Residual Stress and Debonding of DLC Films on Metallic Substrates

8/13/2019 Residual Stress and Debonding of DLC Films on Metallic Substrates

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D oAMONDRELATEDMATERIALS

LS VI R D iamo n d an d R e la t ed M ate r i a l s 7 ( 1 9 9 8 ) 9 4 4 - 9 5 0

R e s i d u a l s tr es s a n d d e b o n d i n g o f D L C f il m s o n m e t a ll ic s u b s tr a te sX . L . P e n g , T . W . C l y n e *

Departmen t of Materials Sciences and Metallurgy Cambridge University Pembroke Street Cambridge CB2 3QZ UKReceived 21 July 1997 : accepte d 13 No vem ber 1997

bs trac t

Hy droge nated diamond- l ike carbon DL C) f i lms have been prepared us ing 13.56 MH z capaci tively coupled RF glow dischargein a methane a tmosphere . F i lm s t ructure was character i sed by FTIR and Raman spect roscopy. Deposi t ion temperatures , whichwere mea sured using irreversible self-stick therm ax, varied fr om 40 to 182 C. Stress levels were measu red by m onito ring thecurv ature changes o f thin 125 gin) Ti foil after deposit ing thick several lain) DL C fi lms, using an A1 bon ding layer. This intrinsicdeposi t ion) s tress was compress ive and var ied f rom -0 .3 to -2 .0 G Pa. I t increased ini tia lly wi th the average energy of thebom barding ions , reached a max imum and then decreased. Debo nding b ehaviou r of DL C coat ings on fo ur metall ic substra tes lwith the interfaces being prep ared in variou s ways, has been investigated. Films with a range o f thicknesses hav e been prep aredand specimens have be en subjected to changes in tem perature . Fro m a knowledge of the residual stress levels in these fi lms as afunct ion of temperature , the s t ra in energy re lease ra te for inter facia l debonding has been moni tored dur ing deposi t ion andsubsequent temperature changes. The value of the strain energy release rate at a point when interfacial debonding occurred hasbeen taken as the interfaciai toughness fractu re energy). I t is conc luded from these observation s that the interracial toughness islow for DL C fi lms dep osited on to mechanically polished surfaces o f t i tanium , mild steel or stainless steel , but quite high onaluminium. Preclean ing by bo mb ardm ent with energetic arg on ions raises the interracial toughness fo r the steels and fo r aluminium,but no t for t i t anium. T hese observat ions have been re la ted to the nature of the oxide f ilms on these substra tes . A thin sput teredinterlayer of alum inium w as fo und to raise the interfacial toughness significantly fo r the steels and for t i tanium. 1998 ElsevierScience S.A.Keywords Diam ond- l ike carbon DL C); RF discharge; Int rinsic s t ress; Inter facia l toughness

1 I n t r o d u c t i o n

D i a m o n d - l i k e c a r b o n f il m s h y d r o g e n a t e d o r n o n -h y d r o g e n a t e d ) h a v e b e e n t h e s u b j ec t o f i n te n s iv er e s e a r c h o v e r th e p a s t 2 0 y e a r s o r s o . V a r i o u s d e p o s i t i o nt e c h n i q u e s [ 1 - 6 ] h a v e b e e n e m p l o y e d . B o m b a r d m e n t b ye n e r g e t i c s p e c ie s d u r i n g g r o w t h w h i c h is n e c e s s a r y f o ro b t a i n i n g D L C s t r u c t u r e s ) t e n d s t o g e n e r a t e h i g h i n t r i n -s i c c o m p r e s s i v e s t r e s s e s . T h i s i s p r i m a r i l y a r e s u l t o fi m p l a n t a t i o n o f c a r b o n a t o m s i n t o t h e g r o w i n g f il md u r i n g b o m b a r d m e n t . T h i s i n t r in s i c s tr e s s is e x p e c t e d t ob e s e n s i t iv e t o t h e i m p a c t e n e r g i e s [ 7 , 8 ] , A l t h o u g h Z e l e z[ 9 ] r e p o r t e d r e l a t i v e l y l o w i n t ri n s i c st r e ss le v e ls + 7 0 t o- 7 5 0 M P a ) , m o s t s t u d ie s [ 7 ,1 0 - 1 4 ] h a v e i n d i c a te d h ig hc o m p r e s s i v e s tr e s s l e v el s i n t h e r a n g e - 0 . 5 t o- 1 2 . 5 G P a . D i f f er e n t ia l t h e r m a l c o n t r a c t i o n d u r i n g* C o r r es p o n d in g au th o r . F ax : 0 0 4 4 I 2 2 3 3 3 4 5 6 7;e - ma i l : tw c l0 @cam. ac . u k

T h i s p a p e r w a s p r e s e n t e d a t t h e D i a m o n d 1 9 9 7 C o n f e r e nc e i nEd in b u r g h , S co t l an d , A u g u s t 3 - 8, 1 9 97 .

0925-9635/98/ 19.00 1998 Elsevier Science B.V. All r ights reserved.P H S 0 9 2 5 - 9 6 3 5 ( 9 7 ) 0 0 3 3 1 - 2

c o o l i n g a l s o t e n d s t o m a k e t h e s t r e s s i n t h e f i l m m o r ec o m p r e s s i v e , p a r t i c u l a r l y w i t h m e t a l li c s u b s t r a te s . T h e s eh i g h s t r e s s l e v e l s h a v e s e v e r a l c o n s e q u e n c e s i n t e r m s o ft h e p e r f o r m a n c e o f t h e c o at i ng . P r o m i n e n t a m o n g t h e s eis t h e f a c t t h a t , p a r t i c u l a r l y w h e n t h e c o a t i n g i s r e l a t iv e l yt h i c k , t h e r e i s a s t r o n g d r i v i n g f o r c e f o r i t t o b e c o m ed e t a c h e d . T h i s d r i v i n g f o r c e i s b e s t e x p r e s s e d a s t h es t r a in e n e r g y r e l e a s e r a te f o r i n t e r fa c i a l d e b o n d i n g , G i,a n d / o r t h e a s s o c i a t e d s t r e s s i n t e n s i t y f a c t o r , K i . W h e nt h e s e p a r a m e t e r s r e a c h a p p r o p r i a t e v a l u e s , w h i c h c h a r -a c t e r is e th e t o u g h n e s s o f th e i n t e r f a c e , t h e n d e b o n d i n gis e x p e c te d t o o c c u r [ 1 5 - 1 9 ] . F o r e x a m p l e , d e b o n d i n gis e x p e c t e d t o o c c u r w h e n G i r e a c h e s t h e c r i t i c a l i n t e r-f a c i a l s t r a i n e n e r g y r e l e a s e r a t e , o r i n t e r f a c i a l f r a c t u r ee n e r g y , G i c . I n p r a c t i c e , t h e r e m a y b e s o m e b a r r i e r t oi n i t i a ti o n o f th e i n t e r f a c i a l c r a c k [ 2 0 - 2 2 ] , s o t h a t ah i g h e r d r i v i n g f o r c e t h a n t h a t p r e d i c t e d m i g h t , i n f a c t ,b e n e c e s sa r y . H o w e v e r , i n i t ia t i o n o f d e b o n d i n g m a yo c c u r r e a d i l y f r o m e d g e s , w h e r e f l a w s a r e i n e v i t a b l yp r e s e n t [ 2 2 ] . T h e r e a r e a l s o v a r i o u s i s s u e s c o n c e r n i n g


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X.L. Peng. T, W. Clyne / Diamondand Related Materials 7 (1998) 944-950 945t h e e x a c t m e c h a n i s m s b y w h i c h t h e f i l m m a y b e c o m ed e t a c h e d , i n c l u d i n g t h e p o s s i b i li t y o f g a s a c c u m u l a t i o na t th e i n t e r f a c e p r o m o t i n g t h e in i t ia l d e b o n d i n g [ 2 3 -2 7 ] . D L C f i l m s h a v e a l s o b e e n o b s e r v e d [ 2 8 ] t o b u c k l ew h e n e x p o s e d t o a i r f o r p r o l o n g e d p e r io d s . T h e r e h a sa l so b e e n i n te r e s t [ 2 9 -3 1 ] i n t h e d e v e l o p m e n t o f t h r o u g h -t h i c k n e s s c r a c k s w i t h i n c o a t i n g s a s a r e s u l t o f r e s id u a ls t re s s es . T h e f o r m a t i o n o f t h e s e m a y a l s o a f f e c t t h eo n s e t o f d e b o n d i n g .

T h e r e h a s b e e n v e r y li tt le s y s t e m a t i c w o r k o n t h er e s i d u a l s t r e ss l e v e ls , c o r r e s p o n d i n g s t r a i n e n e r g y r e l e a s er a te s a n d i n t er r a ci a l t o u g h n e ss e s o f s u b s t r a t e / D L C c o a t -i n g s y s t e m s . T h e r e h a v e , h o w e v e r , b e e n v a r i o u s a t t e m p t st o e x p l o r e i n a q u a l i t a t i v e o r s e m i - q u a n t i t a t i v e w a y t h ef a c t o r s w h i c h a f f e c t i n t e r r a c ia l a d h e s i o n . F o r e x a m p l e ,i t h a s b e e n s h o w n t h a t a d h e s i o n o f D L C f il m s t o c e r t a ins u b s t r a t e s c a n b e i m p r o v e d b y s u r f a c e p r e t r e a t m e n t s[ 3 2 ] a n d b y u s i n g t h i n i n t e r l a y e r s s u c h a s s i li c o n [ 3 3 ] o ra l u m i n i u m [ 3 4 , 3 5 ] , o r b y g e n e r a t i n g D L C / m e t a l m u l t i -l a y e r s tr u c t u r e s [ 3 6 ] . I n t h e p r e s e n t p a p e r , t h e m e c h a n -i ca l s t a b il i ty o f D L C f il m s p r e p a r e d b y R F m e t h a n eg l o w d i s c h a r g e is s t u d ie d o v e r a r a n g e o f t e m p e r a t u r ea n d a s a f u n c t i o n o f t h e r e s i d u a l s t re s s l ev e l s a n di n t e r f a c i a l s t r u c t u r e .

2. Experimental procedures2.1. Spec nen preparation and structural exam Tation

D L C f i lm s h a v e b e e n p r e p a r e d u s i n g 1 3 . 5 6 M H zc a p a c i ti v e ly c o u p l e d R F g l o w d i s c h a rg e i n a m e t h a n ea t m o s p h e r e . T a b l e 1 l is ts t h e d e p o s i t i o n p a r a m e t e r s .S u b s tr a te s o f t it a n iu m ( S x 3 0 x 2 m m ) , a l u m i n i u m( 8 x 30 x 1 .5 r am ) , 316 s t a i n l e s s s t ee l, m i l d s t ee l ( b o t h8 x 3 0 x 1 .5 m m ) a n d S i w a f e r ( 8 x 3 0 x 0 . 3 8 m m ) w e r ep o l i s h e d t o a m i r r o r f in i sh ( 0 .2 5 g m d i a m o n d ) , u l t r a s o n -i c a ll y c l e a n e d f o r 2 0 r a i n i n a c e t o n e , b l o w n d r y a n dp l a c e d i n t h e d e p o s i t i o n c h a m b e r . S e l e c t e d s u b s t r a t e sw e r e t h e n s u b je c te d t o b o m b a r d m e n t f r o m a n A r p l a sm af o r 1 0 r a in a n d / o r d e p o s i t i o n o f a n A 1 b o n d i n g l a y e r,u s i n g a D C m a g n e t r o n s p u t t e r in g s o u r c e in a n A ra t m o s p h e r e . T h e s p u t t e r i n g t a r g e t w a s a l u m i n i u m , o f9 9 . 9 p u r i t y , c le a n e d b y i n s i tu p r e s p u t t e r i n g f o r t 0 m i n .

S o m e o f t h e A l - c o a t e d s u b s tr a t es w e r e t h e n e x p o s e d t oa i r . T h e d e p o s i t i o n c h a m b e r w a s p u m p e d d o w n t ob e l o w 1 0 . 6 P a a n d a p u r e m e t h a n e a t m o s p h e r e i n t r o -d u c e d f o r D L C d e p o s i ti o n .

T h e D L C f il m t h ic k n e s s w a s m e a s u r e d d i r e c tl y o n t h ec r o s s - s e c t i o n , u s i n g i m a g e d i s p l a y s o f t w a r e o n a J O E L5 8 0 0 L V S E M . S p e c i m e n s w e r e a l s o e x a m i n e d a t v a r i o u ss t a g e s u s i n g o p t i c a l m i c r o s c o p y , S E M , R a m a n a n dF T I R s p e c t r o sc o p y . M o s t o f t h e D L C d e p o s i t io n f o rd e b o n d i n g s t u d ie s w a s c a r r ie d o u t a t - 1 0 5 V n e g a t iv eb i a s a n d 1 0 P a p r e s s u r e , s in c e t h e se p a r a m e t e r s g e n e r -a t e d t h e h i g h e s t in t r i n s ic s tr e s s ( a r o u n d - 2 G P a ) a n dt h e r e f o r e t h e h i g h e s t d r i v i n g f o r c e f o r d e b o n d i n g a t ag i ven f i l m t h i ckness .2.2. Deduction of stresses fr om curvature measurements

T h e S t o n e y e q u a t i o n [ 37 ] is c o m m o n l y u s e d t o c a l c u -l a t e a f i l m s t r e s s f r o m a m e a s u r e d c u r v a t u r e . H o w e v e r ,t h e e q u a t i o n i s a c c u r a t e o n l y i n t h e l i m i t w h e r e t h e f i l mt h i c k n e s s t e n d s t o b e z e r o . U n f o r t u n a t e l y , i n t h i s l i m i tt h e c u r v a t u r e m u s t a l s o t e n d t o b e z e r o . I t h a s b e e np o i n t e d o u t b y B r e n n e r a n d S e n d e r o f f [ 3 8] t h a t t h ee x p e r i m e n t a l e r r o r a r i si n g f r o m t h e c u r v a t u r e b e i n g t o os m a l l t o m e a s u r e a c c u r a t e l y t e n d s t o e x c e e d t h e e r r o ri n t r o d u c e d v i a t h e a p p r o x i m a t i o n i n c o r p o r a t e d i n t o t h eS t o n e y e q u a t i o n , w h e n t h e r a t i o o f t h e t h ic k n e s s o f t h ec o a t i n g t o t h a t o f t h e s u b s t r a t e is l es s t h a n a b o u t 5 .I n t h e c u r r e n t w o r k t h e t h i c k n e s s r a t io s a n d h e n c ec u r v a t u r e s , w e r e r e l a t i v e ly l a rg e , b u t t h e s t re s s w a sc a l c u l a t e d u s i n g t h e e x a c t e x p r e s s i o n s [ 2 2 ] w h i c h r e l a t ec u r v a t u r e s a n d s t r e s s d i s t r i b u t i o n s t o t h e t h i c k n e s s e sa n d p h y s i c a l p r o p e r ti e s o f su b s t r a te a n d c o a t i n g - - s e eT ab l e 2 ,2.3. Meas ure ment of inte~faciaI toughness

R e s i d u a l s t r e ss e s , a n d h e n c e s t r a i n e n e r g y r e l e a s e r a t e sf o r i n t e r f a c i a l d e b o n d i n g , a r e g e n e r a t e d d u r i n g d e p o s -i t i o n a n d a l s o d u r i n g s u b s e q u e n t t e m p e r a t u r e c h a n g e s( p r o v i d e d t h e c o a t i n g a n d s u b s t r a t e h a v e d i f f e r e n t t h e r -ma l expan s i v i t i e s ) . T he s t r a i n ene r gy r e l ease r a t e , G i , i sa l s o d e p e n d e n t o n t h e t h i c k n e s s o f t h e fi lm ( a n d o f t h es u b s t r a t e , u n l e s s i t c a n b e t a k e n a s b e i n g i n f i n i te l y th i c k ) .

Table 1Experimental conditions used for all D LC depositionRF power (W) 5-150Cathode area (cma)Base pressure (Pa)Pre-cleaning gasWorking gasGas flow rate (sccm)Working pressure (Pa)Deposition time (min)

20


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946 X.L. Peng, T.W. Clyne / Diamond and Related M ater ials 7 (1998) 944-950The following expression [39] can be used.

fh o'(Y)2 f Gr(y)i = - - d y - - - d y 1 )- H E ( y ) -H E'(y)where h and H are the thicknesses of film and substrate,E ( y ) is the effective Young's modulus as a function ofdepth and ~ ( y ) , a t ( y ) are the stress distributions beforeand after debonding. In the absence of comprehensivedata on this, the effective Young's moduli of all theDLC films produced in the present work have beentaken here as 174 GPa, which is the value reported byBlech and Wood [40]. For an effectively infinite substrate(h 3 GPa) obtainedin some studies [10-12, 14] have not been recorded.The intrinsic stress is plotted in Fig. 1 as a functionof negative bias voltage. Initially, the stress rapidlybecomes strongly compressive as I~ rises. This is theresult of pronounced implantation of bombardingcarbon ions, once they have enough energy to penetratethe structure. Further increases in bombardment ener-gies lead to intensive local heating (thermal spike), anincrease in the substrate temperature and consequentreduction in the compressive stress as the structureundergoes thermal relaxation. It was also found thatthe stress level progressively reduces as the methanepressure is increased. Increasing this pressure leads to asharp decrease in the mean free path o f ions bombardingthe specimen. This tends to inhibit implantation byreducing the energy of ions reaching the specimen sur-face and hence to reduce the stress levels. These resultsare similar to those reported by Zou et al. [10].3 . 2. D e b o n d i n g b e h a v i o u r

Observations on the conditions under which debond-ing occurred are summarised in Table 4, which givesinferred values of, or bounds on, Gic (taken as equal toGi at the po int o f debonding). These results are alsopresented in Fig. 2. Several features are immediatelyapparent. For the titanium and for both types of steel,the toughness of the interface formed by depositingDLC directly onto the substrate is low (Gi~ ~ 7 J m-2).It is very difficult to deposit films of more than about


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X . L . P e n g , T . W , C l y n e / D i a m o n d a n d R e l a t e d M a t e r i a l s 7 1 9 9 8 ) 9 4 4 - 9 5 0 9 4 7T a b l e 3E x p e r i m e n t a l d a t a f o r f i h n t h i c k n e s s , r o o m t e m p e r a t u r e c u r v a t u r e , c a l c u l a t e d m i s f it s t r a in s a n d i n t r i n s i c s t re s s b y a s s u m i n g i t i s e n t i r e l ya c c o m m o d a t e d i n t he f i lm ( i nf i n it e s u b s t r a t e c as e ) f o r v a ri o u s D L C f il m s d e p o s i t e d o n t o 1 2 5 / a m t h i ck C P - T i s u b s t r a t e sS p e c . B i a s T , D L C M e a s u r e d N e t m i s f it T h e r m a l I n t r in s i c m i s f it I n t r in s i cc o d e v o l t a g e ' :C t h i c k n e s s c u r v a t u r e s t r a i n , 2 xe m i s f i t s t r a i n , s t r a i n , k e~ s t r e s s

( V ) h ( ~ tm ) K ( m - 1 ) ( m i l l i s t ra i n ) A % ( m i l l i s tr a i n ) ( m i t l i s t ra i n ) cri ( G P a )T 2 4 - 4 0 2 1 2 . 4 - 2 . 3 6 - 2 . 5 9 0 - 2 . 5 9 - 0 . 4 5T 2 5 - 4 0 2 1 2 . 4 - 3 . 1 9 - 3 . 5 0 0 - 3 . 5 0 - 0 . 6 iT 2 6 - 4 0 2 1 2 . 4 - 2 . 2 1 - 2 . 4 3 0 - 2 . 4 3 - 0 . 4 2T 7 - 1 0 5 4 0 1 . 0 5 - 4 . 7 - 1 1 . 4 0 - 0 . 0 9 - 1 1 . 3 1 - 1 . 9 7T 1 8 - t 0 5 4 0 1 . 03 - 5 . 0 1 - 1 2 . 4 0 - 0 . 0 9 - 1 2 ,3 1 - 2 . I 4T 3 I - I 0 5 4 0 1 . 7 - 6 . 8 2 - 1 0 . 4 0 - 0 . 0 9 - 1 0 . 3 1 - 1 . 7 9T 1 3 - 1 9 5 7 1 5 . 8 - 1 7 . 3 5 - 8 , 5 7 - 0 . 2 8 - 8 . 2 9 - 1 . 4 4T 1 4 - 1 9 5 7 1 5 . 6 - 1 0 . 6 7 - 5 . 4 3 - 0 . 2 8 - 5 . 1 5 - 0 . 9 0T 5 - 2 8 5 1 I 8 4 . 1 6 - 7 . 3 - 4 . 8 3 - 0 . 5 8 - 4 . 2 5 - 0 . 7 4T 6 - 2 8 5 I 1 8 4 . 0 3 - 9 . 2 - 6 . 2 6 - 0 . 5 8 - 5 . 6 8 - 0 . 9 9T 8 - 3 4 5 1 8 2 6 - 1 0 - 4 . 8 0 - 0 . 9 7 - 3 . 8 2 - 0 . 6 6T t 1 - 3 4 5 i8 2 5 .8 - 5 . 8 2 - 2 , 8 7 - 0 . 9 7 - 1 .9 0 - 0 . 3 3T 1 2 - 3 4 5 1 8 2 5 . 2 - - 1 0 . 6 - 5 . 7 5 - 0 . 9 7 - 4 . 7 8 - 0 . 8 3

0.5 lam in thickness without them spontaneously debond-ing. On aluminium, however, the adhesion is muchbetter, with a value of Gio in excess of 100 J m -2. Thisis probably due to the different nature of the oxide filmon aluminium, which is normally very thin and coherent.

In the case of titanium, this tow toughness was notsignificantly affected by introduction of an Ar ion bom-bardmen t precleaning of the substrate. However, for thetwo steels, and especially for the stainless steel, suchprecteaning effected a large increase in Gic. This observa-tion is presumably related to the nature of the surfaceoxide films formed in these systems. Such a film isapparently difficult to eliminate from titanium by asimple bombardment process for a relative short time10 min); in fact, it is probable that it is at least partly

removed, but a relatively thick oxide film reformsquickly. For aluminium, the high value of Gio observedin the absence of precleaning was increased significantlyvia argon bombardment to over 350Jm-2: This is

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r r i r P r r I ~ ] r I I I I I I p 1 1 1 , , I I I I I . . . .0 0 50 1(30 I50I o n e n e r g y , E = 0 . 5 e V b ( e V )

F i g . 1 . I n t r i n s i c s t r e s s le v e ls m e a s u r e d e x p e r i m e n t a l l y , f o r f il m s d e p o s -i t e d a t 1 0 P a p r e s s u r e w i t h v a r i o u s n e g a t i v e b i a s p r e s s u r e s .

presumably associated with removal of general contami-nation and possibly with a small reduction in oxide filmthickness.

In view of the excellent adhesion exhibited on alumin-ium, an A1 interlayer offers promise as a method ofimproving the bonding. It was indeed found that a thin80 nm) layer of A1 deposited on the substrate prior to

DLC deposition raised the interfacial toughness substan-tially for titanium and for both steels. The importanceof surface oxide in contact with the D LC was highlightedfor this case also by introducing a stage in which thealuminium interlayer was exposed to air, allowing for-mation of a thin surface oxide. In fact, in view of thevery high affinity of A1 for oxygen, and the unavoidablepresence of at least some oxygen in the chamber, therewould probably be at least a monolayer of oxide presentin any event, but this would tend to thicken appreciablyon exposure to air.) I ntroduc tion of the oxidation stageresulted in the measured interfacial toughness falling tovalues close to those in the absence of the interlayer orprecleaning step.

In fact, there is evidence that oxidation of the A1interlayer changed the location of the debonding event.Fig. 3 shows EDX spectra taken from free surfacesbefore deposition of the DLC film and after it haddebonded, for specimens with and wi thout the interlayeroxidation stage. It can be seen that an A1 peak hasdisappeared after debonding with the unoxidised A1interlayer--see Fig. 3a, while this A1 peak is still presentwhen the interlayer was previously oxidised--seeFig. 3b. It follows that debonding occurred between theinterlayer and the t itanium substrate with the unoxidisedA1 interlayer, but that the adhesion between theinterlayer and the DLC film was weakened by theoxidation, and the interlayer was left adhering to thetitanium substrate in this case. The measured toughness


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9 4 8 X . L . P e n g , T . W. C ly n e / D ia m o n d a n d R e la t e d Ma te r ia l s 7 t 9 9 8 ) 9 4 4 - 9 5 0T a b l e 4V a l u e s o f a n d b o u n d s o n t h e c r i t i c a l s t r a i n e n e r g y r e le a s e r a t e o f t h e i n t e r fa c e , G , ~, o b t a i n e d b y o b s e r v i n g t h e c o n d i t i o n s u n d e r w h i c h d e b o n d i n go c c u r re d d u r i n g d e p o s i t i o n o r s u b s e q u e n t c o o l i n gS p e c i m e n d e t a i l s S p e c i m e n s t a t e a t d e b o n d i n gS u b s t r a t e A r A 1 O x i d i s e D L C T e m p . A e a f G ~ D e b .( m m ) p r e c l e a n ( g m ) ( g i n ) ( ~ 'C ) ( m i l l i st r a i n ) ( G P a ) ( J m - ) l o c a t i o nT i , 2 . 0 x 0 - - 0 . 3 0 4 0 - - 1 1 .3 1 - 1 . 9 7 6 . 7 D L C / T iT i , 2 . 0 ~ 0 - - 0 . 4 0 2 5 - 1 1 . 4 - 1 . 9 8 9 . 0 D L C / ~ -T i , 2 . 0 ~ 0 . 0 8 x 1 . 8 1 > 2 5 > - 1 1 . 4 > - I . 9 8 < 4 i A I / T iT i , 2 . 0 t , ~ 0 . 0 8 x 1 . 2 0 < - 1 9 3 < - 1 2 . 7 5 < - 2 . 2 2 > 3 4 -T i , 2 . 0 ~ 0 . 0 8 ~ 0 . 5 0 4 0 - 1 1 . 3 1 - 1 . 9 7 8 . 1 D L C / A 1 -

M S , 1 . 5 x 0 - - 0 . 3 3 4 0 - 1 1 . 31 - 1 . 9 7 7 . 3 D L C / M SM S , 1 . 5 t ~ 0 - - 9 . 5 6 4 0 - 1 1 . 3 i - 1 . 9 7 2 1 3 D L C / M SM S , 1 . 5 t ~ 0 . 0 8 ~ 0 . 9 3 4 0 - 1 1 . 3 1 - 1 . 9 7 2 0 . 7 D L C / A IM S , t . 5 t l 0 . 0 8 x 1 . 86 > 2 5 > - 1 1 . 4 6 > - 1 . 99 < 4 2 . 5 A I / M SM S , 1 ,5 ~ 0 . 0 8 x 1 .3 < - 1 9 3 < - I 3 . 6 2 < - 2 . 3 7 > 4 1 , 9 -S S , 1 . 5 x 0 - - 0 . 3 0 4 0 - 1 1 . 3 i - 1 . 9 7 6 . 7 D L C / S SS S , 1 . 5 I ,, , 0 - - 1 3 . 5 4 0 - 1 1 . 3 1 - 1 . 9 7 3 0 1 D L C / S SS S , 1 . 5 ~ 0 . 0 8 t l 0 . 5 0 4 0 - - 1 1 . 3 1 - 1 . 9 7 8 .1 D L C / S SS S , 1 .5 ~ 0 . 0 8 2 .1 > 2 5 > - 1 1 . 6 5 > - 2 . 0 3 < 4 9 . 7 A 1 / S SA 1 , 1 .5 ~ 0 - - 1 6 . 0 4 0 - 1 1 . 3 1 - 1 . 9 7 3 5 7 A I / D L CA 1 , 1 . 5 x 0 - - 5 . 1 4 0 - I 1 , 3 1 - I . 9 7 1 1 4 A I / D L CR e s i d u a l s t r e s s le v e l s a n d h e n c e s t r a i n e n e r g y r e l ea s e r a te s ) w e r e c a l c u l a t e d u s i n g d e p o s i t i o n s t r e s s v a l u e s s e e T a b l e 3 ) a n d t h e f o l l o w i n g t h e r m a le x p a n s i v i t y d a t a [ 4 1 ]: . X D L C= 2 . 3 X t 0 - 6 K - ~ , e v i = 8 . 5 x 1 0 - - 6 K - z , ~ Ms = 1 2 . 2 x 1 0 - 6 K - t , ~ s s = I 6 x 1 0 - 6 K - 1 , c ~ a a = 2 4 x I 0 - ~ K - 1 . T h e D L Cd e p o s i t i o n c o n d i t i o n s f o r a l l o f t h e s e s p e c i m e n s w e r e : i 0 P a a n d - 1 0 5 V b i a s v o l t a g e .

4 0 0 ,3 5 03O022 5 0

e-

g 200e1 5 0

'U 10o5 0

T iM SAI

As- polish ed Pre-cleaned A1 InterlayerI n t e r f a c e

Oxidised

F i g . 2 . E x p e r i m e n t a l d a t a s h o w i n g t h e e f f ec t o f v a r io u s t r e a t m e n t s o ft h e s u b s t r a t e p r i o r t o d e p o s i t i o n o f D L C o n t h e i n t e r f a c i a l t o u g h n e s s ,f o r d i f f e r e n t s u b s t r a t e s .

w a s t h u s o n e b e t w e e n t h e D L C a n d a n o x i d e l a y e r , a sf o r t h e c a s e s o f m e t a l s u b s t r a t e s n o t s u b j e c t e d t o p r e -c l e a n i n g o r i n t e r l a y e r d e p o s i t i o n .

4 . C o n c l u s i o n sT h e f o l l o w i n g c o n c l u s i o n s c a n b e d r a w n f r o m t h i s

w o r k .1 ) D L C f i l m s d e p o s i t e d f r o m m e t h a n e R F g l o w d i s -

c h a r g e s h o w e d h i g h i n t r i n s i c s t r e s s i n t h e r a n g ef r om - 0 . 5 t o - 2 . 0 G P a , w h i c h i n c r e a s e d w i t h t h e

a v e r a g e e n e r g y o f t h e im p i n g i n g i o n s a t f ir s t a n dt h e n d e c r e a s e d .

( 2 ) W h e n D L C c o a t in g s a r e d e p o s i te d u n d e r s t an d a r dc o n d i t i o n s o n t o m e c h a n i c a l l y p o l i s h e d s u b s t r a t e s o ft i t a n i u m , m i l d s t e e l o r s ta i n l e s s s t e e l , d e b o n d i n g t e n d sto ta k e p l a c e r e a d i l y . T h e i n t e r r a c i a l t o u g h n e s s fo ra l l th e s e c a s e s h a s b e e n e s t i m a t e d a t a b o u t 7 J m - 2 .T h i s l o w v a l u e i s a t t r i b u t e d t o t h e p r e s e n c e o f o x i d el a y er s . F o r a l u m i n i u m , o n t h e o t h e r h a n d , t h e t o u g h -n e s s w a s q u i t e h ig h , a t o v e r 1 0 0 J m - : . I n t h is c a s e ,t h e o x i d e l a y e r w a s p r e s u m a b l y v e r y t h i n a n d c o h e r -e n t , s o t h a t i t d i d n o t r e a d i l y p r o m o t e d e b o n d i n g .

( 3 ) F o r t h e t w o s t e e l s , a n i n s i t u p r e c l e a n i n g p r o c e s sa p p l ie d t o t h e s u b st r a te , i n v o l v i n g b o m b a r d m e n t b ya r g o n i o n s , r a i s e d t h e i n t er f a c i a l t o u g h n e s s c o n s i d e r -a b l y , t o v a l u e s i n e x c e s s o f 2 0 0 J m - a . T h i s i sp r e s u m e d t o b e a s s o c i a t e d w i t h t h e e f f e c t i v e r e m o v a lo f t h e o x i d e l a y e r s. I n t h e c a s e o f a l u m i n i u m , t h et o u g h n e s s w a s r a i s e d t o a v e r y h i g h v a l u e o f o v e r3 0 0 J m - a . T h i s m a y h a v e b e e n c a u s e d b y a s m a l lr e d u c t i o n i n o x i d e l a y e r t h i c k n e s s a n d g e n e r a lr e m o v a l o f c o n t a m i n a t io n . F o r t h e t it a n i u m s u b -s t r a t e , o n t h e o t h e r h a n d , t h e p r e c l e a n i n g p r o c e s sd i d n o t s i g n i f i c a n t ly a f f e c t t h e i n t e r f a c i a l t o u g h n e s s .A p p a r e n t l y , a r e la t i v e ly t h i c k o x i d e t e n d s t o r e f o r mq u i c k l y i n t h i s s y s t e m , i n h i b i t i n g g o o d b o n d i n g .

( 4 ) F o r t h e t i t a n i u m a n d t h e t w o s t e e ls , t h e i n t r o d u c t i o no f a n a l u m i n i u m i n t e r la y e r b y s p u t t e r d e p o s i t i o nh a s b e e n f o u n d t o e f f e c t a s i g n i f i c a n t i n c r e a s e i n t h ei n t e r f a c ia l t o u g h n e s s , t o a r o u n d 4 0 - 5 0 J m - z . T h i s


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X,L. Peng, T.W. Clyne / Diamond and Relat ed Afaterials 7 1998) 94 4-950 949

T.

i I I i ,a) T i A f t e r d e b ~? n d i n g o f D L C f i l m. . . . . . . B e f o r e d e p o s i t i o n o f D L C f i l m I i

T iTi A1,.A,.

: ' . . . . . . . . " ' 1 . . . . . . . . . . . . . . . . . 1 ' ~ : ' . . . . . . . . . I . . . . . . . . . . . . . . . . . . . . .2 4 6 8En e rg y k e V)

10

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>

Ib ) T i. . . . . . . . . B e f o r e d e p o s i t i o n () t D L C fi l m- - A f te r d e b on d l ng o f D L C f ii m "~

T iT i A I

i , L . . . . . . . . : " -. . . . . . . . . . . F ' ' " . . . . . . . . I . . . . . . . . . . . . . . . r . . . . .o _ 4 6 8 10Energy keV)Fig . 3 . EDX spect ra from free surfaces produce d wi th t i tan ium sub-st ra tes and a luminium in terlayers a) wi thout and b) wi th prior ox ida-t ion of the in terlayer prior to DLC deposi t ion .

is consistent with the good bonding observedbetween DLC and A1 substrates and with a sput-tered A1 layer adhering well to a cleaned metallicsubstrate. However, when the sputtered A1 layerwas exposed to air before the DLC was deposited,then the interfacial toughness was reduced to valuesfairly close to those in the absence of any precleaningor interlayer deposition.

Ackn ow l e d g em en tFinancial support has been provided for one of theauthors XLP) by the Cambridge Overseas Trust COT)

and ORS.

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Documents

Residual Stress and Debonding of DLC Films on Metallic Substrates