
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PLASTIC PLOUGHINGTMEb .- -.
F.
The i n t e r n a l s h e a r a n g l e 8 i s a l s o shown t o represent chip formation.
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c) POLYMER w,
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BRITTLE TEARING
physt due t o h y s t e r e s i s l o s s e s was
SCOELASTIC HYSTERESIS
determined by Greenwood and Tabor4 and i s given by : -
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p:
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where a i s t h e f r a c t i o n of a p p l i e d work d i s s i p a t e d d u r i n g s l i d i n g and has a v a l u e which i s t y p i c a l l y i n t h e range 0 . 3 - 0 . 4 f o r elastomers. I f t h e flow s t r e s s of t h e m a t e r i a l i s exceeded t h e n a permanent groove i s formed by t h e cone. The f i r s t o r d e r a n a l y s i s f o r t h e p l a s t i c deformation, due t o Bowden and. Tabor ( 8 ) , l e a d s t o t h e following e x p r e s s i o n f o r t h e c o e f f i c i e n t of f r i c t i o n pp:-
pp
=
2 tan 0 ' / n
=
tan O ' / n
PLOUGING
Ftot= Fvep t F p t Fbf F i g u r e 2 . Schematic r e p r e s e n t a t i o n s of t h e f r i c t i o n t r a c e s observed f o r d i f f e r i n g m a t e r i a l s t o g e t h e r w i t h fundamental components of t h e f r i c t i o n a l work for each c a s e .
(2)
When t h e r e i s s i g n i f i c a n t recovery a s s o c i a t e d with p l a s t i c grooving than t h e corresponding v i s c o e l a s t i c - p l a s t i c c o e f f i c i e n t of f r i c t i o n pvep i s given b y ( 2 ): -
pvep
& VISCOELASTIC-PLASTIC
(3)
These modes of s l i d i n g a r e r e p r e s e n t e d by Figure 2 t o g e t h e r w i t h t h e t y p e of f r i c t i o n a l t r a c e s obseved which a r e continuous. I n t h e c a s e of g l a s s y polymers o r e l a s t o m e r s , b r i t t l e f r a c t u r e can occur e i t h e r adjacent t o t h e groove(9) o r a s t r a c t i o n c r a c k s beneath t h e s l i d i n g i n d e n t o r ( l 0 ) . I t has been shown t h a t t h e c o e f f i c i e n t of f r i c t i o n &,f f o r t h i s p r o c e s s t a k e s t h e following f o r m ( 3 ) : -
where k i s a p r o p o r t i o n a l i t y c o n s t a n t . B r i t t l e f r a c t u r e i s accompanied by a modest d e g r e e of s t i c k - s l i p a s shown schematically i n Figure 2 . Another damage mode which i s commonly observed i s c u t t i n g o r c h i p forming. I n t h i s c a s e , c r a c k p r o p a g a t i o n o c c u r s through an i n t e r n a l s h e a r p l a n e ( s e e F i g u r e 1) and t h e c o e f f i c i e n t of f r i c t i o n i s not s p e c i f i c a l l y a f u n c t i o n of t h e cone g e o m e t r y ( l 1 ) . I t i s g e n e r a l l y found t h a t b r i t t l e f r a c t u r e i s e v i d e n t f o r b l u n t e r cones and t h e r e i s a b r i t t l e ductile transition a t a c r i t i c a l cutting a n g l e which corresponds t o t h e m i n i m u m angle f o r chip formation. I t h a s been e s t a b l i s h e d f o r m a t e r i a l s e x h i b i t i n g mixed mode
I42
deformation under t h e a c t i o n of s l i d i n g point contacts, t h a t the coefficient of f r i c t i o n i s g i v e n by t h e sum o f t h e i n d i v i d u a l components c o r r e s p o n d i n g t o e a c h d e f o r m a t i o n mode ( 2 , 3 ) . T h i s was e x e m p l i f i e d b y e x p e r i m e n t s w i t h PTFE e x p o s e d t o d i f f e r i n g l e v e l s o f yi r r a d i a t i o n , which h a s t h e e f f e c t o f transforming a d u c t i l e p l a s t i c materials i n t o a b r i t t l e glasslike material with i n c r e a s i n g d o s a g e l e v e l . Thus t h e o b s e r v e d v a l u e of p i s p r o p o r t i o n a l t o t a n 0 ' e x c e p t where c h i p f o r m i n g i s o b s e r v e d . I n t h i s case, t h e r e w i l l be a marked r e d u c t i o n i n t h e g r a d i e n t o f p against tan 0' a t the c r i t i c a l cutting angle. 2 . E X P E R I M E N T A L PROCEDURES.
2 . 1 Thin Film P r e D a r a t i o n .
The p o l y m e r s were s u p p l i e d i n a n e m u l s i o n f o r m ( c o n c e n t r a t i o n c a . 1 0 % by v o l u m e ) a n d were c a s t o n t o f l o a t g l a s s m i c r o s c o p e s l i d e s . T h i n f i l m s were p r e p a r e d b y u s i n g a 1000 f o l d d i l u t i o n o f t h e o r i g i n a l s o l u t i o n i n d i s t i l l e d water f o l l o w e d b y f i l t r a t i o n t h r o u g h m i l l i p o r e membranes t o remove a l l p a r t i c u l a t e m a t t e r w i t h a s i z e o f >1.2pm. d i a . C l e a n e d f l o a t g l a s s m i c r o s c o p e s l i d e s were immersed i n t h e d i l u t e d e m u l s i o n s and withdrawn, t h e excess s o l u t i o n being allowed t o d r a i n away. The c o a t e d s l i d e s were t h e n d r i e d a t 40C f o r one h o u r f o l l o w e d by a f u r t h e r h o u r u n d e r vacuum. T h i s p r o c e d u r e p r o d u c e d u n i f o r m f i l m s of a b o u t 4 p m . t h i c k n e s s a s m e a s u r e d by a l a s e r profilometer. 2 . 2 Thick F i l m P r e D a r a t i o n . T h i c k f i l m s were c a s t u s i n g u n d i l u t e d e m u l s i o n s a n d t h e s e were c o n t a i n e d b y a s i l i c o n e rubber b a r r i e r around t h e edges of t h e f l o a t g l a s s s l i d e s . These f i l m s were d r i e d i n a s i m i l a r manner t o t h e t h i n f i l m s . T h e r e was some n o n - u n i f o r m i t y i n t h e i r t h i c k n e s s but t h e average value a s m e a s u r e d by m i c r o m e t e r was 500pm. A t t e m p t s t o p r o d u c e t h i c k e r f i l m s were unsuccessful. 2 . 3 Thin Film F r i c t i o n Measurements. I n o r d e r t o d e t e r m i n e an i n t e r f a c i a l component v a l u e o f t h e c o e f f i c i e n t o f f r i c t i o n , t h e t h i n f i l m s p r e p a r e d were sheared by a flamed g l a s s hemispherical i n d e n t o r o f r a d i u s 4.93mm. u n d e r a n a p p l i e d l o a d s i n t h e r a n g e 0 . 0 1 - 0.5N. The m e a s u r e m e n t s were c a r r i e d o u t a t ambient t e m p e r a t u r e w i t h a s l i d i n g s p e e d o f O . l S m m / s e c . The e q u i p m e n t u s e d f o r t h e s e measurements h a s been d e s r i b e d i n d e t a i l elsewhere (6) . 2 . 4 Thick F i l m S c r a t c h i n s ExDeriments.
Low l o a d s c r a t c h i n g e x p e r i m e n t s were carried o u t f o r t h e t h i c k f i l m specimens u s i n g t h e same a p p a r a t u s d e s c r i b e d i n Section 2.3, with t h e exception t h a t t h e s p h e r e u s e d t h e r e was r e p l a c e d w i t h a
s e r i e s o f c o n e s , T h e s e were f a b r i c a t e d from e i t h e r t u n g s t e n c a r b i d e o r s a p p h i r e and h a v i n g a range of i n c l u d e d a n g l e s (20) o f 4 5 , 6 0 , 9 0 , 1 0 5 , a n d 1 5 5 d e g r e e s . The s c r a t c h i n g was p e r f o r m e d a t a s l i d i n g s p e e d o f 0.15mm/sec a n d a t a n a p p l i e d l o a d o f 0.01N, w i t h t h e e x p e r i m e n t a l r e s u l t s b e i n g r e c o r d e d on a c h a r t recorder. In order t o generate stick-slip b e h a v i o u r , a T a b o r - E l d r e d g e d e v i c e was used, which i s c a p a b l e of b e i n g o p e r a t e d a t h i g h e r n o r m a l l o a d s . Here a s t e e l c o n e o f i n c l u d e d a n g l e 45 d e g r e e s was u s e d w i t h a n a p p l i e d n o r m a l l o a d o f 1N a n d a s l i d i n g s p e e d o f 0 . 2 2 m m / s e c . The m e a s u r e m e n t s were a g a i n c a r r i e d o u t a t ambient temperature and t h e f r i c t i o n / t i m e t r a c e recorded. 3.RESULTS. 3 . 1 T h i n F i l m A d h e s i o n Component Experiments. No s i g i f i c a n t t r e n d s were o b s e r v e d f o r the i n t e r f a c i a l f r i c t i o n data obtained f r o m t h e low l o a d s l i d i n g s p h e r e experiments f o r t h e various films s t u d i e d ; t h e c o e f f i c i e n t s of f r i c t i o n w e r e a l l within t h e range 0.38 & 0 . 0 7 a t t h e n o r m a l l o a d o f 0 . 0 1 N . However t h e r e were r e l a t i v e l y l a r g e v a r i a t i o n s i n t h e d a t a w h i c h was t h o u g h t t o a r i s e f r o m d i f f i c u l t i e s i n obtaining reproducible evenly t h i n films. A t t h e dilution f i n a l l y u s e d t h e r e w a s no d e t e c t a b l e s l i d i n g damage p r o d u c e d a s j u d g e d f r o m s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) examination of t h e f i l m s . P r e l i m i n a r y work u s i n g f i l m s d e r i v e d from 1 0 f o l d d i l u t i o n of t h e o r i g i n a l s o l u t i o n r e s u l t e d i n some e v i d e n c e o f t e a r i n g ; f i l m s c a s t from 100 f o l d d i l u t i o n s g a v e t r a c k s t h a t were c l e a r l y v i s b l e b u t t h i s deformation appeared t o be s o l e l y due t o ploughing p r o c e s s e s . P r e v i o u s work h a s e s t a b l i s h e d t h a t f i l m s s h o u l d be l e s s t h a n a b o u t 200nm f o r t h e f r i c t i o n t o be i n d e p e n d e n t o f t h e t h i c k n e s s ( 6 ) . T h i s a r i s e s from t h e "penumbral e f f e c t " i n which m a t e r i a l outside t h e Hertzian contact area is u n d e r s u f f i c i e n t p r e s s u r e , due t o t h e g e o m e t r y o f t h e s p h e r e , t h a t it makes a significant contribution t o the frictional traction. 3 . 2 Low Load T h i c k F i l m E x D e r i m e n t s . The f r i c t i o n t r a c e s f o r t h e s c r a t c h i n g e x p e r i m e n t s on t h i c k f i l m s a t t h e l o w e r n o r m a l l o a d o f 0.01N showed no o b v i o u s e v i d e n c e o f s t i c k - s l i p b e h a v i o u r . I t was f o u n d t h a t t h e r e were two d i s t i n c t t r e n d s i n t h e v a r i a t i o n of t h e f r i c t i o n c o e f f i c i e n t (p) w i t h t a n 0 ' d e p e n d i n g on t h e r e l a t i v e proportion of t h e rubbery and g l a s s y b l o c k s i n t h e copolymer. A t y p i c a l s e t o f d a t a a r e shown i n F i g u r e 3 f o r o n e of t h e more g l a s s y p o l y m e r s . I n g e n e r a l f o r t h i s g r o u p o f p o l y m e r s , 1.1 i n c r e a s e d g r a d u a l l y from a v a l u e o f a b o u t 0 . 5 f o r t h e b l u n t e s t cone t o about u n i t y f o r t h e s h a r p e s t . However f o r t h e more
143
r u b b e r y polymers ( t y p i c a l r e s u l t s shown i n F i g u r e 4 ) t h e f r i c t i o n was much g r e a t e r with values of about 1 . 5 f o r t h e b l u n t e r c o n e s and i n c r e a s i n g t o h i g h e r v a l u e s of u p t o c a . 4 i n some c a s e s f o r t h e sharper cones. The damage p a t t e r n s r e s u l t i n g from t h e s e e x p e r i m e n t s were examined u s i n g an SEM and were s i m i l a r i n c h a r a c t e r t o t h o s e found a t h i g h e r normal l o a d ( s e e l a t e r ) , b u t were s m a l l e r i n s c a l e . T h e f r i c t i o n t r a c e s were c o n t i n u o u s i n a l l c a s e s even though i n some i n s t a n c e s t h e s h a r p e r c o n e s c l e a r l y produced t h e c l a s s i c " a r r o w head" t e a r s ( F i g u r e 5 a ) . For t h e b l u n t e r c o n e s , t h e t e a r i n g damage was c o n s i d e r a b l y l e s s s e v e r e w i t h some e v i d e n c e of e l a s t i c - p l a s t i c p l o u g h i n g j u d g i n g by t h e p a r t i a l r e c o v e r y of t h e g r o o v e s ( F i g u r e 5 b ) . The r e c o v e r y was g r e a t e r f o r t h e more r u b b e r y p o l y m e r s . 3 . 3 H i g h e r Load Thick F i l m E x p e r i m e n t s . A t t h e h i g h e r normal l o a d of
1N t h e r e was
a g r o s s s t i c k - s l i p r e s p o n s e and i t was a l s o possible t o discern a greater g r a d a t i o n i n t h e b e h a v i o u r of t h e polymers from t h e s e e x p e r i m e n t s . F o r t h e most r u b b e r y p o l y m e r s , t h e s t i c k p h a s e s o f t h e f r i c t i o n t r a c e s t e n d e d t o be rounded and t h e d i s t a n c e between p e a k s was g r e a t e s t w i t h a v a l u e i n t h e r a n g e 18mm; f o r c o n v e n i e n c e t h i s c o m p a r a t i v e measure o f d i s t a n c e r e f e r r e d t o h e r e was t h a t r e c o r d e d on t h e c h a r t t r a c e a t a constant speed. This w i l l correspond t o a much s m a l l e r d i s p l a c e m e n t o f t h e cone on t h e t h i c k f i l m . The mean peak v a l u e of p was a l s o t h e g r e a t e s t f o r t h i s g r o u p a t a b o u t 0 . 9 . The d e f o r m a t i o n s p r o d u c e d by t h e cone were o f t h e c h a r a c t e r i s t i c a r r o w head t y p e but q u i t e e l o n g a t e d a s though c o n s i d e r a b l e e l a s t i c r e c o v e r y had t a k e n place (Figure 6 a ) . The polymers w i t h i n t e r m e d i a t e rubbery g l a s s p r o p e r t i e s e x h i b i t e d s h a r p e r and h i g h e r f r e q u e n c y s t i c k - s l i p b e h a v i o u r , where t h e mean d i s t a n c e between p e a k s on t h e c h a r t p a p e r was a b o u t 4 m m . The mean peak v a l u e o f p was n e a r l y a s g r e a t a s i n t h e p r e v i o u s group a t a b o u t 0.85. The d e f o r m a t i o n p a t t e r n was a l s o o f t h e a r r o w head t y p e b u t l e s s elongated indicating l e s s e l a s t i c recovery (Figure 6 b ) . In a d d i t i o n , t h e t e a r s were more e r r a t i c b o t h i n s i z e and s p a c i n g ; t h e y were o f t e n s u p e r i m p o s e d . The most g l a s s y group of polymers c a u s e d a v e r y uneven ' s t i c k - s l i p ' of s m a l l e r a m p l i t u d e and h i g h e r f r e q u e n c y . The s p a c i n g was i n t h e r a n g e 3.0-3.4mm. and t h e mean peak v a l u e of p was a b o u t 0 . 5 5 . The t e a r s were much s q u a t t e r and more rounded; i n some c a s e s t h e t e a r s a p p e a r e d t o merge i n t o one c o n t i n u o u s deformation zones of varying width (Figure 6 c ) .
4.ANALYSIS OF RESULTS. S c r a t c h i n g may be c h a r a c t e r i s e d by measuring t h e c o e f f i c i e n t of f r i c t i o n a s a f u n c t i o n of t h e cone a n g l e ; a c t u a l l y t h e p a r a m e t e r u s e d i s t a n @ I , where @ I i s t h e a t t a c k a n g l e which i s shown i n
1
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Figure 3. F r i c t i o n c o e f f i c i e n t a s a f u n c t i o n of t a n @ ' f o r one of t h e h a r d e r polymers.(
@ I =
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cone a t t a c k a n g l e ) .
F i g u r e 1 . T h i s e n a b l e s t h e t y p e of m a t e r i a l r e s p o n s e t o b e c h a r a c t e r i s e d and c e r t a i n o t h e r parameters such a s t h e c r ; . t i c a l c u t t i n g a n g l e which c o r r e s p o n d s t o t h e t r a n s i t i o n from d u c t i l e p l o u g h i n g t o c h i p formation t o be a s s e s s e d . 4 . 1 how J,oad S c r a t c h i n a E x D e r i m e n t s .
The c u r r e n t d a t a do n o t , on f i r s t c o n s i d e r a t i o n , seem t o c o r r e s p o n d t o t h e t r e n d s d i s c u s s e d above a n d , i n p a r t i c u l a r , t o t h e work c a r r i e d o u t on P T F E . I n t h e c a s e of PTFE, t h e i n t e r f a c i a l f r i c t i o n a l work i s s m a l l compared w i t h t h e t o t a l d e f o r m a t i o n work, and was t h e r e f o r e n e g l e c t e d . F o r t h e s y s t e m s h e r e , t h i s component of f r i c t i o n i s r e l a t i v e l y h i g h and h a s t o be i n c l u d e d . Thus i n o r d e r t o d e t e r m i n e t h e i n t r i n s i c v a r i a t i o n of p w i t h t a n 0 ' f o r t h e d e f o r m a t i o n modes o n l y , t h e i n t e r f a c i a l c o e f f i c i e n t of f r i c t i o n must b e s u b t r a c t e d from t h e o b s e r v e d v a l u e s .
144
Figure 5a. SEM photomicrograph of lower load thick film experiments. Demostrating "arrow head" tears.
Figure 6a. SEM photomicrograph of harder thick film specimen after high load experiment.
Figure 5b. SEM photomicrograph of lower load thick film experiments. Demostrating partial recovery in grooves.
Figure 6b. SEM photomicrograph of intermediate thick film specimen after high load experiment.
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F i g u r e 8 . The d a t a f o r F i g u r e 4 w i t h t h e i n t e r f a c i a l f r i c t i o n s u b t r a c t e d from t h e measured v a l u e s . ( l i n e h a s g r a d i e n t 2/n)
F i g u r e 6c. SEM p h o t o m i c r o g r a p h of s o f t e r t h i c k f i l m specimen a f t e r h i g h l o a d experiment, T h i s h a s been done assuming s i m p l e a d d i t i v i t y f o r t h e d a t a shown i n F i g u r e s 3 a n d 4 ; t h e r e s u l t i n g p l o t s a r e shown i n F i g u r e s 8 and 9 r e s p e c t i v e l y . H a r d e r P o l v m e r s : The d a t a i n F i g u r e 8 r e p r e s e n t s t h e harder polymers and i s p e r h a p s more s t r a i g h t f o r w a r d t o i n t e r p r e t . The c o e f f i c i e n t o f f r i c t i o n i n scratching experiments f o r simple glassy p o l y m e r s s u c h a s v i r g i n PTFE c a n b e a s c r i b e d t o t h e sum o f t h e h y s t e r e s i s 1.0 1
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F i g u r e 7 . The d a t a f o r F i g u r e 3 w i t h t h e i n t e r f a c i a l f r i c t i o n s u b t r a c t e d from t h e measured v a l u e s . ( l i n e h a s g r a d i e n t 2/n)
losses and v i s c o e l a s t i c - p l a s t i c d e f o r m a t i o n . Thus a c c o r d i n g t o e q u a t i o n s (1) a n d ( 3 ) t h e g r a d i e n t o f t h e d a t a i n F i g u r e 8 s h o u l d be 2 / n . T h i s l i n e i s shown a n d it c a n b e s e e n t h a t t h e d a t a a t low a t t a c k a n g l e s i s somewhat l e s s t h a n e x p e c t e d on t h i s b a s i s . A t t h e h i g h e r a t t a c k a n g l e s t h e r e i s a marked d i v e r g e n c e from t h e l i n e s i n c e p i s no l o n g e r s e n s i t i v e t o t h e a t t a c k a n g l e . The l a t t e r behaviour i s very similar t o t h a t o b s e r v e d when t h e r e i s c h i p f o r m i n g t a k i n g p l a c e , o n l y i n t h i s case t e a r i n g (an analogous p r o c e s s ) i n t e r v e n e s i n t h e p l o u g h i n g p r o c e s s . Thus it seems f o r t h e h a r d e r polymers t h a t t e a r i n g r e p r e s e n t s a n e n e r g e t i c a l l y more f a v o u a b l e p a t h f o r accommodating s l i d i n g t h a n d o e s p l a s t i c ploughing. I t was commonly o b s e r v e d f o r t h e s e harder polymers t h a t t h e c o e f f i c i e n t of f r i c t i o n a t t h e h i g h e r a t t a c k a n g l e s , was e v e n l e s s t h a n e x p e c t e d on t h e b a s i s o f h y s t e r e s i s l o s s e s ( e q u a t i o n 1) which i s t h e m a j o r component o f t h e t e a r i n g e n e r g y . However, t h e model f o r h y s t e r e s i s was b a s e d on s l i d i n g o v e r a v i s c o e l a s t i c h a l f - s p a c e . For t h e s h a r p e r c o n e s , t h e p e n e t r a t i o n w i l l be g r e a t e r and hence sub-surface deformation i n these coatings i s u n l i k e l y t o be a c o n t r i b u t o r y f a c t o r . The m a j o r h y s t e r e s i s l o s s e s w i l l a r i s e f r o m d e f o r m a t i o n a t t h e c r a c k t i p s of t h e t e a r s . This w i l l be a f u n c t i o n of t h e l e n g t h of t h e c r a c k s a n d n o t n e c e s s a r i l y t h e cone a n g l e , I n summary, f o r t h e s e h a r d e r polymers only t h e d a t a a t t h e lower c u t t i n g angles tended t o t h e expected r e s u l t i s t h e c o n s e q u e n c e o f two f a c t o r s . F i r s t l y , penetration of t h e t h i c k f i l m w i l l be markedly less f o r t h e b l u n t e r cones and hence t h e s l i d i n g c o n d i t i o n s w i l l c o r r e s p o n d more c l o s e l y t o t h e c o n d i t i o n s assumed i n t h e d e r i v a t i o n o f ( e q u a t i o n 1) . S e c o n d l y , SEM e x a m i n a t i o n showed t h a t t h e r e was c o n s i d e r a b l y l e s s t e a r i n g f o r these cones; t h a t i s p l o u g h i n g seemed t o be t h e d o m i n a n t damage p r o c e s s .
146
S o f t e r P o l y m e r s : Moving o n t o a c o n s i d e r a t i o n o f t h e more r u b b e r y p o l y m e r s , F i g u r e 7 c l e a r l y shows t h e much g r e a t e r c o e f f i c i e n t s of f r i c t i o n f o r t h e s e p o l y m e r s , which i s p a r t l y t h e r e s u l t of t h e i r g r e a t e r f r a c t u r e t o u g h n e s s . T h i s i s t h e r e s u l t of g r e a t e r h y s t e r e s i s l o s s e s a t t h e crack t i p s of t h e t e a r s . However, a n o t h e r s i g n i f i c a n t contributory factor i s t h a t generally t h e t e a r i n g was o b s e r v e d t o be n o t continuous, with t h e r e s u l t t h a t t h e c o n e s would have t o s l i d e o u t o f t h e d e p r e s s i o n once t e a r i n g was a r r e s t e d . This i s equivalent t o introducing a Coulombic t y p e i n t e r a c t i o n i n t o t h e c o n t a c t which c a n g r e a t l y i n c r e a s e s l i d i n g f r i c t i o n between two b o d i e s ( l 2 ) . The c r a c k a r r e s t a r i s e s from t h e way i n which t h e c r o s s - s e c t i o n a l a r e a of t h e t e a r increases with t h e s l i d i n g d i s t a n c e a s t h e cone g r a d u a l l y i n d e n t s f u r t h e r i n t o t h e coating. Since t h e crack resistance is proportional t o the crosss e c t i o n a l a r e a of t h e c r a c k , t h e t e a r i n g e n e r g y e v e n t u a l l y becomes g r e a t e r t h a n t h a t r e q u i r e d f o r Coulombic f a i l u r e . To a f i r s t o r d e r , t h i s i s only a f u n c t i o n of t h e cone a n g l e and n o t t h e p e n e t r a t i o n d e p t h a s t h e a d h e s i o n component o f f r i c t i o n h a s a l r e a d y been s u b t r a c t e d from t h e d a t a . For t h e b l u n t e r c o n e s , t h e d e f o r m a t i o n t r a c k s were r e l a t i v e l y continuous so t h a t t h e c o e f f i c i e n t of f r i c t i o n values represent t h e almost continuous t e a r i n g of t h e polymers, w i t h no a d d i t i o n a l Coulombic c o n t r i b u t i o n . 4 . 2 P i a h e r Load E x p e r i m e n t s ,
The d a t a from t h e h i g h e r normal l o a d t h i c k f i l m measurements s i m p l y p r o v i d e s a d d i t i o n a l e v i d e n c e o f t h e above p h y s i c a l i n t e r p r e t a t i o n s of t h e low normal l o a d t h i c k f i l m d a t a . There was a l a r g e difference i n t h e p values f o r t h e harder and s o f t e r p o l y m e r s . T h i s d i f f e r e n c e , w i t h t h e h a r d e r g l a s s i e r polymers being a b o u t a f a c t o r of two l e s s t h a t t h e s o f t e r p o l y m e r s , i s e x a c t l y what would be e x p e c t e d on t h e b a s i s of t h e c r a c k r e s i s t a n c e a r g u m e n t s p r e s e n t e d above and corresponds t o t h e observed d i f f e r e n c e a t t h e low normal l o a d . A major a d v a n t a g e o f c a r r y i n g o u t s c r a t c h i n g a t a h i g h e r normal l o a d i s t h a t it i s p o s s i b l e t o e s t i m a t e t h e t e a r i n g e n e r g y of t h e c o a t i n g . The c a l c u l a t i o n of t h e c r a c k r e s i s t a n c e would i n v o l v e d i v i d i n g t h e t e a r i n g work by t h e c r o s s - s e c t i o n a l a r e a of t h e t e a r s . The t e a r i n g work i s p r o p o r t i o n a l t o t h e a r e a u n d e r a s t i c k p h a s e a s shown i n F i g u r e 9 , a s mentioned p r e v i o u s l y t h i s c o u l d be converted i n t o t h e a b s o l u t e value of t h e t e a r i n g work from a knowledge o f t h e c o m p l i a n c e o f t h e f o r c e t r a n s d u c e r . Crude e s t i m a t e s of t h e c r a c k s u r f a c e c o u l d b e o b t a i n e d from e l e c t r o n m i c r o s c o p y . However, t h i s would p r o b a b l y r e s u l t i n an o v e r e s t i m a t e of t h e c r a c k r e s i s t a n c e s i n c e t h e f r a c t u r e process w i l l occur i n t h e s l i p p h a s e and c o r r e s p o n d s t o u n s t a b l e f r a c t u r e . The problem i s t h e n t h a t t h e energy s t o r e d i n t h e s t i c k phase i s i n e x c e s s of t h a t r e q u i r e d t o
Elastic Energy
A Figure 9.Schematic s t i c k - s l i p t r a c e f o r an e l a s t o m e r i c body, where t h e s h a d e d a r e a under t h e s t i c k phases i s proportional t o t h e t e a r i n g angle.
p r o p a g a t e c r a c k s and i s d i s s i p a t e d a s k i n e t i c energy. These c a l c u l a t i o n s were n o t c a r r i e d o u t h e r e b u t i n t e g r a t i o n of t h e discontinuous p a r t s of t h e c h a r t o u t p u t s i n d i c a t e d t h a t t h e f r a c t u r e energy o € t h e s o f t e r p o l y m e r s was a b o u t a f a c t o r o f two g r e a t e r t h a n t h e medium h a r d n e s s p o l y m e r s and a b o u t a f a c t o r o f t h r e e g r e a t e r t h a n t h e h a r d e s t polymers. This t r e n d i s e v i d e n t from t h e d e c r e a s e i n magnitude of t h e s t i c k d i s t a n c e a n d a m p l i t u d e of t h e s t i c k p h a s e w i t h i n c r e a s i n g h a r d n e s s . The g r e a t e r compliance and f r a c t u r e toughness of t h e s o f t e r p o l y m e r s a l l o w s g r e a t e r s t r a i n s t o b e s t o r e d p r i o r t o Coulombic f a i l u r e when t h e s l i p p h a s e commences. For t h e h a r d e r p o l y m e r s , t h e s l i p p h a s e i s i n i t i a t e d by f u r t h e r t e a r i n g which produces t h e o v e r l a p p i n g arrow head tears. 4 . 3 A b r a s i v e Wea r , The aim o f t h i s s t u d y was t o d e t e r m i n e t h e damage o r wear r e s i s t a n c e of r e p r e s e n t a t i v e o r g a n i c c o a t i n g s . One o f t h e more common damage r e g i m e s i s a b r a s i v e w e a r . The r a t e of a b r a s i v e wear of p o l y m e r s u s u a l l y c o r r e l a t e s q u i t e w e l l with t h e i n v e r s e of t h e i r f r a c t u r e toughness (13) . For t h e presenc system, t h i s would imply t h a t t h e a b r a s i v e wear would i n c r e a s e w i t h t h e h a r d n e s s of t h e p o l y m e r s and c o n s e q u e n t l y w i t h a r e d u c i n g c o e f f i c i e n t of f r i c t i o n . A s i m i l a r dependence was found f o r t h e PTFE s y s t e m s s t u d i e d by B r i s c o e e t a l ( 3 ) . T h i s was i n t e r p r e t e d i n t e r m s o f damage e f f i c i e n c y . In t h e c u r r e n t context, t h e Coulombic f r i c t i o n a l component of t h e s o f t e r p o l y m e r s can b e r e g a r d e d a s n o t c o n t r i b u t i n g t o t h e damage work.
147
5.CONCLUSIONS. Coatings w i t h a spectrum o f materials b e h a v i o u r c a n be o b t a i n e d b y u s i n g copolymers comprising of rubbery and g l a s s y b l o c k s , T h e damage t o l e r a n c e o f s u c h c o a t i n g s i n terms o f t h e i r r e s p o n s e t o i s o l a t e d s l i d i n g p o i n t c o n t a c t s has b e e n a s s e s s e d . T h e n a t u r e of t h e damage is analogous t o that observed for bulk elastomer s p e c i m e n s which d e p e n d s o n their hardness. A t l o w normal loads, continuous s l i d i n g contact occurs with cones and an a n a l y s i s c a n be made of t h e way i n w h i c h t h e n o n - a d h e s i v e component o f t h e c o e f f i c i e n t o f f r i c t i o n (p) v a r i e s w i t h the tangent of the attack angle (tan 0 ' ) . The r e s u l t s c l e a r l y d i s c r i m i n a t e between soft r u b b e r y polymers w i t h a h i g h f r a c t u r e toughness and harder glassier p o l y m e r s w i t h a lower f r a c t u r e t o u g h n e s s . For s l i d i n g w i t h cones a t higher normal loads, s t i c k - s l i p b e h a v i o u r i s observed. The a m p l i t u d e o f t h e s t i c k p h a s e s decreases w i t h i n c r e a s i n g h a r d n e s s of t h e p o l y m e r s d u e t o t h e r e d u c t i o n i n the f r a c t u r e toughness. I n addition, t h e d u r a t i o n of t h e s t i c k p h a s e s decreases with harness because of the reduction i n t h e c o m p l i a n c e . The f r i c t i o n a l work a s s o c i a t e d w i t h t h e s t i c k phases c a n be u s e d t o estimate t h e t e a r i n g e n e r g y , On t h e b a s i s o f t h e s e s t u d i e s w e a r e a b l e t o c o n c l u d e t h a t the three b r o a d i n v e s t i g a t i v e methods p r o v i d e a m e a n s of estimating the durability of organic f i l m s , particularly their sensitivity to f r a c t u r e damage. T h e s u b j e c t i v e SEM assessment is g e n e r a l l y c o n s i s t e n t w i t h t h e i n t e r p r e t a t i o n of b o t h t h e s t r a i n ( t a n 0 ' ) a n d load d e p e n d e n c e o f t h e f r i c t i o n and t h e e x t e n t o f t h e measured stick-slip contribution to the frictional force. References
(1) B E N J A M I N , P . a n d WEAVER, C .
' M e a s u r e m e n t of A d h e s i o n of T h i n F i l m s ' . P r o c . Roy. SOC. Lond., 1960, 163. (2) BRISCOE, B . J . , LAKCASTER, J . K . a n d EVANS, P . D . ' D u c t i l e t o B r i t t l e Transitions i n the Single Point C o n t a c t D e f o r m a t i o n a n d A b r a s i o n of y - i r r a d i a t e d PTFE i n P o i n t C o n t a c t s ' . P r o c . 1 2 t h Leeds-Lyon T r i b o l o g y Symp. ,p p 3 9 - 4 6 , B u t t e r w o r t h s , Guildford, (1985). ( 3 ) BRISCOE, B . J . , EVANS, P . D . a n d LANCASTER, J . K . ' S i n g l e P o i n t D e f o r m a t i o n a n d A b r a s i o n of y - i r r a d -iated PTFE'. J Phys. D : Appl. Phys., 2Q, 1 9 8 7 , 3 4 6 . ( 4 ) GREENWOOD, J . A . a n d TABOR, D . ' T h e F r i c t i o n o f Hard S l i d e r s o n L u b r i c a t e d R u b b e r : T h e I m p o r t a n c e of Deformation Losses'. P r o c . Phys. S O C . , 71,1 9 5 8 , 9 8 9 . ( 5 ) SCHALLAMACH, D . ' A b r a s i o n of R u b b e r b y a Needle' . J . P o l y m . S c i . , 9, 1952, 385.
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( 6 ) SCRUTON, B . P h . D T h e s i s , T r i n i t y C o l l e g e , Cambridge, ( 7 ) BRISCOE, B . J . a n d TABOR, D . ACS P r e p r . , 21, 1 9 7 6 , 1 0 . ( 8 ) BOWDEN, F . P a n d TABOR, D . ' T h e Friction and Lubrication of Solids', Clarendon P r e s s , Oxford, ( 1 9 5 0 ) . ( 9 ) LAMY, B . ' E f f e c t of B r i t t l e n e s s I n d e x and S l i d i n g S p e e d o n t h e M o r p h o l o g y of S u r f a c e S c r a t c h i n g i n A b r a s i v e o r Erosive Processes'. Tribology I n t l . , 17, 1 9 8 4 , 3 5 . (lO)BETHUNE, B . J . ' T h e S u r f a c e C r a c k i n g of G l a s s y P o l y m e r s u n d e r a S l i d i n g S p h e r i c a l I n d e n t o r ' . Mat. S c i . , 11, 1976, 199. ( l l ) T R E N T , E M . i n 'Metal C u t t i n g ' , 2 n d . Edition. Butterworths, Guildford, (1984). ADAMS, M . J . , BRISCOE, B . J a n d WEE, T K . 'The D i f f e r e n t i a l F r i c t i o n E f f e c t of K e r a t i n F i b r e s ' . T o be p u b l i s h e d i n J Phys. D . : Appl. Phys. 1969, LANCASTER, J . K . Wear, 2 2 3 - 2 3 9 , ' A b r a s i v e Wear of P o l y m e r s ' .
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2,
Paper V (iii)
Assessing the durability of organic coatings M.J. Adams, B.J. Briscoe, A.L. Carter and P.J. Tweedale
T h i s p a p e r d e s c r i b e s an e x p e r i m e n t a l and a n a l y t i c a l s t u d y of t h e m e c h a n i c a l p r o p e r t i e s of a group of o r g a n i c c o a t i n g s . The s t u d y h a s sought t o s i m u l a t e t h e m e c h a n i c a l damage e n c o u n t e r e d i n p r a c t i c e by t h e u s e of model p o i n t c o n t a c t s . The p a p e r r e p o r t s measurements on t h e f r i c t i o n and damage c h a r a c t e r i s t i c s g e n e r a t e d by a r a n g e o f m a i n l y c o n i c a l i n d e n t o r s o r s l i d e r s of v a r y i n g i n c l u d e d a n g l e . The damage p r o d u c e d , where a p p a r e n t , h a s been s u b j e c t i v e l y a s s e s s e d . The f r i c t i o n a l d a t a have been compared w i t h t h e p r e d i c t i o n s p r o v i d e d by v a r i o u s models i n o r d e r t o i d e n t i f y t h e l i k e l y damage I n a d d i t i o n , t h e dynamic c h a r a c t e r of t h e f r i c t i o n f o r c e p r o c e s s e s and t h e i r e x t e n t . h a s been s t u d i e d i n e a r l i e r c a s e s t o i d e n t i f y t h e c o n t r i b u t i o n from f r a c t u r e and tearing. The e x p e r i m e n t a l methods d e s c r i b e d combined w i t h t h e v a r i o u s a n a l y s e s p r o v i d e a u s e f u l f i r s t o r d e r e s t i m a t e o f t h e m e c h a n i c a l d u r a b i l i t y of o r g a n i c c o a t i n g s .
1.INTRODUCTION.
Organic c o a t i n g s a r e used i n a v a r i e t y of applications i n order t o preserve t h e m e c h a n i c a l and c h e m i c a l i n t e g r i t y o f t h e s u b s t r a t e b e n e a t h . Such c o a t i n g s may b e commonly s u b j e c t e d t o a r a n g e o f m e c h a n i c a l a b u s e which i n v a r i a b l y r e d u c e s their efficiency. D e t a i l e d m e c h a n i c a l s t u d i e s of c o a t i n g s have b e e n l a r g e l y c o n f i n e d t o t h o s e f i l m s t h a t a r e r e l a t i v e l y hard and t o u g h I n g e n e r a l , t h e m a j o r a r e a of i n t e r e s t has been t h e i r r e s i s t e n c e t o d e t a c h m e n t . The most common method u s e d i n v o l v e s t h e s l i d i n g of a s t y l u s a c r o s s t h e c o a t i n g t o determine t h e c r i t i c a l detachment l o a d ; t h i s method was p o p u l a r i s e d by Weaver (1). I n c o n t r a s t , more c o m p l i a n t c o a t i n g s s u c h a s o r g a n i c s y s t e m s have r e c e i v e d c o n s i d e r a b l y l e s s a t t e n t i o n . For such s y s t e m s , t h e adhesion is o f t e n l e s s o f a problem b e c a u s e t h e thermodynamic f o r c e s a c t i n g a t t h e i n t e r f a c e a r e s i g n i f i c a n t l y augmented by the associated viscoelastic or plastic detachment work. F u r t h e r m o r e , t h e i r g r e a t e r c o m p l i a n c e means t h a t r e s i d u a l e l a s t i c s t r a i n s developed d u r i n g t h e coating process a r e generally q u i t e small compared w i t h t h e i r i n t e r f a c i a l o r i n d e e d b u l k r u p t u r e s t r a i n s . A much more s e r i o u s a r e a of c o n c e r n i s t h e i r t o l e r a n c e t o o t h e r damage, p a r t i c u l a r l y c o h e s i v e , p r o c e s s e s r e c e i v e d d u r i n g mechanical a b u s e , which would c l e a r l y a f f e c t t h e chemical o r p h y s i c a l p r o p e r t i e s . The aim o f t h e p r e s e n t s t u d y i s t o d e s c r i b e a p r e l i m i n a r y e v a l u a t i o n on t h e d u r a b i l i t y of compliant o r g a n i c c o a t i n g s u s i n g simple m e c h a n i c a l s i m u l a t i o n s . The technique used c o n s i s t e d of s c r a t c h i n g t h e c o a t i n g s w i t h c o n i c a l i n d e n t o r s of
known i n c l u d e d a n g l e w h i l s t m e a s u r i n g t h e f r i c t i o n f o r c e and a s s e s s i n g t h e damage c a u s e d . The f r i c t i o n a l work done i n t h e o r g a n i c f i l m p r o d u c e s damage and u l t i m a t e l y a t t r i t i o n o r wear o f t h e c o a t i n g . A m a j o r a d v a n t a g e of t h i s t y p e of e n d e a v o u r i s t h a t t h e r e s p o n s e t o s l i d i n g of such i s o l a t e d stress i n d e n t o r s h a s been s t u d i e d i n d e t a i l f o r b u l k s u b s t r a t e s of s i m i l a r p r o p e r t i e s . I n p a r t i c u l a r , c o n s i d e r a b l e a d v a n c e s have b e e n made r e c e n t l y w i t h r e s p e c t t o t h e b e h a v i o u r of o r g a n i c p o l y m e r s which a r e c a p a b l e o f e x h i b i t i n g a wide s p e c t r u m o f response c h a r a c t e r i s t i c s ( 2 , 3 ) . The d a t a and a n a l y s i s c o n t a i r e d in t h e p a p e r i s p r e s e n t e d and d i s c u s s e d i n t h r e e b r o a d s e c t i o n s . The a p p r o a c h developed i n each s e c t i o n r e l i e s h e a vi l y upon t h e p r e c e d e n t s d e v e l o p e d i n t h e s t u d y o f b u l k polymer f r i c t i o n and w e a r . One s e c t i o n d e a l s s p e c i f i c a l l y w i t h t h e s u b j e c t i v e e v a l u a t i o n o f t h e damage observed using i n d e n t o r s of varying i n d e n t e d cone a n g l e . The two r e m a i n i n g s e c t i o n s d e a l w i t h t h e i n t e r p r e t a t i o n of friction data. A f i r s t o r d e r a n a l y s i s o f t h e mean f r i c t i o n a l forces provides a useful b a s i s f o r e s t i m a t i n g t h e damage p r o d u c e d by t h e s l i d i n g work. A more d e t a i l e d s t u d y o f the fluctuations i n the frictional forces ( s t i c k - s l i p ) o f f e r s a means of e s t i m a t i n g t h e e x t e n t of t e a r i n g o r b r i t t l e f r a c t u r e w i t h t h e f i l m s . The c r a c k i n g o r t e a r i n g of a f i l m i s r e g a r d e d a s a p o t e n t i a l l y s e r i o u s form o f damage and h e n c e i s worthy o f s p e c i a l a t t e n t i o n . I n g e n e r a l t h e r e w i l l n o t be a c o r r e l a t i o n between t h e m a g n i t u d e of t h e f r i c t i o n and t h e e x t e n t o f damage. However w i t h i n a g i v e n c l a s s o f m a t e r i a l s of t h e s o r t s t u d i e d , a p o s i t i v e
140
c o r r e l a t i o n w i l l be e x p e c t e d . The q u a l i t y of t h i s c o r r e l a t i o n i s examined f o r t h e p r e s e n t systems i n t h e concluding s e c t i o n s of t h e p a p e r . The c o a t i n g s s t u d i e d h e r e were a range of p r o p r i e t o r y co-polymers comprised of rubbery and g l a s s y b l o c k s . The a s s o c i a t i o n of t h e g l a s s y blocks l e a d s t o a continuous rubbery phase, which i s e f f e c t i v e l y c r o s s - l i n k e d . The g r o s s mechanical p r o p e r t i e s t h e r e f o r e a r e c h a r a c t e r i s t i c of e l a s t o m e r s w i t h a hardness t h a t i s depenedent on t h e r e l a t i v e p r o p o r t i o n s of t h e two block types. I t i s however important t o emphasise a t t h i s s t a g e t h a t t h e organic materials used i n t h i s s t u d y , while producing t h i n coherent f i l m s , cannot be prepared i n t h e form of m o n o l i t h i c specimens. The d e s o l u t i o n p r o c e s s e s induce very s u b s t a n t i a l shrinkage s t r a i n s which r u p t u r e t h i c k samples. I n a d d i t i o n i t i s b e l i e v e d t h a t t h e p r o c e s s i b i l i t y of t h e i n t e r f a c e has a marked i n f l u e n c e on t h e morphology of t h e m a t e r i a l and hence conveys p e c u l i a r p r o p e r t i e s t o t h e f i l m . 1 . 1 F r i c t i o n and Damaae
i n P o i n t Contacts
This s t u d y has r e l i e d h e a v i l y upon t h e p r e c e d e n t s developed t o i n t e r p r e t t h e f r i c t i o n and damage produced i n polymers during s l i d i n g , p a r t i c u l a r l y f o r t h e c a s e s i n v o l v i n g t h e use of i s o l a t e d p o i n t c o n t a c t s . A b r i e f review of t h i s s u b j e c t i n t r o d u c e s t h e approach adopted i n t h i s s t u d y . Two s o r t s of experiment a r e u s u a l l y performed t o e s t i m a t e t h e adhesion and deformation components of the f r i c t i o n a l work(2,3). I n the l a t t e r c a s e , r e l a t i v e l y t h i c k polymeric f i l m s o r bulk polymers a r e used and an attempt i s made t o d i s t i n g u i s h t h e n a t u r e of t h e d i s s i p a t i o n processes involved. I n v a r i a b l y , v i s c o e l a s t i c work i s done but t h e c o n t r i b u t i o n from t e a r i n g and p l a s t i c flow may a l s o be e s t i m a t e d . 1 . 1 . 1 U i c a t e d Contacts The understanding of t h e response of viscoelastomers t o s l i d i n g i s o l a t e d s t r e s s i n d e n t o r s i s w e l l advanced. Greenwood and T a b o r ( 4 ) have shown t h a t i n t h e c a s e of s p h e r e s o r b l u n t cones s l i d i n g a c r o s s t h e s u r f a c e of l u b r i c a t e d specimens, t h e n t h e e l a s t o m e r i s compressed a t t h e f r o n t of t h e c o n t a c t and r e l a x e s a t t h e r e a r . The a s s o c i a t e d sub-surface c y c l i c compressive and s h e a r deformations l e a d t o v i s c o e l a s t i c h y s t e r e s i s l o s s e s , which c r e a t e a f r i c t i o n a l r e s i s t a n c e t o s l i d i n g but r e s u l t s i n no permanent damage i f t h e y e i l d s t r e s s i s not exceeded. These f r i c t i o n a l work c o n t r i b u t i o n s may be computed w i t h some confidence f o r bulk samples a t l e a s t .
1 . 1 . 2 U u b r i c a t e d Contacts Schallamach ( 5 ) i n h i s c l a s s i c study i n v e s t i g a t e d u n l u b r i c a t e d s l i d i n g where t h e f r i c t i o n a l t r a c t i o n r e s u l t i n g i s much g r e a t e r . When t h e t e n s i l e s t r e s s e s developed a t t h e r e a r of t h e p o i n t c o n t a c t may r i s e s u f f i c i e n t l y t o then cause r u p t u r e . The
corresponding t e a r s observed were i n a d i r e c t i o n p e r p e n d i c u l a r t o t h e motion, o f t e n t h e r e were r a i s e d l i p s a t t h e f r o n t of t h e t e a r . Much g r e a t e r damage occured i f s h a r p cones were used; t h e e x a c t n a t u r e of t h e damage depended on t h e h a r d n e s s of t h e e l a s t o m e r and a p p l i e d normal l o a d . The t e a r i n g work c o n t r i b u t i o n s t o t h e damage were found t o be r e l a t e d t o t h e s t i c k - s l i p motion. For s o f t e l a s t o m e r s , t h e s u r f a c e s appeared a s a s e r i e s of r e l a t i v e l y widely spaced p i t s w i t h p r o t u b e r a n c e s of rubber a t t h e e x i t s of t h e cone c o n t a c t p a t c h e s . The damage was more e x t e n s i v e f o r e l a s t o m e r s w i t h an i n t e r m e d i a t e h a r d n e s s . I n t h i s c a s e , a s e r i e s of e l o n g a t e d t e a r s were formed w i t h much g r e a t e r volumes of dislodged m a t e r i a l a t t h e e x i t . For hard e l a s t o m e r s even more e x t e n s i v e damage occured i n which t h e t e a r s had c h a r a c t e r i s t i c arrow-head geometries p o i n t i n g i n t h e reverse d i r e c t i o n t o t h a t of motion. I n c r e a s i n g t h e normal l o a d l e d t o a h i g h e r volume deformation and hence g r e a t e r s p a c i n g between t h e damage d i s c o n t i n u i t i e s . A t t h e normal l o a d s used i n Schallamach's studies, the f r i c t i o n trace exhibited g r o s s s t i c k s l i p behaviour which i n c r e a s e d w i t h t h e magnitude of t h e normal l o a d . While t h i s p r o v i d e s u s e f u l d a t a , a s w i l l be d i s c u s s e d l a t e r , n o t h i n g i s known about t h e c h a r a c t e r i s t i c response a t low normal l o a d s where t h e f r i c t i o n a l f o r c e might be expected t o appear v i r t u a l l y c o n t i n u o u s . The p r e s e n t work has sought t o a d d r e s s both of t h e s e response a r e a s by t a k i n g measurements a t both h i g h and low normal l o a d s . The f r i c t i o n a l 1.1.3 c h a r a c t e r i s t i c s of t h i n o r g a n i c f i l m s s h e a r e d between hard e l a s t i c s u b s t r a t e s has been e x t e n s i v e l y s t u d i e d ( 6 ) . The experimental method u s u a l l y c o n f i n e s t h e f i l m between a s p h e r i c a l i n d e n t o r and a p l a t e . The t e c h n i q u e p r o v i d e s a measure of t h e adhesion component of t h e f r i c t i o n . I f simple a d d i v i t y of t h e two normal f r i c t i o n p r o c e s s e s , adhesion and ploughing components, i s assumed then an e s t i m a t e of t h e r e l a t i v e c o n t r i b u t i o n s of t h e two may be roughly e s t i m a t e d f o r c e r t a i n c o n t a c t s ( 7 , 8 ) . This e x e r c i s e i s undertaken i n t h i s p r e s e n t work. ,
I
1 . 1 . 4 T h e General I n t e r w r e t a t i o n of The adhesion component of f r i c t i o n when s u b t r a c t e d from t h e t o t a l f r i c t i o n p r o v i d e s a f i r s t o r d e r measure of t h e deformation components. The r e s u l t i n g deformation components may then b e i n t e r p r e t e d a s f o l l o w s . A more d e t a i l e d d e s c r i p t i o n of t h i s t y p e of a n a l y s i s i s given elsewhere ( 2 ,3 ) . The way i n which t h e c o e f f i c i e n t of f r i c t i o n v a r i e s with a t t a c k a n g l e ( 0 ' ) i s c h a r a c t e r i s t i c of t h e m a t e r i a l s behaviour and does not always l e a d t o permanent damage of t h e s u b s t r a t e . T h i s can be t h e c a s e f o r v i s c o e l a s t i c materials, a s discussed i n t h e i n t r o d u c t i o n . The c o e f f i c i e n t of f r i c t i o n ,
I
141
a) NATURAL RUBBER
$1 ( A
a
E
=
=
OELASTIC HYSTERERSIS VISCOELASTIC TIME
'pb) LEAD
E F i g u r e 1. Schematic r e p r e s e n t a t i o n of a cone s c r a t c h i n g a v i s c o e l a s t i c body of included a n g l e 2 0 and a t t a c k a n g l e 0 ' .
zI2
Fvep = 3 W tans tan@'
$-F
PLASTIC PLOUGHINGTMEb .- -.
F.
The i n t e r n a l s h e a r a n g l e 8 i s a l s o shown t o represent chip formation.
\k F
F
c) POLYMER w,
W2 tan
p 7 r
0
1
BRITTLE TEARING
physt due t o h y s t e r e s i s l o s s e s was
SCOELASTIC HYSTERESIS
determined by Greenwood and Tabor4 and i s given by : -
E
p:
Ll
where a i s t h e f r a c t i o n of a p p l i e d work d i s s i p a t e d d u r i n g s l i d i n g and has a v a l u e which i s t y p i c a l l y i n t h e range 0 . 3 - 0 . 4 f o r elastomers. I f t h e flow s t r e s s of t h e m a t e r i a l i s exceeded t h e n a permanent groove i s formed by t h e cone. The f i r s t o r d e r a n a l y s i s f o r t h e p l a s t i c deformation, due t o Bowden and. Tabor ( 8 ) , l e a d s t o t h e following e x p r e s s i o n f o r t h e c o e f f i c i e n t of f r i c t i o n pp:-
pp
=
2 tan 0 ' / n
=
tan O ' / n
PLOUGING
Ftot= Fvep t F p t Fbf F i g u r e 2 . Schematic r e p r e s e n t a t i o n s of t h e f r i c t i o n t r a c e s observed f o r d i f f e r i n g m a t e r i a l s t o g e t h e r w i t h fundamental components of t h e f r i c t i o n a l work for each c a s e .
(2)
When t h e r e i s s i g n i f i c a n t recovery a s s o c i a t e d with p l a s t i c grooving than t h e corresponding v i s c o e l a s t i c - p l a s t i c c o e f f i c i e n t of f r i c t i o n pvep i s given b y ( 2 ): -
pvep
& VISCOELASTIC-PLASTIC
(3)
These modes of s l i d i n g a r e r e p r e s e n t e d by Figure 2 t o g e t h e r w i t h t h e t y p e of f r i c t i o n a l t r a c e s obseved which a r e continuous. I n t h e c a s e of g l a s s y polymers o r e l a s t o m e r s , b r i t t l e f r a c t u r e can occur e i t h e r adjacent t o t h e groove(9) o r a s t r a c t i o n c r a c k s beneath t h e s l i d i n g i n d e n t o r ( l 0 ) . I t has been shown t h a t t h e c o e f f i c i e n t of f r i c t i o n &,f f o r t h i s p r o c e s s t a k e s t h e following f o r m ( 3 ) : -
where k i s a p r o p o r t i o n a l i t y c o n s t a n t . B r i t t l e f r a c t u r e i s accompanied by a modest d e g r e e of s t i c k - s l i p a s shown schematically i n Figure 2 . Another damage mode which i s commonly observed i s c u t t i n g o r c h i p forming. I n t h i s c a s e , c r a c k p r o p a g a t i o n o c c u r s through an i n t e r n a l s h e a r p l a n e ( s e e F i g u r e 1) and t h e c o e f f i c i e n t of f r i c t i o n i s not s p e c i f i c a l l y a f u n c t i o n of t h e cone g e o m e t r y ( l 1 ) . I t i s g e n e r a l l y found t h a t b r i t t l e f r a c t u r e i s e v i d e n t f o r b l u n t e r cones and t h e r e i s a b r i t t l e ductile transition a t a c r i t i c a l cutting a n g l e which corresponds t o t h e m i n i m u m angle f o r chip formation. I t h a s been e s t a b l i s h e d f o r m a t e r i a l s e x h i b i t i n g mixed mode
I42
deformation under t h e a c t i o n of s l i d i n g point contacts, t h a t the coefficient of f r i c t i o n i s g i v e n by t h e sum o f t h e i n d i v i d u a l components c o r r e s p o n d i n g t o e a c h d e f o r m a t i o n mode ( 2 , 3 ) . T h i s was e x e m p l i f i e d b y e x p e r i m e n t s w i t h PTFE e x p o s e d t o d i f f e r i n g l e v e l s o f yi r r a d i a t i o n , which h a s t h e e f f e c t o f transforming a d u c t i l e p l a s t i c materials i n t o a b r i t t l e glasslike material with i n c r e a s i n g d o s a g e l e v e l . Thus t h e o b s e r v e d v a l u e of p i s p r o p o r t i o n a l t o t a n 0 ' e x c e p t where c h i p f o r m i n g i s o b s e r v e d . I n t h i s case, t h e r e w i l l be a marked r e d u c t i o n i n t h e g r a d i e n t o f p against tan 0' a t the c r i t i c a l cutting angle. 2 . E X P E R I M E N T A L PROCEDURES.
2 . 1 Thin Film P r e D a r a t i o n .
The p o l y m e r s were s u p p l i e d i n a n e m u l s i o n f o r m ( c o n c e n t r a t i o n c a . 1 0 % by v o l u m e ) a n d were c a s t o n t o f l o a t g l a s s m i c r o s c o p e s l i d e s . T h i n f i l m s were p r e p a r e d b y u s i n g a 1000 f o l d d i l u t i o n o f t h e o r i g i n a l s o l u t i o n i n d i s t i l l e d water f o l l o w e d b y f i l t r a t i o n t h r o u g h m i l l i p o r e membranes t o remove a l l p a r t i c u l a t e m a t t e r w i t h a s i z e o f >1.2pm. d i a . C l e a n e d f l o a t g l a s s m i c r o s c o p e s l i d e s were immersed i n t h e d i l u t e d e m u l s i o n s and withdrawn, t h e excess s o l u t i o n being allowed t o d r a i n away. The c o a t e d s l i d e s were t h e n d r i e d a t 40C f o r one h o u r f o l l o w e d by a f u r t h e r h o u r u n d e r vacuum. T h i s p r o c e d u r e p r o d u c e d u n i f o r m f i l m s of a b o u t 4 p m . t h i c k n e s s a s m e a s u r e d by a l a s e r profilometer. 2 . 2 Thick F i l m P r e D a r a t i o n . T h i c k f i l m s were c a s t u s i n g u n d i l u t e d e m u l s i o n s a n d t h e s e were c o n t a i n e d b y a s i l i c o n e rubber b a r r i e r around t h e edges of t h e f l o a t g l a s s s l i d e s . These f i l m s were d r i e d i n a s i m i l a r manner t o t h e t h i n f i l m s . T h e r e was some n o n - u n i f o r m i t y i n t h e i r t h i c k n e s s but t h e average value a s m e a s u r e d by m i c r o m e t e r was 500pm. A t t e m p t s t o p r o d u c e t h i c k e r f i l m s were unsuccessful. 2 . 3 Thin Film F r i c t i o n Measurements. I n o r d e r t o d e t e r m i n e an i n t e r f a c i a l component v a l u e o f t h e c o e f f i c i e n t o f f r i c t i o n , t h e t h i n f i l m s p r e p a r e d were sheared by a flamed g l a s s hemispherical i n d e n t o r o f r a d i u s 4.93mm. u n d e r a n a p p l i e d l o a d s i n t h e r a n g e 0 . 0 1 - 0.5N. The m e a s u r e m e n t s were c a r r i e d o u t a t ambient t e m p e r a t u r e w i t h a s l i d i n g s p e e d o f O . l S m m / s e c . The e q u i p m e n t u s e d f o r t h e s e measurements h a s been d e s r i b e d i n d e t a i l elsewhere (6) . 2 . 4 Thick F i l m S c r a t c h i n s ExDeriments.
Low l o a d s c r a t c h i n g e x p e r i m e n t s were carried o u t f o r t h e t h i c k f i l m specimens u s i n g t h e same a p p a r a t u s d e s c r i b e d i n Section 2.3, with t h e exception t h a t t h e s p h e r e u s e d t h e r e was r e p l a c e d w i t h a
s e r i e s o f c o n e s , T h e s e were f a b r i c a t e d from e i t h e r t u n g s t e n c a r b i d e o r s a p p h i r e and h a v i n g a range of i n c l u d e d a n g l e s (20) o f 4 5 , 6 0 , 9 0 , 1 0 5 , a n d 1 5 5 d e g r e e s . The s c r a t c h i n g was p e r f o r m e d a t a s l i d i n g s p e e d o f 0.15mm/sec a n d a t a n a p p l i e d l o a d o f 0.01N, w i t h t h e e x p e r i m e n t a l r e s u l t s b e i n g r e c o r d e d on a c h a r t recorder. In order t o generate stick-slip b e h a v i o u r , a T a b o r - E l d r e d g e d e v i c e was used, which i s c a p a b l e of b e i n g o p e r a t e d a t h i g h e r n o r m a l l o a d s . Here a s t e e l c o n e o f i n c l u d e d a n g l e 45 d e g r e e s was u s e d w i t h a n a p p l i e d n o r m a l l o a d o f 1N a n d a s l i d i n g s p e e d o f 0 . 2 2 m m / s e c . The m e a s u r e m e n t s were a g a i n c a r r i e d o u t a t ambient temperature and t h e f r i c t i o n / t i m e t r a c e recorded. 3.RESULTS. 3 . 1 T h i n F i l m A d h e s i o n Component Experiments. No s i g i f i c a n t t r e n d s were o b s e r v e d f o r the i n t e r f a c i a l f r i c t i o n data obtained f r o m t h e low l o a d s l i d i n g s p h e r e experiments f o r t h e various films s t u d i e d ; t h e c o e f f i c i e n t s of f r i c t i o n w e r e a l l within t h e range 0.38 & 0 . 0 7 a t t h e n o r m a l l o a d o f 0 . 0 1 N . However t h e r e were r e l a t i v e l y l a r g e v a r i a t i o n s i n t h e d a t a w h i c h was t h o u g h t t o a r i s e f r o m d i f f i c u l t i e s i n obtaining reproducible evenly t h i n films. A t t h e dilution f i n a l l y u s e d t h e r e w a s no d e t e c t a b l e s l i d i n g damage p r o d u c e d a s j u d g e d f r o m s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) examination of t h e f i l m s . P r e l i m i n a r y work u s i n g f i l m s d e r i v e d from 1 0 f o l d d i l u t i o n of t h e o r i g i n a l s o l u t i o n r e s u l t e d i n some e v i d e n c e o f t e a r i n g ; f i l m s c a s t from 100 f o l d d i l u t i o n s g a v e t r a c k s t h a t were c l e a r l y v i s b l e b u t t h i s deformation appeared t o be s o l e l y due t o ploughing p r o c e s s e s . P r e v i o u s work h a s e s t a b l i s h e d t h a t f i l m s s h o u l d be l e s s t h a n a b o u t 200nm f o r t h e f r i c t i o n t o be i n d e p e n d e n t o f t h e t h i c k n e s s ( 6 ) . T h i s a r i s e s from t h e "penumbral e f f e c t " i n which m a t e r i a l outside t h e Hertzian contact area is u n d e r s u f f i c i e n t p r e s s u r e , due t o t h e g e o m e t r y o f t h e s p h e r e , t h a t it makes a significant contribution t o the frictional traction. 3 . 2 Low Load T h i c k F i l m E x D e r i m e n t s . The f r i c t i o n t r a c e s f o r t h e s c r a t c h i n g e x p e r i m e n t s on t h i c k f i l m s a t t h e l o w e r n o r m a l l o a d o f 0.01N showed no o b v i o u s e v i d e n c e o f s t i c k - s l i p b e h a v i o u r . I t was f o u n d t h a t t h e r e were two d i s t i n c t t r e n d s i n t h e v a r i a t i o n of t h e f r i c t i o n c o e f f i c i e n t (p) w i t h t a n 0 ' d e p e n d i n g on t h e r e l a t i v e proportion of t h e rubbery and g l a s s y b l o c k s i n t h e copolymer. A t y p i c a l s e t o f d a t a a r e shown i n F i g u r e 3 f o r o n e of t h e more g l a s s y p o l y m e r s . I n g e n e r a l f o r t h i s g r o u p o f p o l y m e r s , 1.1 i n c r e a s e d g r a d u a l l y from a v a l u e o f a b o u t 0 . 5 f o r t h e b l u n t e s t cone t o about u n i t y f o r t h e s h a r p e s t . However f o r t h e more
143
r u b b e r y polymers ( t y p i c a l r e s u l t s shown i n F i g u r e 4 ) t h e f r i c t i o n was much g r e a t e r with values of about 1 . 5 f o r t h e b l u n t e r c o n e s and i n c r e a s i n g t o h i g h e r v a l u e s of u p t o c a . 4 i n some c a s e s f o r t h e sharper cones. The damage p a t t e r n s r e s u l t i n g from t h e s e e x p e r i m e n t s were examined u s i n g an SEM and were s i m i l a r i n c h a r a c t e r t o t h o s e found a t h i g h e r normal l o a d ( s e e l a t e r ) , b u t were s m a l l e r i n s c a l e . T h e f r i c t i o n t r a c e s were c o n t i n u o u s i n a l l c a s e s even though i n some i n s t a n c e s t h e s h a r p e r c o n e s c l e a r l y produced t h e c l a s s i c " a r r o w head" t e a r s ( F i g u r e 5 a ) . For t h e b l u n t e r c o n e s , t h e t e a r i n g damage was c o n s i d e r a b l y l e s s s e v e r e w i t h some e v i d e n c e of e l a s t i c - p l a s t i c p l o u g h i n g j u d g i n g by t h e p a r t i a l r e c o v e r y of t h e g r o o v e s ( F i g u r e 5 b ) . The r e c o v e r y was g r e a t e r f o r t h e more r u b b e r y p o l y m e r s . 3 . 3 H i g h e r Load Thick F i l m E x p e r i m e n t s . A t t h e h i g h e r normal l o a d of
1N t h e r e was
a g r o s s s t i c k - s l i p r e s p o n s e and i t was a l s o possible t o discern a greater g r a d a t i o n i n t h e b e h a v i o u r of t h e polymers from t h e s e e x p e r i m e n t s . F o r t h e most r u b b e r y p o l y m e r s , t h e s t i c k p h a s e s o f t h e f r i c t i o n t r a c e s t e n d e d t o be rounded and t h e d i s t a n c e between p e a k s was g r e a t e s t w i t h a v a l u e i n t h e r a n g e 18mm; f o r c o n v e n i e n c e t h i s c o m p a r a t i v e measure o f d i s t a n c e r e f e r r e d t o h e r e was t h a t r e c o r d e d on t h e c h a r t t r a c e a t a constant speed. This w i l l correspond t o a much s m a l l e r d i s p l a c e m e n t o f t h e cone on t h e t h i c k f i l m . The mean peak v a l u e of p was a l s o t h e g r e a t e s t f o r t h i s g r o u p a t a b o u t 0 . 9 . The d e f o r m a t i o n s p r o d u c e d by t h e cone were o f t h e c h a r a c t e r i s t i c a r r o w head t y p e but q u i t e e l o n g a t e d a s though c o n s i d e r a b l e e l a s t i c r e c o v e r y had t a k e n place (Figure 6 a ) . The polymers w i t h i n t e r m e d i a t e rubbery g l a s s p r o p e r t i e s e x h i b i t e d s h a r p e r and h i g h e r f r e q u e n c y s t i c k - s l i p b e h a v i o u r , where t h e mean d i s t a n c e between p e a k s on t h e c h a r t p a p e r was a b o u t 4 m m . The mean peak v a l u e o f p was n e a r l y a s g r e a t a s i n t h e p r e v i o u s group a t a b o u t 0.85. The d e f o r m a t i o n p a t t e r n was a l s o o f t h e a r r o w head t y p e b u t l e s s elongated indicating l e s s e l a s t i c recovery (Figure 6 b ) . In a d d i t i o n , t h e t e a r s were more e r r a t i c b o t h i n s i z e and s p a c i n g ; t h e y were o f t e n s u p e r i m p o s e d . The most g l a s s y group of polymers c a u s e d a v e r y uneven ' s t i c k - s l i p ' of s m a l l e r a m p l i t u d e and h i g h e r f r e q u e n c y . The s p a c i n g was i n t h e r a n g e 3.0-3.4mm. and t h e mean peak v a l u e of p was a b o u t 0 . 5 5 . The t e a r s were much s q u a t t e r and more rounded; i n some c a s e s t h e t e a r s a p p e a r e d t o merge i n t o one c o n t i n u o u s deformation zones of varying width (Figure 6 c ) .
4.ANALYSIS OF RESULTS. S c r a t c h i n g may be c h a r a c t e r i s e d by measuring t h e c o e f f i c i e n t of f r i c t i o n a s a f u n c t i o n of t h e cone a n g l e ; a c t u a l l y t h e p a r a m e t e r u s e d i s t a n @ I , where @ I i s t h e a t t a c k a n g l e which i s shown i n
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Figure 3. F r i c t i o n c o e f f i c i e n t a s a f u n c t i o n of t a n @ ' f o r one of t h e h a r d e r polymers.(
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F i g u r e 1 . T h i s e n a b l e s t h e t y p e of m a t e r i a l r e s p o n s e t o b e c h a r a c t e r i s e d and c e r t a i n o t h e r parameters such a s t h e c r ; . t i c a l c u t t i n g a n g l e which c o r r e s p o n d s t o t h e t r a n s i t i o n from d u c t i l e p l o u g h i n g t o c h i p formation t o be a s s e s s e d . 4 . 1 how J,oad S c r a t c h i n a E x D e r i m e n t s .
The c u r r e n t d a t a do n o t , on f i r s t c o n s i d e r a t i o n , seem t o c o r r e s p o n d t o t h e t r e n d s d i s c u s s e d above a n d , i n p a r t i c u l a r , t o t h e work c a r r i e d o u t on P T F E . I n t h e c a s e of PTFE, t h e i n t e r f a c i a l f r i c t i o n a l work i s s m a l l compared w i t h t h e t o t a l d e f o r m a t i o n work, and was t h e r e f o r e n e g l e c t e d . F o r t h e s y s t e m s h e r e , t h i s component of f r i c t i o n i s r e l a t i v e l y h i g h and h a s t o be i n c l u d e d . Thus i n o r d e r t o d e t e r m i n e t h e i n t r i n s i c v a r i a t i o n of p w i t h t a n 0 ' f o r t h e d e f o r m a t i o n modes o n l y , t h e i n t e r f a c i a l c o e f f i c i e n t of f r i c t i o n must b e s u b t r a c t e d from t h e o b s e r v e d v a l u e s .
144
Figure 5a. SEM photomicrograph of lower load thick film experiments. Demostrating "arrow head" tears.
Figure 6a. SEM photomicrograph of harder thick film specimen after high load experiment.
Figure 5b. SEM photomicrograph of lower load thick film experiments. Demostrating partial recovery in grooves.
Figure 6b. SEM photomicrograph of intermediate thick film specimen after high load experiment.
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F i g u r e 8 . The d a t a f o r F i g u r e 4 w i t h t h e i n t e r f a c i a l f r i c t i o n s u b t r a c t e d from t h e measured v a l u e s . ( l i n e h a s g r a d i e n t 2/n)
F i g u r e 6c. SEM p h o t o m i c r o g r a p h of s o f t e r t h i c k f i l m specimen a f t e r h i g h l o a d experiment, T h i s h a s been done assuming s i m p l e a d d i t i v i t y f o r t h e d a t a shown i n F i g u r e s 3 a n d 4 ; t h e r e s u l t i n g p l o t s a r e shown i n F i g u r e s 8 and 9 r e s p e c t i v e l y . H a r d e r P o l v m e r s : The d a t a i n F i g u r e 8 r e p r e s e n t s t h e harder polymers and i s p e r h a p s more s t r a i g h t f o r w a r d t o i n t e r p r e t . The c o e f f i c i e n t o f f r i c t i o n i n scratching experiments f o r simple glassy p o l y m e r s s u c h a s v i r g i n PTFE c a n b e a s c r i b e d t o t h e sum o f t h e h y s t e r e s i s 1.0 1
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F i g u r e 7 . The d a t a f o r F i g u r e 3 w i t h t h e i n t e r f a c i a l f r i c t i o n s u b t r a c t e d from t h e measured v a l u e s . ( l i n e h a s g r a d i e n t 2/n)
losses and v i s c o e l a s t i c - p l a s t i c d e f o r m a t i o n . Thus a c c o r d i n g t o e q u a t i o n s (1) a n d ( 3 ) t h e g r a d i e n t o f t h e d a t a i n F i g u r e 8 s h o u l d be 2 / n . T h i s l i n e i s shown a n d it c a n b e s e e n t h a t t h e d a t a a t low a t t a c k a n g l e s i s somewhat l e s s t h a n e x p e c t e d on t h i s b a s i s . A t t h e h i g h e r a t t a c k a n g l e s t h e r e i s a marked d i v e r g e n c e from t h e l i n e s i n c e p i s no l o n g e r s e n s i t i v e t o t h e a t t a c k a n g l e . The l a t t e r behaviour i s very similar t o t h a t o b s e r v e d when t h e r e i s c h i p f o r m i n g t a k i n g p l a c e , o n l y i n t h i s case t e a r i n g (an analogous p r o c e s s ) i n t e r v e n e s i n t h e p l o u g h i n g p r o c e s s . Thus it seems f o r t h e h a r d e r polymers t h a t t e a r i n g r e p r e s e n t s a n e n e r g e t i c a l l y more f a v o u a b l e p a t h f o r accommodating s l i d i n g t h a n d o e s p l a s t i c ploughing. I t was commonly o b s e r v e d f o r t h e s e harder polymers t h a t t h e c o e f f i c i e n t of f r i c t i o n a t t h e h i g h e r a t t a c k a n g l e s , was e v e n l e s s t h a n e x p e c t e d on t h e b a s i s o f h y s t e r e s i s l o s s e s ( e q u a t i o n 1) which i s t h e m a j o r component o f t h e t e a r i n g e n e r g y . However, t h e model f o r h y s t e r e s i s was b a s e d on s l i d i n g o v e r a v i s c o e l a s t i c h a l f - s p a c e . For t h e s h a r p e r c o n e s , t h e p e n e t r a t i o n w i l l be g r e a t e r and hence sub-surface deformation i n these coatings i s u n l i k e l y t o be a c o n t r i b u t o r y f a c t o r . The m a j o r h y s t e r e s i s l o s s e s w i l l a r i s e f r o m d e f o r m a t i o n a t t h e c r a c k t i p s of t h e t e a r s . This w i l l be a f u n c t i o n of t h e l e n g t h of t h e c r a c k s a n d n o t n e c e s s a r i l y t h e cone a n g l e , I n summary, f o r t h e s e h a r d e r polymers only t h e d a t a a t t h e lower c u t t i n g angles tended t o t h e expected r e s u l t i s t h e c o n s e q u e n c e o f two f a c t o r s . F i r s t l y , penetration of t h e t h i c k f i l m w i l l be markedly less f o r t h e b l u n t e r cones and hence t h e s l i d i n g c o n d i t i o n s w i l l c o r r e s p o n d more c l o s e l y t o t h e c o n d i t i o n s assumed i n t h e d e r i v a t i o n o f ( e q u a t i o n 1) . S e c o n d l y , SEM e x a m i n a t i o n showed t h a t t h e r e was c o n s i d e r a b l y l e s s t e a r i n g f o r these cones; t h a t i s p l o u g h i n g seemed t o be t h e d o m i n a n t damage p r o c e s s .
146
S o f t e r P o l y m e r s : Moving o n t o a c o n s i d e r a t i o n o f t h e more r u b b e r y p o l y m e r s , F i g u r e 7 c l e a r l y shows t h e much g r e a t e r c o e f f i c i e n t s of f r i c t i o n f o r t h e s e p o l y m e r s , which i s p a r t l y t h e r e s u l t of t h e i r g r e a t e r f r a c t u r e t o u g h n e s s . T h i s i s t h e r e s u l t of g r e a t e r h y s t e r e s i s l o s s e s a t t h e crack t i p s of t h e t e a r s . However, a n o t h e r s i g n i f i c a n t contributory factor i s t h a t generally t h e t e a r i n g was o b s e r v e d t o be n o t continuous, with t h e r e s u l t t h a t t h e c o n e s would have t o s l i d e o u t o f t h e d e p r e s s i o n once t e a r i n g was a r r e s t e d . This i s equivalent t o introducing a Coulombic t y p e i n t e r a c t i o n i n t o t h e c o n t a c t which c a n g r e a t l y i n c r e a s e s l i d i n g f r i c t i o n between two b o d i e s ( l 2 ) . The c r a c k a r r e s t a r i s e s from t h e way i n which t h e c r o s s - s e c t i o n a l a r e a of t h e t e a r increases with t h e s l i d i n g d i s t a n c e a s t h e cone g r a d u a l l y i n d e n t s f u r t h e r i n t o t h e coating. Since t h e crack resistance is proportional t o the crosss e c t i o n a l a r e a of t h e c r a c k , t h e t e a r i n g e n e r g y e v e n t u a l l y becomes g r e a t e r t h a n t h a t r e q u i r e d f o r Coulombic f a i l u r e . To a f i r s t o r d e r , t h i s i s only a f u n c t i o n of t h e cone a n g l e and n o t t h e p e n e t r a t i o n d e p t h a s t h e a d h e s i o n component o f f r i c t i o n h a s a l r e a d y been s u b t r a c t e d from t h e d a t a . For t h e b l u n t e r c o n e s , t h e d e f o r m a t i o n t r a c k s were r e l a t i v e l y continuous so t h a t t h e c o e f f i c i e n t of f r i c t i o n values represent t h e almost continuous t e a r i n g of t h e polymers, w i t h no a d d i t i o n a l Coulombic c o n t r i b u t i o n . 4 . 2 P i a h e r Load E x p e r i m e n t s ,
The d a t a from t h e h i g h e r normal l o a d t h i c k f i l m measurements s i m p l y p r o v i d e s a d d i t i o n a l e v i d e n c e o f t h e above p h y s i c a l i n t e r p r e t a t i o n s of t h e low normal l o a d t h i c k f i l m d a t a . There was a l a r g e difference i n t h e p values f o r t h e harder and s o f t e r p o l y m e r s . T h i s d i f f e r e n c e , w i t h t h e h a r d e r g l a s s i e r polymers being a b o u t a f a c t o r of two l e s s t h a t t h e s o f t e r p o l y m e r s , i s e x a c t l y what would be e x p e c t e d on t h e b a s i s of t h e c r a c k r e s i s t a n c e a r g u m e n t s p r e s e n t e d above and corresponds t o t h e observed d i f f e r e n c e a t t h e low normal l o a d . A major a d v a n t a g e o f c a r r y i n g o u t s c r a t c h i n g a t a h i g h e r normal l o a d i s t h a t it i s p o s s i b l e t o e s t i m a t e t h e t e a r i n g e n e r g y of t h e c o a t i n g . The c a l c u l a t i o n of t h e c r a c k r e s i s t a n c e would i n v o l v e d i v i d i n g t h e t e a r i n g work by t h e c r o s s - s e c t i o n a l a r e a of t h e t e a r s . The t e a r i n g work i s p r o p o r t i o n a l t o t h e a r e a u n d e r a s t i c k p h a s e a s shown i n F i g u r e 9 , a s mentioned p r e v i o u s l y t h i s c o u l d be converted i n t o t h e a b s o l u t e value of t h e t e a r i n g work from a knowledge o f t h e c o m p l i a n c e o f t h e f o r c e t r a n s d u c e r . Crude e s t i m a t e s of t h e c r a c k s u r f a c e c o u l d b e o b t a i n e d from e l e c t r o n m i c r o s c o p y . However, t h i s would p r o b a b l y r e s u l t i n an o v e r e s t i m a t e of t h e c r a c k r e s i s t a n c e s i n c e t h e f r a c t u r e process w i l l occur i n t h e s l i p p h a s e and c o r r e s p o n d s t o u n s t a b l e f r a c t u r e . The problem i s t h e n t h a t t h e energy s t o r e d i n t h e s t i c k phase i s i n e x c e s s of t h a t r e q u i r e d t o
Elastic Energy
A Figure 9.Schematic s t i c k - s l i p t r a c e f o r an e l a s t o m e r i c body, where t h e s h a d e d a r e a under t h e s t i c k phases i s proportional t o t h e t e a r i n g angle.
p r o p a g a t e c r a c k s and i s d i s s i p a t e d a s k i n e t i c energy. These c a l c u l a t i o n s were n o t c a r r i e d o u t h e r e b u t i n t e g r a t i o n of t h e discontinuous p a r t s of t h e c h a r t o u t p u t s i n d i c a t e d t h a t t h e f r a c t u r e energy o € t h e s o f t e r p o l y m e r s was a b o u t a f a c t o r o f two g r e a t e r t h a n t h e medium h a r d n e s s p o l y m e r s and a b o u t a f a c t o r o f t h r e e g r e a t e r t h a n t h e h a r d e s t polymers. This t r e n d i s e v i d e n t from t h e d e c r e a s e i n magnitude of t h e s t i c k d i s t a n c e a n d a m p l i t u d e of t h e s t i c k p h a s e w i t h i n c r e a s i n g h a r d n e s s . The g r e a t e r compliance and f r a c t u r e toughness of t h e s o f t e r p o l y m e r s a l l o w s g r e a t e r s t r a i n s t o b e s t o r e d p r i o r t o Coulombic f a i l u r e when t h e s l i p p h a s e commences. For t h e h a r d e r p o l y m e r s , t h e s l i p p h a s e i s i n i t i a t e d by f u r t h e r t e a r i n g which produces t h e o v e r l a p p i n g arrow head tears. 4 . 3 A b r a s i v e Wea r , The aim o f t h i s s t u d y was t o d e t e r m i n e t h e damage o r wear r e s i s t a n c e of r e p r e s e n t a t i v e o r g a n i c c o a t i n g s . One o f t h e more common damage r e g i m e s i s a b r a s i v e w e a r . The r a t e of a b r a s i v e wear of p o l y m e r s u s u a l l y c o r r e l a t e s q u i t e w e l l with t h e i n v e r s e of t h e i r f r a c t u r e toughness (13) . For t h e presenc system, t h i s would imply t h a t t h e a b r a s i v e wear would i n c r e a s e w i t h t h e h a r d n e s s of t h e p o l y m e r s and c o n s e q u e n t l y w i t h a r e d u c i n g c o e f f i c i e n t of f r i c t i o n . A s i m i l a r dependence was found f o r t h e PTFE s y s t e m s s t u d i e d by B r i s c o e e t a l ( 3 ) . T h i s was i n t e r p r e t e d i n t e r m s o f damage e f f i c i e n c y . In t h e c u r r e n t context, t h e Coulombic f r i c t i o n a l component of t h e s o f t e r p o l y m e r s can b e r e g a r d e d a s n o t c o n t r i b u t i n g t o t h e damage work.
147
5.CONCLUSIONS. Coatings w i t h a spectrum o f materials b e h a v i o u r c a n be o b t a i n e d b y u s i n g copolymers comprising of rubbery and g l a s s y b l o c k s , T h e damage t o l e r a n c e o f s u c h c o a t i n g s i n terms o f t h e i r r e s p o n s e t o i s o l a t e d s l i d i n g p o i n t c o n t a c t s has b e e n a s s e s s e d . T h e n a t u r e of t h e damage is analogous t o that observed for bulk elastomer s p e c i m e n s which d e p e n d s o n their hardness. A t l o w normal loads, continuous s l i d i n g contact occurs with cones and an a n a l y s i s c a n be made of t h e way i n w h i c h t h e n o n - a d h e s i v e component o f t h e c o e f f i c i e n t o f f r i c t i o n (p) v a r i e s w i t h the tangent of the attack angle (tan 0 ' ) . The r e s u l t s c l e a r l y d i s c r i m i n a t e between soft r u b b e r y polymers w i t h a h i g h f r a c t u r e toughness and harder glassier p o l y m e r s w i t h a lower f r a c t u r e t o u g h n e s s . For s l i d i n g w i t h cones a t higher normal loads, s t i c k - s l i p b e h a v i o u r i s observed. The a m p l i t u d e o f t h e s t i c k p h a s e s decreases w i t h i n c r e a s i n g h a r d n e s s of t h e p o l y m e r s d u e t o t h e r e d u c t i o n i n the f r a c t u r e toughness. I n addition, t h e d u r a t i o n of t h e s t i c k p h a s e s decreases with harness because of the reduction i n t h e c o m p l i a n c e . The f r i c t i o n a l work a s s o c i a t e d w i t h t h e s t i c k phases c a n be u s e d t o estimate t h e t e a r i n g e n e r g y , On t h e b a s i s o f t h e s e s t u d i e s w e a r e a b l e t o c o n c l u d e t h a t the three b r o a d i n v e s t i g a t i v e methods p r o v i d e a m e a n s of estimating the durability of organic f i l m s , particularly their sensitivity to f r a c t u r e damage. T h e s u b j e c t i v e SEM assessment is g e n e r a l l y c o n s i s t e n t w i t h t h e i n t e r p r e t a t i o n of b o t h t h e s t r a i n ( t a n 0 ' ) a n d load d e p e n d e n c e o f t h e f r i c t i o n and t h e e x t e n t o f t h e measured stick-slip contribution to the frictional force. References
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' M e a s u r e m e n t of A d h e s i o n of T h i n F i l m s ' . P r o c . Roy. SOC. Lond., 1960, 163. (2) BRISCOE, B . J . , LAKCASTER, J . K . a n d EVANS, P . D . ' D u c t i l e t o B r i t t l e Transitions i n the Single Point C o n t a c t D e f o r m a t i o n a n d A b r a s i o n of y - i r r a d i a t e d PTFE i n P o i n t C o n t a c t s ' . P r o c . 1 2 t h Leeds-Lyon T r i b o l o g y Symp. ,p p 3 9 - 4 6 , B u t t e r w o r t h s , Guildford, (1985). ( 3 ) BRISCOE, B . J . , EVANS, P . D . a n d LANCASTER, J . K . ' S i n g l e P o i n t D e f o r m a t i o n a n d A b r a s i o n of y - i r r a d -iated PTFE'. J Phys. D : Appl. Phys., 2Q, 1 9 8 7 , 3 4 6 . ( 4 ) GREENWOOD, J . A . a n d TABOR, D . ' T h e F r i c t i o n o f Hard S l i d e r s o n L u b r i c a t e d R u b b e r : T h e I m p o r t a n c e of Deformation Losses'. P r o c . Phys. S O C . , 71,1 9 5 8 , 9 8 9 . ( 5 ) SCHALLAMACH, D . ' A b r a s i o n of R u b b e r b y a Needle' . J . P o l y m . S c i . , 9, 1952, 385.
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( 6 ) SCRUTON, B . P h . D T h e s i s , T r i n i t y C o l l e g e , Cambridge, ( 7 ) BRISCOE, B . J . a n d TABOR, D . ACS P r e p r . , 21, 1 9 7 6 , 1 0 . ( 8 ) BOWDEN, F . P a n d TABOR, D . ' T h e Friction and Lubrication of Solids', Clarendon P r e s s , Oxford, ( 1 9 5 0 ) . ( 9 ) LAMY, B . ' E f f e c t of B r i t t l e n e s s I n d e x and S l i d i n g S p e e d o n t h e M o r p h o l o g y of S u r f a c e S c r a t c h i n g i n A b r a s i v e o r Erosive Processes'. Tribology I n t l . , 17, 1 9 8 4 , 3 5 . (lO)BETHUNE, B . J . ' T h e S u r f a c e C r a c k i n g of G l a s s y P o l y m e r s u n d e r a S l i d i n g S p h e r i c a l I n d e n t o r ' . Mat. S c i . , 11, 1976, 199. ( l l ) T R E N T , E M . i n 'Metal C u t t i n g ' , 2 n d . Edition. Butterworths, Guildford, (1984). ADAMS, M . J . , BRISCOE, B . J a n d WEE, T K . 'The D i f f e r e n t i a l F r i c t i o n E f f e c t of K e r a t i n F i b r e s ' . T o be p u b l i s h e d i n J Phys. D . : Appl. Phys. 1969, LANCASTER, J . K . Wear, 2 2 3 - 2 3 9 , ' A b r a s i v e Wear of P o l y m e r s ' .
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2,