7 Applications of Ion Beam Methods to Characterization of Adhesive Bonding Materials
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W. L. BAUN Wright-Patterson AFB, Materials Laboratory, OH 45433 Adhesive bonding technology brings the surfaces of both plastics and metals together to form an inter face. Physical and chemical treatments change the morphology and the chemistry of these surfaces and govern whether bonding actually takes place at the interface of these materials. Therefore, there is an increasing need for sensitive analytical tech niques to characterize such surfaces and interphases. Empirical bondability methods such as the water break test may indicate wettability and subsequent bondability but it tells us little of long time durability of the bond which may depend on surface chemistry. A number of analytical techniques have been developed for characterizing solid surfaces. Some of these techniques using electrons and photons as probes of the surface chemistry have been described in this symposium by other authors. In this paper methods of surface analyses using beams of ions w i l l be described. Emphasis is placed on ion scattering spectrometry (ISS) and secondary ion mass spectrom etry (SIMS). Examples are shown for adhesive bon ding applications including determination of locus of failure, contamination, cleaning and thermal and chemical pretreatments. In an effort to develop strong, light and corrosion resis tant structures, the aerospace industry has gone more and more to adhesive bonding. The automotive industry is quickly follow ing along using different structural alloys. In these fields, bonded structures must be strong and possess long-time durability. Both strength and durability depend on many factors of bond prep aration and fundamental properties of the adhesive and adherend.
This chapter not subject to U.S. copyright. Published 1982 American Chemical Society. In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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One i m p o r t a n t i n f l u e n c e i n t h e f o r m a t i o n o f a good a d h e s i v e b o n d i s surface or i n t e r f a c i a l chemistry. In the broader sense, i n w h i c h two s u b s t a n c e s a r e h e l d t o g e t h e r by i n t e r f a c i a l f o r c e s , a d h e s i o n i s o f i m p o r t a n c e i n many t e c h n o l o g i e s s u c h as i n t h i n f i l m s and s e m i c o n d u c t o r s . I t i s the purpose of t h i s paper to d i s c u s s i o n beam methods o f s u r f a c e c h a r a c t e r i z a t i o n a p p l i c a b l e t o t h e b r o a d a r e a o f a d h e s i o n w i t h e m p h a s i s on a d h e s i v e b o n d i n g .
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Experimental
Considerations
The q u e s t i o n i s o f t e n a s k e d : "Which i s t h e b e s t s u r f a c e c h e m i s t r y t o o l f o r r e s e a r c h on a d h e s i v e b o n d i n g ? " This question i s d i f f i c u l t to answer b e c a u s e i t depends on the a s p e c t o f adhesion which i s being s t u d i e d . Often a combination of i n s t r u ments must be u s e d t o t a k e a d v a n t a g e o f t h e s t r o n g p o i n t s o f each. T a b l e I shows t h e f a c e t s o f b o n d i n g a n d some o f t h e c h a r a c t e r i z a t i o n methods w h i c h a r e a p p l i c a b l e i n t h e s e a r e a s . Ion beams e i t h e r p r o v i d e t h e p r i m a r y i n t e r a c t i o n w i t h t h e s u r f a c e o r a r e u s e d w i t h o t h e r methods t o g i v e i n f o r m a t i o n w i t h d e p t h a s t h e s u r f a c e i s e r o d e d away by t h e i o n beam. M e t h o d s shown i n T a b l e I u s i n g an i o n beam as an a n a l y t i c a l p r o b e a r e shown u n d e r l i n e d , w h i l e t e c h n i q u e s u s i n g an i o n beam f o r e l e m e n t a l d e p t h p r o f i l i n g w i t h a d i f f e r e n t p r o b e s u c h as an e l e c t r o n o r p h o t o n a r e shown u n d e r l i n e d w i t h a dashed l i n e . The a c r o n y m s u s e d h e r e a r e t h e Table I A s p e c t s o f A d h e s i v e B o n d i n g and A p p l i c a b l e S u r f a c e C h a r a c t e r i z a t i o n Methods Adherend Chemistry AEAPS, AEM, A E S , APS, B I S , C I S , C L , EM, E S , EXAFS, IMMA, I S , I S S , L M P , P E S , R B S , SIMS, SXAPS, SXES A d h e r e n d S t r u c t u r e and M o r p h o l o g y AEM, E L L , EM, HEED, IMMA, L E E D , SEM, S U M S , XRD Adhesive Chemistry A E S , A I M , ASW, A T R , E S R , H A , 1 R S , I S S ,
I1RS,
S R S , STEM,
L S , P E S , SIMS,
IIXS,
T E M , XEM,
UPS, XPS,
ISP A d h e s i v e S t r u c t u r e and M o r p h o l o g y ATR, I R , UV, RAMAN, SEM I n t e r a c t i o n of Polymers w i t h Metals A E S , A I M , AWS, C P D , E L L , E E L S , E S D I , Ε S D N , F D , F D S , H A , 1 R S , ISS, I S P , L S , P D , S C , SIMS, U P S , X P S , RAMAN, I I R S , I I X S F a i l u r e Surfaces (locus of f a i l u r e ) A E S , A T R , E L L , I S S , SIMS, P E S , X P S , SEM, SXES,
SCAPS,
IR,
SRS, U P S ,
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
Ion Beam Methods to Characterize Adhesive Bonding Materials 123
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o n e s i n c u r r e n t common u s a g e and a r e d e f i n e d b y P o w e l l ( 1 ) . Many o f t h e s e methods w e r e d e s c r i b e d by P a r k ( 2 ) , i n a c o m p r e h e n s i v e r e v i e w i n w h i c h he c a t e g o r i z e d t h e t e c h n i q u e s a c c o r d i n g to t h e k i n d of i n f o r m a t i o n they p r o v i d e . T h i s d i s c u s s i o n i n c l u d e d what i s b e i n g p r o b e d , such as v i b r a t i o n a l s t a t e s , the probe i t s e l f , s u c h a s m o n o e n e r g e t i c e l e c t r o n s , and what was a c t u a l l y b e i n g m e a s u r e d , s u c h as t h e e l e c t r o n e m i s s i o n . When an i o n beam i s u s e d t o i n t e r a c t w i t h t h e s u r f a c e , there a r e s e v e r a l phenomena w h i c h may be s t u d i e d t o c h a r a c t e r i z e a s o l i d s u r f a c e as shown i n F i g u r e 1. O f t h e i n t e r a c t i o n s shown, t h e u s e o f s c a t t e r e d and s p u t t e r e d i o n s (ISS a n d SIMS, r e s p e c t i v e l y ) p r o v i d e s the h i g h e s t s u r f a c e s e n s i t i v i t y . High energy i o n methods i n t h e MeV r e g i o n ( s u c h a s RBS) p r o v i d e v a l u a b l e q u a n t i t a t i v e information with depth. Since such h i g h energy t e c h n i q u e s a r e d i s c u s s e d e l s e w h e r e i n t h i s volume ( 3 ) , the p r e s e n t work w i l l c o n c e r n o n l y low e n e r g y methods ( 0 , 1 t o 5KeV) a n d e m p h a s i z e i o n s c a t t e r i n g s p e c t r o m e t r y and s e c o n d a r y i o n mass spectrometry. A l s o shown i n F i g u r e 1 i s a n i o n beam w h i c h may be u s e d w i t h o t h e r s u r f a c e t e c h n i q u e s s u c h as XPS a n d AES as d e s c r i b e d b y B r u n d l e (4) t o p r o d u c e an e l e m e n t a l d e p t h p r o f i l e analysis. Ion S c a t t e r i n g S p e c t r o m e t r y (ISS) Use o f low energy b a c k s c a t t e r e d i o n s to c h a r a c t e r i z e a s u r f a c e i s a r e l a t i v e l y r e c e n t development. T h e method h a s b e e n r e v i e w e d b y Buck ( 5 ) . High energy i o n s had been used i n the p a s t to a n a l y z e s u r f a c e s but i t was n o t u n t i l S m i t h (6) u s e d l o w e n e r g y (I KeV) n o b l e gas i o n s t o probe the s u r f a c e of a v a r i e t y of m a t e r i a l s that the technique came i n t o p o p u l a r u s e . I t was f o u n d f r o m t h i s work and o t h e r s t h a t when t h e e n e r g y o f i o n s was l o w e r e d t h e s c a t t e r e d i o n s p e c t r a became s h a r p e r and a p p r o a c h e d t h e b e h a v i o r e x p e c t e d on t h e b a s i s o f a b i n a r y s c a t t e r i n g e v e n t f r o m a s i n g l e s u r f a c e atom. T h e r e f o r e , t h e e n e r g y E ^ r e t a i n e d b y an i o n o f m a s s , M ^ w i t h a n i n c i d e n t e n e r g y E Q a f t e r s c a t t e r i n g f r o m a n atom o f mass M t h r o u g h an a n g l e θ i s g i v e n b y e q u a t i o n (1) w h i c h i s b a s e d on t h e c o n s e r v a t i o n o f k i n e t i c e n e r g y and momentum ( h e r e Μ ^ is smaller than M ). o
n
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For 9 0 ° s c a t t e r i n g which i s very simple r e l a t i o n s h i p :
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In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
an
124
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SURFACE
Figure 1.
Figure 2.
ANALYSIS
The use of ion beams for analysis of surfaces.
Equipment configurations for ion scattering. Key: A, 90° scattering using electrostatic analyzer; and B, 138° scattering using CM A.
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
BAUN
Adhesive Bonding Materials
125
electrostatic analyzer. R e c e n t l y a g r e a t improvement i n s e n s i t i v i t y was g a i n e d b y t h e a d a p t a t i o n o f a c y l i n d r i c a l m i r r o r a n a l y z e r (CMA) s u b s t i t u t e d f o r t h e o r i g i n a l e l e c t r o s t a t i c s e c t o r as shown i n F i g u r e 2 b . T h e g e o m e t r y o f t h e CMA r e s u l t s i n a s c a t t e r i n g a n g l e o f 1 3 8 ° . The m a j o r a d v a n t a g e o f low e n e r g y i o n s c a t t e r i n g i s t h e e x t r e m e l y f i n e s u r f a c e s e l e c t i v i t y when low energy i o n s c o l l i d e w i t h the s u r f a c e atom. The p r o b a b i l i t y f o r n e u t r a l i z a t i o n i s v e r y h i g h because of the l o n g r e s i d e n c e time (10~ to 1 C T sec). O n l y a b o u t one i n 1 0 of the s c a t t e r e d p a r t i c l e s r e t a i n a p o s i t i v e c h a r g e e v e n a f t e r one c o l l i s i o n . T h e r e f o r e , the p r o b a b i l i t y t h a t an i o n i s s t i l l i n the charged s t a t e a f t e r two o r more c o l l i s i o n s i s v e r y s m a l l . S i n c e the d e t e c t o r responds o n l y to charged p a r t i c l e s , c o n t r i b u t i o n s from p a r t i c l e s w h i c h s c a t t e r more t h a n o n c e a r e a l m o s t c e r t a i n l y n e u t r a l a n d n o t c o u n t e d b y an i o n d e t e c t o r . An i n h e r e n t f e a t u r e o f i o n s c a t t e r i n g w h i c h may be c o n s i d e r e d an a d v a n t a g e o r a d i s a d v a n t a g e i s t h e s i m u l t a n e o u s s p u t t e r i n g o f , t h e s u r f a c e as e n e r g y i n t r a n s f e r r e d t o t h e s u r f a c e atoms f r o m t h e i o n beam. I t i s an advantage i n t h a t the c o n c e n t r a t i o n o f the v a r i o u s atomic s p e c i e s may be f o l l o w e d w i t h d e p t h . On t h e o t h e r h a n d i t i s a d i s a d v a n t a g e b e c a u s e damage i s b e i n g p r o d u c e d by t h e s p u t t e r i n g . Once t h e atom s p u t t e r s f r o m t h e s u r f a c e t h e s a m p l e i s c h a n g e d , and an e x a c t e x p e r i m e n t on t h a t s p o t may n o t b e r e p e a t e d . One p o s i t i v e f e a t u r e o f i o n s c a t t e r i n g c o m p a r e d t o most o t h e r spectroscopies i s the s i m p l i c i t y o f the s p e c t r a . Binary ion scattering gives one p e a k f o r e a c h i s o t o p e o f an e l e m e n t p r e s e n t . For instance, the s c a t t e r i n g o f h e l i u m from aluminum o x i d e r e s u l t s i n the a p p e a r a n c e o f o n l y two p e a k s i n t h e s p e c t r u m , one f o r o x y g e n a n d one f o r a l u m i n u m . E a c h p e a k i s s e n s i t i v e t o t h e amount p r e s e n t but a b s o l u t e q u a n t i t a t i v e a n a l y s e s c a n be d i f f i c u l t s i n c e the s c a t t e r e d y i e l d d e p e n d s on t h e s c a t t e r i n g c r o s s s e c t i o n and n e u t r a l i z a t i o n e f f i c i e n c y , n e i t h e r o f w h i c h i s w e l l known f o r most e l e m e n t s . D u r i n g t h e i o n s c a t t e r i n g e x p e r i m e n t atoms a r e s p u t t e r e d from the s u r f a c e , a l l o w i n g depth p r o f i l i n g a n a l y s i s from the removal o f the s u r f a c e l a y e r s by the p r o b e i o n d u r i n g the a n a l y s i s . Use o f h e l i u m i o n s g i v e s a v e r y slow r a t e o f s u r f a c e r e m o v a l and w h i l e n e o n and a r g o n p r o v i d e much h i g h e r s p u t t e r i n g r a t e s , t h e i o n beam may b e f o c u s e d and r a s t e r e d on t h e s u r f a c e to r e d u c e s p u t t e r i n g w h i l e t h e s i g n a l i s g a t e d f r o m t h e c e n t e r o f t h e c r a t e r t o r e d u c e c r a t e r edge e f f e c t s . The s i g n a l may be c o l l e c t e d f r o m t h e s u r f a c e u s i n g t h e r a s t e r e d beam t o g i v e a l a t e r a l a n a l y s i s of the s u r f a c e . Therefore ion scattering prov i d e s a c o m b i n a t i o n o f i n d e p t h a n a l y s e s and l a t e r a l a n a l y s e s to g i v e a t h r e e d i m e n s i o n a l p i c t u r e o f t h e c h e m i c a l makeup o f t h e surface with depth. 1 4
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Ion Beam Methods to Characterize
1 6
3
S e c o n d a r y I o n Mass S p e c t r o m e t r y (SIMS) When a l o w e n e r g y i o n s t r i k e s t h e s o l i d s u r f a c e i t u n d e r g o e s and p r o d u c e s a number of i n t e r a c t i o n s . The s p u t t e r e d s p e c i e s w h i c h a r e removed f r o m t h e s u r f a c e a r e b o t h p o s i t i v e and n e g a t i v e i o n s as w e l l as
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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neutral particles. N e u t r a l p a r t i c l e s h a v e much more a b u n d a n c e t h a n i o n i c s p e c i e s and h a v e a l s o b e e n u s e d f o r s u r f a c e a n a l y s i s . S u r f a c e a n a l y s i s by SIMS f a l l s i n t o two c a t e g o r i e s , l o w c u r r e n t d e n s i t y s p u t t e r i n g and h i g h c u r r e n t d e n s i t y s p u t t e r i n g . Cate g o r i e s a r e d e t e r m i n e d by the c h a r a c t e r i s t i c s o f the p r i m a r y i o n beam. A low c u r r e n t d e n s i t y s p u t t e r i n g a n a l y s i s r e s u l t s i n a v e r y small f r a c t i o n of the s u r f a c e being d i s t u r b e d , a r e s u l t that approaches a b a s i c r e q u i r e m e n t of a t r u e s u r f a c e a n a l y s i s method. T h i s i s g e n e r a l l y known as t h e S t a t i c SIMS (SSIMS) m e t h o d . The SSIMS method u s e s s m a l l c u r r e n t d e n s i t i e s o f 10~9 A / c m ^ s p r e a d o v e r an a r e a o f 0 . 1 cm^ o r more ( 7 ) . High current density s p u t t e r i n g removes more m a t e r i a l and i s r e q u i r e d f o r p r e p a r i n g elemental depth p r o f i l e s . In the h i g h c u r r e n t d e n s i t y method, c h a n g e s a r e s e e n i n t h e s u r f a c e and n e a r s u r f a c e r e g i o n s . E q u i p m e n t f o r SIMS may be as s i m p l e a s t h a t shown i n F i g u r e 3, o r a s c o m p l e x as t h e i o n m i c r o p r o b e ( 8 ) . In a simple system a SIMS e x p e r i m e n t r e q u i r e s a vacuum chamber t o h o u s e t h e e x p e r i ment, a s a m p l e h o l d e r , an i o n s o u r c e , an e n e r g y a n a l y z e r a n d a mass a n a l y z e r . I n s u c h s i m p l e s y s t e m s t h e n o b l e o r r e a c t i v e gas f i l l s t h e s y s t e m and t h e e n t i r e chamber i n c l u d i n g t h e i o n gun and s a m p l e a r e a a r e a t a p p r o x i m a t e l y 1-5 χ 10"-* t o r r . A more c o m p l i c a t e d t y p e o f i n s t r u m e n t i s one i n w h i c h t h e p e r f o r m a n c e i s i m p r o v e d t h r o u g h t h e u s e o f a d i f f e r e n t i a l l y pumped vacuum s y s t e m t o p r o d u c e u l t r a h i g h vacuum i n t h e v i c i n i t y o f t h e s a m p l e . T h i s a l s o a l l o w s t h e e n t r y o f a r e a c t i v e gas i n t h e s a m p l e chamber a r e a w h i l e s p u t t e r i n g w i t h a n o b l e i o n f o r s t u d y i n g c h e m i c a l changes o r r e a c t i o n s on the s u r f a c e . S t i l l another i m p r o v e m e n t and a d d e d c o m p l e x i t y may be made t o t h e SIMS i n s t r u ment by t h e mass a n a l y s i s o f t h e p r i m a r y beam. The e n e r g y f i l t e r i s g e n e r a l l y made up o f s e v e r a l e l e m e n t s whose f u n c t i o n i s t o o p t i m i z e c o l l e c t i o n o f t h e s e c o n d a r y i o n s and t o f i l t e r a n d f o c u s t h e i o n s a t t h e e n t r a n c e t o t h e mass a n a l y z e r . T h e mass a n a l y z e r i n simple systems i s u s u a l l y a quadrupole f i l t e r . I t i s r e c o g n i z e d t h a t SIMS h a s b e e n u s e d s u c c e s s f u l l y as a s t a n d - a l o n e t e c h n i q u e t o s o l v e many s u r f a c e p r o b l e m s . However, i t a p p e a r s t h a t t h e a r e a o f g r e a t e s t u s e o f SIMS i s as a c o m p l e ment t o o t h e r s u r f a c e c h a r a c t e r i z a t i o n m e t h o d s . The e x t r e m e l y h i g h s e n s i t i v i t y f o r some e l e m e n t s c a n be t a k e n a d v a n t a g e o f by u s i n g SIMS w i t h o t h e r t e c h n i q u e s i n w h i c h t h e s e e l e m e n t s do n o t show h i g h s e n s i t i v i t y . T h e SIMS t e c h n i q u e i s a l s o i d e a l t o u s e w i t h f u n d a m e n t a l l y l o w r e s o l u t i o n methods s u c h as i o n s c a t t e r i n g , t o s e p a r a t e and i d e n t i f y t h e a d j a c e n t m a s s e s w h i c h may be p r e s e n t a t the sample s u r f a c e . T h e most p o p u l a r c o m b i n a t i o n o f i n s t r u ments u s e d t h u s f a r h a s b e e n I S S - S I M S and A E S - S I M S . SIMS h a s a l s o b e e n u s e d on 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 s , a l l o w i n g h i g h q u a l i t y i m a g i n g a l o n g w i t h l a t e r a l and d e p t h a n a l y s i s o f t h e sample.
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
ENERGY FILTER
Figure 3.
ION DETECTOR
Equipment for a simple SIMS system.
UHV SYSTEM
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idJ
*
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SIMS SPECTRUM
lilllililtlttlllltlt)llUtltiUltllltflM[llllllllllUllllltt1IIW1 10 20 30 40 50
I
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128
SURFACE
A summary o f SIMS a n a l y s i s
is
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and n e g a t i v e
i n Tables II
and
aspects
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ANALYSIS
I S S and
III.
Table I I . P o s i t i v e A s p e c t s o f Low E n e r g y I o n Beam A n a l y s i s M e t h o d s ISS -
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SIMS -
Elemental Analysis S e n s i t i v i t y to F i r s t Monolayer P r o f i l i n g C a p a b i l i t y w i t h Depth P o s s i b l e O p e r a t i o n i n Near S t a t i c
Mode
E l e m e n t a l and M o e l c u l a r A n a l y s i s S e n s i t i v i t y t o F i r s t Few L a y e r s P r o f i l i n g C a p a b i l i t y w i t h Depth Isotopic Analysis E x t r e m e l y S e n s i t i v e t o Many E l e m e n t s S e n s i t i v i t y to Hydrogen
Table I I I . N e g a t i v e A s p e c t s o f Low E n e r g y I o n Beam A n a l y s i s M e t h o d s ISS -
Low R e s o l u t i o n P o o r S e n s i t i v i t y t o Low Ζ E l e m e n t s Specimen C h a r g i n g (Insulators) L i t t l e Chemical Information
SIMS -
Extremely Variable Y i e l d s Specimen C h a r g i n g (Insulators) D i f f i c u l t Data I n t e r p r e t a t i o n
T h e s e two t e c h n i q u e s may b e a p p l i e d i n a g e n t l e manner u s i n g a n e a r s t a t i c i o n beam to p r o d u c e l i t t l e c h a n g e i n t h e s u r f a c e and a l s o i n a mode i n w h i c h c h e m i c a l p r o f i l i n g w i t h d e p t h i s possible. ISS c a n d e t e c t a l l o f t h e e l e m e n t s h e a v i e r t h a n h e l i u m i n the p e r i o d i c t a b l e . The s e n s i t i v i t y v a r i a t i o n a c r o s s t h e p e r i o d i c t a b l e i s p r o b a b l y l e s s t h a n one o r d e r o f m a g n i t u d e . SIMS p r o v i d e s a d i s t i n c t a d v a n t a g e o f b e i n g a b l e to a n a l y z e , i n p r i n c i p l e , a l l of the elements ( i n c l u d i n g hydrogen) i n the periodic table. B e i n g a b l e to i d e n t i f y i s o t o p e s i s a d e f i n i t e advantage. T h e s e n s i t i v i t y o f t h e SIMS t e c h n i q u e c a n v a r y s e v e r a l o r d e r s o f m a g n i t u d e ( p e r h a p s up t o 10^) due t o a r a p i d l y c h a n g i n g s e c o n d a r y i o n y i e l d c a u s e d by m a t r i x and c h e m i c a l effects. The a b i l i t y o f i o n s c a t t e r i n g to r e s o l v e d i f f e r e n t e l e m e n t s i n a c o m p l e t e unknown i s a t t i m e s somewhat l i m i t e d . T h e r e a r e few i n t r i n s i c l i m i t a t i o n s o r s p e c t r a l interferences but the t e c h n i q u e i s f u n d a m e n t a l l y a low r e s o l u t i o n t e c h n i q u e i n w h i c h t h e r e i s some u n c e r t a i n t y a s t o t h e e x a c t i d e n t i t y o f
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
Ion Beam Methods to Characterize
BAUN
Adhesive Bonding Materials
129
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a given l i n e . S p e c i f i c i t y may b e i m p r o v e d b y g o i n g t o a s c a t t e r i n g i o n c l o s e r t o t h e mass o f t h e unknown e l e m e n t , t h a t is we w o u l d u s e h e l i u m f o r t h e l i g h t e l e m e n t s , n e o n f o r i n t e r m e d i a t e mass e l e m e n t s and a r g o n f o r t h e h e a v y e l e m e n t s . I o n s c a t t e r i n g g i v e s v e r y l i t t l e i n f o r m a t i o n on t h e c h e m i c a l combination of the element d e t e c t e d i n the sample, however, r e c e n t l y d i s c o v e r e d y i e l d v a r i a t i o n s (9) and t h e u s e o f o t h e r f i n e f e a t u r e s i n t h e s p e c t r u m (10) g i v e some p o s s i b i l i t y o f u s i n g i o n s c a t t e r i n g to determine c h e m i c a l s p e c i e s at the s u r face. The a p p e a r a n c e o f c l u s t e r i o n s i n t h e s e c o n d a r y i o n mass s p e c t r u m g i v e s a good p o s s i b i l i t y o f u s i n g SIMS t o d e t e r m i n e chemical combinations (11). The i n t e r p r e t a t i o n o f such s p e c t r a i s e x t r e m e l y c o m p l i c a t e d and h a s t o b e t r e a t e d w i t h a g r e a t d e a l of care. M o l e c u l a r i o n s c a n be d i s l o d g e d f r o m t h e s u r f a c e a n d g i v e some i d e a o f t h e c h e m i c a l c o m b i n a t i o n , b u t m o l e c u l a r comp l e x e s may a l s o b e s y t h e s i z e d a t t h e s a m p l e s u r f a c e i n t h e gas phase above the sample s u r f a c e . The p r e s e n c e o f such i o n s i n t h e mass s p e c t r u m d o e s n o t u n e q u i v i c a l l y p r o v e t h e p r e s e n c e o f s u c h s p e c i e s i n o r on t h e s a m p l e i t s e l f . Application
of
I S S and SIMS
C h a n g e s i n C h e m i s t r y Due t o S u r f a c e T r e a t m e n t s . Many c h e m i c a l e t c h i n g and o x i d i z i n g t r e a t m e n t s a r e u s e d on m e t a l and a l l o y s to enhance a v a r i e t y o f p r o p e r t i e s . Numerous t h e r m a l pretreatments f o l l o w i n g f a b r i c a t i o n improve s t r e n g t h , ductility, toughness o r other p r o p e r t i e s . Each of these chemical or thermal t r e a t m e n t s a f f e c t the c o m p o s i t i o n of the s u r f a c e e i t h e r by i n t r o d u c i n g i m p u r i t i e s , o r by i n c r e a s i n g o r d e c r e a s i n g t h e c o n c e n t r a t i o n o f a l l o y i n g elements at the s u r f a c e . Many i m p u r i t i e s a r e p r e s e n t i n t h e raw m a t e r i a l s o r a r e i n t r o d u c e d d u r i n g m a t e r ials processing. F i n a l f a b r i c a t i o n of the m a t e r i a l s i n t o the d e s i r e d s h a p e f o r a d h e s i v e b o n d i n g may a l s o i n t r o d u c e c o n t a m i n a n t s which a r e d e l e t e r i o u s to a d h e s i v e b o n d i n g . F i n a l l y the e n v i r o n m e n t and c a r e l e s s h a n d l i n g b e f o r e and d u r i n g t h e a c t u a l bonding can i n t r o d u c e contaminants o r even p h y s i c a l l y a l t e r the adherend s u r f a c e s . A summary o f t h e s o u r c e s o f c o n t a m i n a n t s b a s e d upon p r e p a r a t i o n h i s t o r y w h i c h c o u l d a f f e c t a d h e s i v e b o n d i n g and bond p r o p e r t i e s i s shown i n T a b l e I V .
Sources
1.
2
U
T a b l e IV o f Contaminants o r S p e c i e s Which C o u l d A d h e s i v e B o n d i n g and Bond P r o p e r t i e s
Affect
Raw M a t e r i a l s P r o c e s s i n g a. Adherend b. Adhesive Prebonding Treatments a. Chemical s o l u t i o n contributions b.
Alloy
constituents
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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130
SURFACE
3.
Bonding
4.
a. D i f f u s i o n of i m p u r i t i e s H a n d l i n g and S t o r a g e
ANALYSIS
V e r y o f t e n t h e m i l l s c a l e t h a t r e m a i n s on a m a t e r i a l f o l l o w i n g f i n a l p r o c e s s i n g o r r o l l i n g has l i t t l e r e l a t i o n to the c o n c e n t r a t i o n s o f each element i n the b u l k . T h e ISS and SIMS s p e c t r a f r o m s u c h an i n i t i a l 2024 a l u m i n u m a l l o y , as shown i n F i g u r e 4, i n d i c a t e h i g h magnesium c o n c e n t r a t i o n a t t h e s u r f a c e . C o n v e n t i o n a l a l k a l i n e c l e a n i n g t r e a t m e n t s do n o t e t c h t h e s u r f a c e a p p r e c i a b l y , l e a v i n g t h e s u r f a c e magnesium r i c h . Such a s u r f a c e , when a d h e s i v e l y b o n d e d , may e x h i b i t good i n i t i a l b o n d a b i l i t y b u t p o o r l o n g t i m e d u r a b i l i t y when compared w i t h b o n d e d s t r u c t u r e s i n w h i c h f o r m a t i o n o f aluminum o x i d e has been a s s u r e d . T h e r e i s an e v e r i n c r e a s i n g demand f o r m a t e r i a l s w h i c h c a n b e u s e d a t h i g h t e m p e r a t u r e s w i t h good c o r r o s i o n r e s i s t a n c e . Such r e q u i r e m e n t s a r e s a t i s f i e d i n many c a s e s by s t a i n l e s s s t e e l s o r t i t a n i u m alloys. C o n v e n t i o n a l methods o f j o i n i n g t h e s e m a t e r i a l s s u c h a s w e l d i n g , b r a x i n g and s o l d e r i n g may b e u s e d b u t r e l i a b l e methods using adhesive bonding are a l s o being developed. Stainless s t e e l following f i n a l processing i s normally covered with a s u r f a c e l a y e r c o n t a i n i n g p r o c e s s i n g a i d s and o x i d a t i o n p r o d u c t s . The s u r f a c e must u n d e r g o an e t c h o r p i c k l e b e f o r e i t may b e u s e d f o r the f i n a l p r o c e s s i n g . I t has been found t h a t g e n e r a l l y t h i s s u r f a c e l a y e r i n i t s o r i g i n a l s t a t e h a s d e l e t e r i o u s e f f e c t on a d h e s i v e bond s t r e n g t h . U n f o r t u n a t e l y , a f t e r a c i d p i c k l i n g the s u r f a c e c a n be e v e n more c o n t a m i n a t e d t h a n i t was o r i g i n a l l y . T h i s c o n t a m i n a t i o n , as i n a l u m i n u m , i s c a l l e d s m u t . The u s u a l method f o r d e s m u t t i n g i s t o w i p e t h e w o r k p i e c e a f t e r r i n s i n g , w h i l e i t i s s t i l l wet o r t o b r u s h m e c h a n i c a l l y w i t h a s t i f f wire b r i s t l e brush. However, i t has been found t h a t o n l y a p o r t i o n o f t h e smut i s removed by t h e s e methods and c h e m i c a l e t c h e s a r e r e q u i r e d to remove a l l o f t h e m a t e r i a l . An example o f smut o n s t a i n l e s s s t e e l i s shown i n F i g u r e 5 ( 1 2 ) , w h e r e a s u r f a c e w h i c h had b e e n t r e a t e d i n h o t s u l f u r i c a c i d i s shown. The e l e m e n t s w h i c h a r e o b s e r v e d a r e s i l i c o n and o x y g e n w i t h some c a r b o n p r e s e n t on t h e s u r f a c e and o n l y a s l i g h t amount o f t h e matrix stainless s t e e l . T h e SIMS s p e c t r a w e r e t a k e n s i m u l t a n e o u s l y w i t h t h e ISS d a t a a n d show p r i m a r i l y s i l i c o n and c l u s t e r p e a k s o f s i l i c o n and o x y g e n t o g e t h e r w i t h a s m a l l amount o f t h e matrix material. E v e n when t h e s u r f a c e o f t h e 304 s t a i n l e s s s t e e l was w i p e d a f t e r r i n s i n g t h e a p p e a r e d v i s a b l y c l e a n some smut r e m a i n e d b e h i n d as d e t e c t e d by b o t h p o s i t i v e SIMS and I S S . The smut was c o m p l e t e l y removed b y d i p p i n g t h e w o r k i n h o t c a u s t i c soda s o l u t i o n or i n chromic a c i d s o l u t i o n c o n v e n t i o n a l l y used i n the i n d u s t r y . The a n a l y s i s o f t h e s m u t t e d s u r f a c e and t h e d e s m u t t e d s u r f a c e b y a v a r i e t y o f t e c h n i q u e s i s shown i n Table V. Each of the t e c h n i q u e s has i t s s t r o n g p o i n t s . In AES, f o r i n s t a n c e , b o t h s u l f u r and c h l o r i n e a r e r e a d i l y d e t e c t e d i n
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
Ion Beam Methods to Characterize Adhesive Bonding Materials 131
BAUN
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4 He
.2
Figure 4.
.3
.4
.5
.6
.7
.8
.9
1.0
ISS/SIMS Data for as received 2024-aluminum alloy showing Mg-rich mill scale.
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
132
SURFACE
ANALYSIS
t h e s p e c t r u m w h e r e a s i n t h e ISS s p e c t r u m n e i t h e r o f t h e s e e l e ments was d e t e c t e d , p r o b a b l y b e c a u s e t h e y o c c u r i n an a r e a o f high scattered ion background. The XPS d a t a show d e f i n i t e l y t h a t s i l i c o n i s i n an o x i d i z e d s t a t e w h i c h w o u l d be e x p e c t e d f r o m t h e f o r m a t i o n i n t h e h i g h l y o x i d a t i v e medium o f t h e a c i d e t c h . Table V S p e c i e s F o u n d on 304 S t a i n l e s s Steel by S u r f a c e A n a l y s i s T e c h n i q u e s
Downloaded by UNIV OF SYDNEY on May 29, 2013 | http://pubs.acs.org Publication Date: September 22, 1982 | doi: 10.1021/bk-1982-0199.ch007
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+
K , Si ,
Cr, Ni
+
+
S ,
+
0H , CH ,
, Ni
C H ', η η
_
Fe,
H^O^-H^CrO^
0
(greatly Cr0 ", 2
, OH , C I , S i O reduced),
Cr0 ", 3
η
CrO ,
Fe0 " 2
2
b
0 , S, S i , N , Fe, Cr, Ni
C,
0,
Fe,
Cr, Ni
form.
form.
Adhesives
and O t h e r
Polymers
As m e n t i o n e d e a r l i e r , i o n beam m e t h o d s h a v e n o t b e e n a p p l i e d e x t e n s i v e l y to o r g a n i c m a t e r i a l s s u c h as a d h e s i v e s and o t h e r polymers. Ion s c a t t e r i n g has been a p p l i e d to a d h e s i v e b o n d i n g m a t e r i a l s t o d e t e r m i n e t h e l o c u s o f f a i l u r e and c o n t a m i n a t i o n effects. D i B e n n e d e t t o and S c o l a (13) h a v e u s e d b o t h I S S a n d SIMS t o c h a r a c t e r i z e s u r f a c e s o f t r e a t e d g l a s s f i b e r s and f i b e r / polymer i n t e r f a c e s . T h e r e s u l t s show how SIMS c a n b e u s e d t o s t u d y the c h e m i c a l s u r f a c e and c h e m i c a l changes on the s u r f a c e and a t i n t e r f a c e s . By w o r k i n g a t l o w c u r r e n t l e v e l s w i t h i n s u l a t o r s u r f a c e s , t h e SIMS a n a l y s i s showed c h a n g e s i n t h e s t r u c t u r e of a p o l y m e r i z e d s i l a n e c o a t i n g as a f u n c t i o n o f d e p t h o f p e n e t r a t i o n i n t o the i n t e r f a c e . T h e c o n c e n t r a t i o n o f n i t r o g e n and
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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7.
BAUN
Ion Beam Methods to Characterize
Adhesive Bonding Materials 133
hydrogen generated from the s u r f a c e m a i n t a i n e d a r e l a t i v e l y c o n s t a n t l e v e l as t h e d i s t a n c e f r o m t h e a i r s i l a n e i n t e r f a c e i n c r e a s e d ; t h e n w i t h i n 160 A i n t o t h e s u r f a c e , a d r a m a t i c i n c r e a s e i n t h e n i t r o g e n l e v e l was n o t e d , t o a d e p t h o f 240 A . In t h i s r e g i o n , t h e s i m p l i c i t y o f t h e SIMS s p e c t r a , w i t h m a j o r p e a k s c o r r e s p o n d i n g to the atomic c o n s t i t u e n t s of y - a m i n o p o r p y l t r i e t h x y s i l a n e , namely, H, C , N , 0, and S i , s u g g e s t e d t h a t l o w - m o l e c u l a r - w e i g h t oligomer was p r e s e n t i n t h i s r e g i o n . T h i s means t h a t t h e s i l a n e c o a t i n g was n o t s u f f i c i e n t l y c u r e d t o p r o v i d e a m e c h a n i c a l l y s t a b l e interface. F i n a l l y , f r o m 240 A t o t h e s i l a n e - g l a s s i n t e r f a c e , the n i t r o g e n and h y d r o g e n g e n e r a t e d from the s u r f a c e r e a c h e d a lower constant l e v e l but about t h r e e times h i g h e r than that gene r a t e d from the a i r - s i l a n e domain. This suggests that the s i l a n e p o l y m e r c o a t i n g a d j a c e n t to t h e g l a s s i n t e r f a c e i s d i f f e r ent from the s i l a n e polymer at the a i r i n t e r f a c e . Thus, i t is c l e a r t h a t the ISS/SIMS t e c h n i q u e can be used to d e f i n e the i n t e r f a c e and i n t e r p h a s e r e g i o n s and a l s o to f o l l o w changes a t t h e i n t e r f a c e due to a c h e m i c a l r e a c t i o n . G a r d e l l a a n d H e r c u l e s (14) h a v e shown SIMS d a t a f o r p o l y ( a l k y l m e t h a c r y l a t e s ) and a l s o I S S d a t a f o r T e f l o n . They found changes i n m o l e c u l a r f r a g m e n t a t i o n p a t t e r n s w i t h o n l y v e r y s l i g h t changes i n polymer p r o c e s s i n g . C a r e f u l examination of core l e v e l XPS d a t a shows no i d e n t i f i a b l e c h a n g e s i n c o r e l e v e l b i n d i n g energies or i n t e n s i t y r a t i o s . Side c h a i n s t r u c t u r e i n the e s t e r p o r t i o n of the p o l y ( a l k y l methacrylates) d r a m a t i c a l l y i n f l u e n c e d s t a t i c SIMS d a t a . T h e r e h a v e b e e n few p a p e r s p u b l i s h e d c o n c e r n e d o n l y w i t h ISS o f p o l y m e r s e x c e p t f o r T h o m a s , e t a l (15) a n d f o r a b s t r a c t s o f m e e t i n g p r e s e n t a t i o n s b y S p a r r o w a n d M i s h m a s h i n 1977 (16) and G a r d e l l a and H e r c u l e s i n 1979 ( 1 7 ) . B o t h ISS and SIMS d a t a appear i n p a p e r s i n which the emphasis i s i n a n o t h e r a r e a , such as t h e c l e a n i n g o f s u r f a c e s by u l t r a v i o l e t l i g h t ( 1 7 ) . Figure 6 shows ISS and SIMS d a t a f r o m p o l y p r o p y l e n e w h i c h h a s b e e n e x p o s e d to u l t r a v i o l e t l i g h t . U s u a l l y , f o l l o w i n g UV e x p o s u r e , slightly more o x y g e n i s o b s e r v e d i n t h e ISS d a t a . SIMS d a t a show some c h a n g e s i n t h e f r a g m e n t a t i o n p a t t e r n and l a r g e r f r a g m e n t s a r e observed. P e r h a p s f u r t h e r s t u d y o f s u c h c h a n g e s may be a t t r i b u t e d t o b o n d b r e a k a g e and c r o s s l i n k i n g e f f e c t s . Thomas a n d c o - w o r k e r s f o u n d i o n s c a t t e r i n g d a t a what n o n i n d i v i d u a l i s t i c , b u t t h e y d e t e r m i n e d t h a t i t u s e f u l f o r determining segregation at the s u r f a c e of T a b l e V I f r o m T h o m a s , e t a l (15) shows t h e v a r i a t i o n oxygen to c a r b o n r a t i o a t the s u r f a c e compared to the values.
t o be somewas v e r y a polymer. i n the bulk
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
134
SURFACE
ANALYSIS
Table VI The C , H and O - c o n t a i n i n g p o l y m e r s s t u d i e d , w i t h t h e t h e o r e t i c a l 0:C r a t i o s d e r i v e d from the f o r m u l a o f t h e r e p e a t i n g c h e m i c a l unit. A l s o l i s t e d a r e t h e m e a s u r e d 0 : C r a t i o s o b t a i n e d by c o m p a r i n g t h e ISS p e a k , 2750eV H e ) a n d t h e r a t i o s a f t e r c o r r e c t i o n f o r t h e d i f f e r e n c e i n s e n s i t i v i t i e s o f t h e two e l e m e n t s based on the s e n s i t i v i t y r a t i o 0:C = 1 8 . 1 : 1 . +
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Polymer
Formula
0:C b u l k
PC
C ,H 0 16 14 3
0.19
PMMA-218 PMMA-XT
D °2 C H 0
PBDDA
C
PET
C H O
1
1 /
o
0:C measured
Corrected
1.9
0.10
η u
0.02
On. Z77/
0.01
H
5
8
r, , 0.4
5
g
1 0
H 0
1 0
2
u
8
4
4
0.4
n 1? O.il
η ι 0.4
11
0.02 °-
0 6
PVA i-o
2
·
8
0
,
1
5
An e x a m p l e o f t h e non i n d i v i d u a l i s t i c n a t u r e o f ISS s p e c t r a f r o m p o l y m e r s i s s e e n i n F i g u r e 7 w h e r e d a t a a r e shown f o r a s i m p l e l i n e a r h y d r o c a r b o n , p o l y e t h y l e n e , and compared t o d a t a o b t a i n e d under i d e n t i c a l c o n d i t i o n s f o r g r a p h i t e . A s c a n be s e e n , one w o u l d be h a r d p r e s s e d t o i d e n t i f y t h e p o l y m e r h e r e . Natural l y , i t w o u l d be e v e n more d i f f i c u l t t o i d e n t i f y i n d i v i d u a l p o l y mers. Specimen
Charging
A s e r i o u s p r o b l e m e n c o u n t e r e d i n i o n beam methods o f a n a l y s i s i s that of specimen c h a r g i n g . Impact o f e n e r g e t i c p o s i t i v e ions c a u s e s d e v e l o p m e n t o f a p o s i t i v e c h a r g e o n t h e s u r f a c e o f an i n sulator. This effect is e s p e c i a l l y prevalent i n polymers, because they are u s u a l l y e x c e l l e n t i n s u l a t o r s . Table VII l i s t s work f u n c t i o n s f o r some t y p i c a l p o l y m e r s . It is questionable w h e t h e r t h e t e r m w o r k f u n c t i o n s h o u l d be u s e d f o r p o l y m e r s , b u t t h e h i g h v a l u e o f t h e s e numbers shows t h e r e a s o n f o r t h e c h a r g i n g e x h i b i t e d by p o l y m e r s .
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
Ion Beam Methods to Characterize Adhesive Bonding Materials 135
BAUN
+ S I MS
I
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10
20
30
40
50
ss
Polypropylene ( UV exposed)
Mr
60
atomic m ass υ π i ts
C H; 3
ss
POLYPROPYLENE
\ c
10
20
30
40
50
60
5
atomic m a s s u n i t s
Figure 6.
.5
.6
Figure 7.
.6
\
Al
0
f
q
^ ?do«
.7
P
.8
.9
ISS/SIMS data for polypropylene UV exposed (A) and polypropylene, untreated (B).
.7
ρ
.8
.9
1.0
·5
.6
.7
P
.8
.9
1.0
ISS data for graphite and polyethylene fiber under identical conditions.
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
136
SURFACE
ANALYSIS
Table VII Work f u n c t i o n o f
polymers
(Ref.
Polymer
18)
Work F u n c t i o n ,
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Teflon Chlortrifluoroethylene Vinyl Chloride Sulfone Styrene Methyl Methacrylate N y l o n 6.6
5.75 5.30 5.13 4-95 4.90 4.68 4.30
T h i s charge w i l l i n f l u e n c e o r even p r e v e n t the e m i s s i o n o f secondary ions. To o v e r c o m e t h i s c h a r g i n g o f i n s u l a t o r s t h e r e h a v e b e e n numerous methods u s e d , as s e e n i n T a b l e V I I I .
Methods o f
Table VIII change n e u t r a l i z a t i o n
(Ref.
17)
(1) (2)
Deposition of a conducting t h i n f i l m or g r i d . U s e o f C s as p r i m a r y i o n s , i n t h i s way a c o n d u c t i n g is continuously deposited
(3)
Compensation of beam
(4)
U s e o f n e u t r a l beams: the of p o s i t i v e primary ions
(5)
A p p l i c a t i o n of special electrodes for draining excessive n e g a t i v e charge S h i f t o f t a r g e t h o l d e r p o t e n t i a l V b y AV i n a d i r e c t i o n o p p o s i t e to the p r e v i o u s c h a r g i n g o f the i n s u l a t o r
(6)
+
the
c h a r g i n g by means charging is
of
an e x t r a
reduced
layer
electron
from the
case
H
Determining Locus of
Failure
F r e q u e n t l y i t i s not s i m p l e u s i n g v i s u a l or even m i c r o s c o p i c e x a m i n a t i o n t o d e t e r m i n e a f t e r t e s t i n g w h e t h e r an a p p a r e n t a d h e s i v e f a i l u r e o c c u r r e d a t t h e i n t e r f a c e due t o i m p r o p e r w e t t i n g o r a t some new i n t e r f a c e , l e a v i n g b e h i n d a t h i n l a y e r o f a d h e s i v e o n the adherend o r o x i d e on the a d h e s i v e . There i s a r e s o l u t i o n l i m i t a t i o n o f a b o u t 100 Â f o r most 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 s (SEMs) w h i c h makes v e r y t h i n o r g a n i c f i l m s d i f f i c u l t t o d e t e c t , e s p e c i a l l y when t h e a d h e s i v e i s a p u r e p o l y m e r c o n t a i n i n g no f i l l e r s o f h i g h e r a t o m i c number t h a n t h e p o l y m e r t o i n c r e a s e c o n trast. O p t i c a l and s t a i n i n g methods h a v e b e e n r e p o r t e d t o d e t e r mine the p r e s e n c e o f a d h e s i v e f i l m s . However, the o p t i c a l t e c h n i q u e s u s e s t h e i n t e r f e r e n c e phenomenon, w h i c h i s a p p l i c a b l e o n l y t o f a i r l y t h i c k f i l m s , c e r t a i n l y n o t t o f i l m s o n l y a few m o l e c u l e s t h i c k o r b o u n d a r y l a y e r s c o n t a i n i n g b o t h a d h e s i v e and a d h e r end c o m p o n e n t s . S t a i n i n g techniques are s e n s i t i v e o n l y to s p e c i f i c compounds p r e s e n t i n t h e u s u a l l y c o m p l e x a d h e s i v e s y s t e m s . S e v e r a l i n v e s t i g a t i v e t e c h n i q u e s on b o t h s i d e s o f a j o i n t failure are n e c e s s a r y to d e t e r m i n e the l o c u s o f f a i l u r e c o n c l u s i v e l y .
thin
It i s films
a l s o important to determine the l o c u s o f f a i l u r e of s u b j e c t e d t o many m e c h a n i c a l t e s t s d e p e n d i n g on p e e l ,
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
Ion Beam Methods to Characterize
BAUN
Adhesive Bonding Materials
137
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s c r a t c h , a b r a s i o n , and d e c e l e r a t i o n as r e v i e w e d by M i t t a l (18). G e n e r a l l y , t h i n p o l y m e r f i l m s on m e t a l s p r e s e n t t h e same p r o b l e m s as e n c o u n t e r e d i n a d h e s i v e b o n d i n g . On t h e o t h e r h a n d , e v a p o r a t e d m e t a l f i l m s , by n a t u r e o f t h e i r s i m p l e r c o m p o s i t i o n and more i d e a l i n t e r f a c e s , p r e s e n t f e w e r d i f f i c u l t i e s . There are u s u a l l y d i f f e r e n c e s i n c o l o r b e t w e e n t h e e v a p o r a t e d f i l m and t h e s u b s t r a t e w h i c h make i t e a s i e r t o d e t e r m i n e t h e mode o f f a i l u r e . H o w e v e r , when b o t h m e t a l s a r e t h e same o r n e a r l y t h e same c o l o r o r when a m e t a l i s d e p o s i t e d on a t h i n o x i d e f i l m o r on a p o l y m e r , t h e r e c a n be p r o b l e m s o f i n t e r p r e t a t i o n o f v i s u a l r e s u l t s . J u s t as i n a d h e s i v e b o n d s , i t i s n e c e s s a r y t o d e s c r i b e where a t h i n f i l m f a i l e d f o l l o w i n g s e r v i c e o r t e s t a s w e l l as how w e l l the f i l m adhered to the s u b s t r a t e . When we u s e s p e c t r o c h e m i c a l t o o l s t o d e t e r m i n e s p e c i e s on t h e s u r f a c e , we p r o b a b l y u s e them s l i g h t l y d i f f e r e n t l y e a c h time, e s p e c i a l l y i n adhesive bonding. E v e n when c h e m i c a l and m o r p h o l o g i c a l i n f o r m a t i o n h a s b e e n c o l l e c t e d , i n t e r p r e t a t i o n may be d i f f i c u l t . J u s t how do we d e c i d e where f a i l u r e h a s o c c u r r e d ? I n t h e t y p i c a l c o m p l e x a d h e s i v e b o n d e d s y s t e m , we h a v e s e v e r a l i n t e r f a c i a l r e g i o n s as shown i n F i g u r e 8. Each of the m a t e r i a l s c o m i n g t o g e t h e r t o f o r m t h e s e i n t e r f a c e s h a s i t s own i n d i v i d u a l chemical signature. The s u b s t r a t e f o r i n s t a n c e u s u a l l y c o n t a i n s a l l o y i n g e l e m e n t s w h i c h v a r y i n c o n t e n t b e t w e e n t h e s u r f a c e and bulk. In a d d i t i o n to a l l o y i n g elements, s u r f a c e treatments l e a v e behind elements c h a r a c t e r i s t i c of each treatment. For instance the p o p u l a r FPL ( F o r e s t P r o d u c t s L a b o r a t o r y ) e t c h f o r aluminum a l l o y s c o n s i s t o f s u l f u r i c a c i d and s o d i u m d i c h r o m a t e i n d i s t i l l e d w a t e r and l e a v e s a d e t e c t a b l e amount o f chromium on t h e alloy. A d d i t i v e s i n t h e p r i m e r o r a d h e s i v e may a l s o b e u s e d t o determine the l o c a t i o n of a f a i l u r e . Primers containing a c o r r o s i o n i n h i b i t o r such as s t r o n t i u m chromate a r e examples o f such m a t e r i a l s . A p u r e p o l y m e r i c a d h e s i v e w i t h no a d d i t i v e s c a n c a u s e p r o b l e m s s i n c e t h e e l e m e n t s p r e s e n t may l o o k j u s t t h e same t o some c h a r a c t e r i z a t i o n methods as o r g a n i c c o n t a m i n a t i o n . It is h e r e t h a t v i b r a t i o n a l s p e c t r o s c o p y o r XPS p r o v i d e s i m p o r t a n t i n f o r m a t i o n on m o l e c u l a r c o n f i g u r a t i o n . The c a p a b i l i t y o f d i f f e r e n t i a t i n g i s o t o p e s w i t h t h e SIMS method a l l o w s d o p i n g o f t h e a d h e s i v e w i t h an i s o t o p e n o t n o r m a l l y f o u n d i n t h e a d h e s i v e s u c h as C or N . 1
3
1
5
F a i l u r e s u r f a c e s f r o m t h e wedge t e s t h a v e p r o v e d t o be i n t e r e s t i n g i l l u s t r a t i o n s o f the ISS-SIMS t e c h n i q u e . T h e wedge t e s t method p r o v i d e s i n f o r m a t i o n a b o u t a d h e r e n d s u r f a c e p r e p a r ation. T h i s c o n f i g u r a t i o n i s s e n s i t i v e to d i f f e r e n t surface p r e p a r a t i o n t r e a t m e n t s and c a n d i s c r i m i n a t e b e t w e e n b o n d i n g p r o c e s s e s t h a t g i v e good and p o o r s e r v i c e p e r f o r m a n c e . The wedge s p e c i m e n c o n s i s t s o f two t h i n a d h e r e n d s w i t h a wedge d r i v e n i n t o the b o n d l i n e . The p o s i t i o n o f t h e c r a c k l e a d i n g edge i s deter-
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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138
SURFACE
ANALYSIS
m i n e d m i c r o s c o p i c a l l y and t h e n t h e s p e c i m e n i s s u b j e c t e d t o v a r i ous e x t e r n a l s t i m u l i s u c h a s c h a n g e s i n t e m p e r a t u r e and r e l a t i v e humidity. The p r o p a g a t i o n o f t h e c r a c k t i p i s f o l l o w e d w i t h t i m e . Sometimes when t h e wedge i s d r i v e n i n t o t h e b o n d l i n e , s e p a r a t i o n o f t h e s p e c i m e n o c c u r s o v e r a p o r t i o n o f t h e b o n d l i n e as i n t h e p i c t u r e s shown i n F i g u r e 9. H e r e t h e wedge was d r i v e n i n as shown f i r s t c a u s i n g t h e c o h e s i v e f a i l u r e i n t h e a d h e s i v e a t t h e left, t h e n a p p a r e n t a d h e s i v e f a i l u r e b e t w e e n a and c d u r i n g t e s t i n g a t 1 6 0 ° F and 95% R . H . , f o l l o w e d a t t h e r i g h t a g a i n by c o h e s i v e f a i l u r e when t h e s p e c i m e n i s o p e n e d f o l l o w i n g t h e t e s t . The a d h e r e n d shows no i n d i c a t i o n o f a d h e s i v e e i t h e r v i s u a l l y o r i n t h e SEM, a l t h o u g h t h e r e a r e s l i g h t r e f l e c t i v i t y differences seen a c r o s s the sample. Such specimens a r e u s u a l l y examined i n several areas of both f a i l u r e surfaces. I n t h e s p e c i m e n shown i n F i g u r e 9 i t a p p e a r e d t h a t t h e mode o f f a i l u r e h a d c h a n g e d from p r i m e r / o x i d e to o x i d e / m e t a l . The a p p e a r a n c e o f chromium i n ISS/SIMS s p e c t r a i n area c suggested f a i l u r e at the o r i g i n a l etched a l l o y s u r f a c e . When t h e o p p o s i t e s i d e ( t h e a d h e s i v e ) was s a m p l e d i n s e v e r a l a r e a s i t was e v e n more e v i d e n t t h a t a m i x e d mode f a i l u r e h a d o c c u r r e d . The I S S / S I M S d a t a f r o m t h r e e a r e a s on t h e a d h e s i v e c o r r e s p o n d i n g t o t h e same t h r e e a r e a s o n t h e a d h e r e n d a r e shown i n F i g u r e 1 0 . These s p e c t r a suggest s t r o n g l y that the c r a c k t i p progresses i n i t i a l l y from the adhes i v e i n t o a boundary l a y e r near the p r i m e r oxide i n t e r f a c e . With time (under i n c r e a s e d t e m p e r a t u r e and h u m i d i t y ) the c r a c k t i p c o n t i n u e s t o a weaker r e g i o n w h i c h f o r t h i s s u r f a c e t r e a t ment a p p e a r s t o be n e a r t h e o x i d e / a l l o y i n t e r f a c e . F i g u r e 11 summarizes the a n a l y s i s o f the bond f a i l u r e f o r t h i s p a r t i c u l a r surface treatment. The i m p o r t a n t a s p e c t h e r e i s t h a t u n d e r i d e n t i c a l c o n d i t i o n s , d i f f e r e n t s u r f a c e p r e p a r a t i o n s show d i f f e r e n t modes o f f a i l u r e . Weak b o u n d a r y l a y e r s a r e n o t d e v e l o p e d u s i n g some t r e a t m e n t / b o n d i n g c o m b i n a t i o n s . Processes have been d e v e l o p e d i n w h i c h the l o c u s o f f a i l u r e remains i n the a d h e s i v e ("a c o h e s i v e f a i l u r e " ) and i t i s n e c e s s a r y t o u s e a m e c h a n i c a l t e s t i n w h i c h e v e n more s t r e s s i s p l a c e d on t h e i n t e r f a c i a l region (19). Conclusions I o n beams p r o v i d e u s e f u l i n f o r m a t i o n e i t h e r a s a d i a g n o s t i c t o o l o r a s a p r e c i s i o n e t c h i n g method i n a d h e s i o n r e s e a r c h . The c o m b i n a t i o n I S S / S I M S method u s e d a l o n g w i t h o t h e r t e c h n i q u e s s u c h as SEM p r o v i d e s a p o w e r f u l t o o l f o r e l e m e n t a l a n a l y s i s o f s u r f a c e composition. T h e s e r e s u l t s , as w e l l a s e a r l i e r w o r k i n t h i s l a b o r a t o r y , i n d i c a t e t h a t t h e s u r f a c e c o m p o s i t i o n c a n be s i g n i f i c a n t l y d i f f e r e n t f r o m t h e b u l k due t o c o n t a m i n a t i o n , s e l e c t i v e c h e m i c a l e t c h i n g and s e g r e g a t i o n . T h e s e same t e c h n i q u e s a l s o p r o v i d e an a n a l y s i s o f t h e mode o f f a i l u r e i n a d h e s i v e j o i n t s . Many f a i l u r e s c l a s s i f i e d a s " a d h e s i v e " o n t h e b a s i s o f v i s u a l i n s p e c t i o n a r e f r e q u e n t l y m i x e d mode f a i l u r e s o r f a i l u r e s a t a new i n t e r f a c e c o n t a i n i n g e l e m e n t s o f b o t h a d h e s i v e s a n d a d h e r e n d .
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
Ion Beam Methods to Characterize
BAUN
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microscopy
Figure 8.
spectroscopy
Model of adhesive bond showing applications of spectroscopy and microscopy.
amu Figure 9.
Adhesive Bonding Materials 139
E/Eo
Wedge failure surface with ISS/SIMS data taken in area C on adherend.
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
140
ANALYSIS
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SURFACE
Figure 10.
ISS/SIMS data from adhesive matching areas to adherend shown in Fig. 9.
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
Ion Beam Methods to Characterize
BAUN
Adhesive Bonding Materials
141
adhesive primer oxide adherend
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Figure 11.
Summary of analysis of the mode of failure from data shown in Figs. 9 and 10.
Literature Cited 1. 2.
Powell, C. J.; Appl. Surf. Sci. 1978, 1, 143. Park, R. L. i n Experimental Methods i n Catalytic Research V o l . I I I , Academic Press, NY, 1976, p. 1. 3. Gossett, C. Robert, this volume. 4. Brundle, C., this volume. 5. Buck, T. M. i n "Methods of Surface Analysis" A. W. Czanderna, Ed., Elsevier Amsterdam p. 75-102 (1975). 6. Smith, D. P. J. Appl. Phys. 1967, 38, 340. 7. Benninghoven, A., Surf. Sci. 1971, 28, 541. 8. McHugh, J. A. i n "Methods of Surface Analysis", A. W. Czanderna, E d . , Elsevier Amsterdam p. 223, (1975). 9. Rusch, T. W. and Erickson, R., J. Vac. Sci. Technol. 1976, 13 374. 10. Baun, W. L. Appl. Surf. Sci. 1977, 1, 81. 11. Benninghoven, A., Surf. Sci. 1975, 53 596. 12. Baun, W. L . Surf. Technol. 1980 11. 385. 13. Dibennedeto, A. T. and Scola, D. A. J. Coll. Inter. Sci. 1978, 64, 480. 14. Gardella, J. A. and Hercules, D. M. Anal. Chem. 1980, 52, 226. 15. Thomas, G. E., van der L i g t , G., L i p p i t s , G., van de Hei, G . , Appl. Surf. Sci., 1980, 6, 204. 16. Sparrow, G. and Mishmash, E . , Abstracts 28th Pittsburgh Conf., Cleveland, Ohio, 1977. 17. Gardella, J. A. and Hercules, D.M.,Abstracts 30th Pitts burgh Conf., Cleveland, Ohio, 1979. 18. M i t t a l , J. Electrocomponent Science and Technology, 1976, 3, 21. 19. N. T. McDevitt and W. L. Baun J. Adhesion, i n press. RECEIVED June 25, 1982
In Industrial Applications of Surface Analysis; Casper, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.