Chapter 36
Intrinsic Bond Strength of Metal Films on Polymer Substrates
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A New Method of Measurement 1
Donald R. Wheeler and Hiroyuld Osaki
Lewis Research Center, National Aeronautics and Space Administration, Cleveland, OH 44135
The b r i t t l e cracking and subsequent debonding of f i l m s deposited on f l e x i b l e substrates subjected to u n i a x i a l s t r a i n i s described t h e o r e t i c a l l y and i l l u s t r a t e d with Ni films evaporated on ion-etched polyethylene terephthalate (PET). It i s shown that, if the materials deform e l a s t i c a l l y , the shear strength of the interface, τo, may be evaluated from the length, lf, of the largest debonded f i l m segment and the t e n s i l e strength, σ , and thickness, t, of the f i l m : o
τo = 4σ o(t/tlf). τo i s the i n t r i n s i c f a i l u r e strength of the f i l m substrate system. It i s shown to be independent of substrate mechanical properties and internal stress i n the f i l m and i s reproducible to ±6%. For Ni on PET, ion-etching doubles the f a i l u r e strength, τo, which reaches the strength of bulk PET.
When studying the mechanism of metal-polymer adhesion and when trying to determine the e f f e c t of surface pretreatment on t h i s mechanism, one would l i k e to measure the i n t r i n s i c i n t e r f a c i a l bond strength. That i s , the strength of the chemical and/or physical forces at the interface. However, adhesion of a f i l m to a substrate i s normally measured by some sort of p u l l - o f f technique which measures the p r a c t i c a l strength of the film-substrate couple. The p r a c t i c a l strength i s affected by the presence of defects, internal stress i n the f i l m and by the p a r t i c u l a r specimen configuration. Thus i t i s , at best, an indirect measure of changes i n the i n t r i n s i c strength.
Current address: Sony Magnetic Products, Inc., 3-4-1 Sakuragi, Tagajo, Miyagi, Japan
0097-6156/90/0440-0500$06.00/0 © 1990 American Chemical Society In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
36. WHEELER AND OSAKI
Intrinsic Bond Strength
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R e c e n t l y , Agrawal and R a j (1) ( h e r e a f t e r , AR) have d e s c r i b e d a method f o r t h e measurement o f t h e i n t r i n s i c i n t e r f a c i a l s t r e n g t h i n c e r a m i c - m e t a l c o u p l e s . Here, we a p p l y t h e same t e c h n i q u e , w i t h some m o d i f i c a t i o n s , t o t h e system o f N i f i l m s on p o l y e t h y l e n e t e r e p h t h a l a t e (PET) s u b s t r a t e s w i t h v a r y i n g d e g r e e s o f A r i o n e t c h i n g p r i o r t o f i l m d e p o s i t i o n . Our o b j e c t i v e i s t o d e s c r i b e t h e t h e o r y b e h i n d t h e method, a s we have adapted i t , v e r i f y i t s a p p l i c a b i l i t y t o the Ni-PET system by d e m o n s t r a t i n g agreement w i t h e x p e r i m e n t a l d e t a i l s and, f i n a l l y t o use t h e method t o measure t h e e f f e c t o f a n i o n - e t c h p r e t r e a t m e n t o f PET on t h e i n t r i n s i c i n t e r f a c i a l s t r e n g t h o f the Ni-PET c o u p l e . Theory The t e s t method c o n s i s t s o f u n i a x i a l l y s t r a i n i n g a sample o f t h e f i l m - s u b s t r a t e c o u p l e a s shown s c h e m a t i c a l l y i n F i g u r e 1. The f i l m t h i c k n e s s i s t , and t h e specimen w i d t h i s w. Under t e n s i l e s t r a i n , an i n t e r f a c i a l shear s t r e s s , τ(χ), i s produced. W h i l e t h e f i l m i s bonded t o t h e s u b s t r a t e , t h e shear s t r e s s , τ(χ), a t t h e i n t e r f a c e causes a t e n s i l e s t r e s s , σ(χ), i n t h e metal f i l m . When t h e s t r a i n i s s u f f i c i e n t , the t e n s i l e s t r e s s w i l l reach the u l t i m a t e t e n s i l e s t r e n g t h o f t h e f i l m , σ . Then, i f t h e f i l m f a i l s b y b r i t t l e f r a c t u r e , i t w i l l develop s t r a i g h t , p a r a l l e l cracks t r a n s v e r s e t o the d i r e c t i o n of s t r a i n . These a r e r e p r e s e n t e d b y t h e d o t t e d l i n e s i n F i g u r e 1. We now f o c u s on one segment o f t h e f i l m . The x - c o o r d i n a t e i s measured from t h e segment edge a s shown i n t h e f i g u r e . I f the f i l m i s t h i n enough t h a t σ(χ) i s n e a r l y c o n s t a n t throughout t h e t h i c k n e s s o f t h e f i l m and t h e p e e l i n g s t r e s s normal t o t h e f i l m may be i g n o r e d , t h e t e n s i l e and shear f o r c e s a t any p o i n t , x, i n t h e f i l m must b a l a n c e , so
(1) The f u n c t i o n a l form, τ(ξ), i s dependent on t h e p r o p e r t i e s o f t h e m a t e r i a l s . AR assume t h a t τ(ξ) i s s i n u s o i d a l b e i n g z e r o a t t h e segment edge and a t a d i s t a n c e λ / 2 from t h e edge. However, e l a s t i c a n a l y s i s o f t h e u n i a x i a l s t r a i n o f a b i l a y e r ( 2 ) shows t h a t τ (ξ) i s maximum a t t h e segment edge and d e c r e a s e s m o n o t o n i c a l l y toward t h e c e n t e r o f t h e segment. T h e r e f o r e , a s a b e t t e r a p p r o x i m a t i o n t o t h e e l a s t i c shear s t r e s s d i s t r i b u t i o n , we l e t τ(ξ) be a l i n e a r f u n c t i o n o f χ w i t h maximum, τ , a t t h e segment edge d e c r e a s i n g t o z e r o a t some m
c h a r a c t e r i s t i c d i s t a n c e , t , from t h e edge.
Such a shear s t r e s s
c
d i s t r i b u t i o n i s shown s c h e m a t i c a l l y a s t h e heavy l i n e i n F i g u r e 2 a l o n g w i t h t h e c o r r e s p o n d i n g t e n s i l e s t r e s s d i s t r i b u t i o n , σ(χ), c a l c u l a t e d a c c o r d i n g t o E q u a t i o n ( 1 ) , a s t h e dashed l i n e . We now c o n s i d e r t h e p r o g r e s s o f c r a c k f o r m a t i o n i n t h e f i l m a s the s t r a i n i n t h e s u b s t r a t e i s i n c r e a s e d . There a r e t h r e e s t a g e s c o r r e s p o n d i n g t o t h e t h r e e p a r t s o f F i g u r e 2 . The f i r s t s t a g e , random c r a c k i n g , b e g i n s when t h e a p p l i e d s t r a i n i s s u f f i c i e n t t o produce c r a c k s i n t h e f i l m . The maximum shear s t r e s s a t t h e i n t e r f a c e , τ , must be l e s s than t h e i n t e r f a c i a l shear s t r e n g t h , τ , m
as shown i n F i g u r e 2 ( a ) , o r t h e f i l m would debond a t t h e edges.
In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
ο
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502
METALLIZATION OF POLYMERS
Figure 1. Schematic diagram of test specimen: Ni f i l m of thickness, t, on PET substrate of width w strained i n d i r e c t i o n of arrows. Dotted l i n e s represent b r i t t l e cracking. The x-coordinate i s measured i n the d i r e c t i o n of s t r a i n from the edge of a f i l m segment.
In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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36. WHEELER AND OSAKI
Intrinsic Bond Strength
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