Applications of Ion Beam Methods to Characterization of Adhesive

7 Applications of Ion Beam Methods to Characterization of Adhesive Bonding Materials

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

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.

122

SURFACE

ANALYSIS

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 .


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

BAUN


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

a

t

o

n

i

ο η

a

t

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Mt cosO +

(M. + M )2 ion atom

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 :

EjE 1

n

0

= (M _

atom

The o r i g i n a l a r r a n g e m e n t

for

2

atom

f r e q u e n t l y used

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(1)

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to

a

(2)

M. )/(M ^ + M. ). ion atom ion 90° scattering

in Figure

2a u s e d


an

124


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.


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


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



126

SURFACE

ANALYSIS

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.



ENERGY FILTER

Figure 3.

ION DETECTOR

Equipment for a simple SIMS system.

UHV SYSTEM


idJ

*

LL ILJ

SIMS SPECTRUM

lilllililtlttlllltlt)llUtltiUltllltflM[llllllllllUllllltt1IIW1 10 20 30 40 50

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128

SURFACE

A summary o f SIMS a n a l y s i s

is

the given

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and n e g a t i v e

i n Tables II

and

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of

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 -


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


7.


BAUN


129


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



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


7.


BAUN


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.


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


Technique

Smutted i n

De-Smutted i n

H S0, 2 4 o

AES

S i , 0 , S, C I , C, Cu, F e , C r , N i

0,

C , S,

ISS

Si, Fe,

0,

Na, F e , Cr

+SIMS

Si ,

0, C , Cr

+

+

Na ,

+

, 3

XPS

ôre Ôxide

a

C , Cu, t h a n one

+

0~, O H " ,

SiO ,

Si0 ", Fe0

S,

C u , CH , F e , 4¥ + C r , N i , Cu

C,H " , b_n Ci

+

Si0 ,

+

0H , + Fe , -SIMS

Cu(?),

Si0

SiF",

2

+

+

Na , Cr

,

SiOF",

+

K , Si ,

Cr, Ni

+

+

S ,

+

0H , CH ,

, Ni

C H ', η η

_

Fe,

HÔ^-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



7.

BAUN


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


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 . +


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 .


7.


BAUN

+ S I MS

I


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.


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 ,


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 ,


7.


BAUN


137


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-



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 .


7.


BAUN


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.


140

ANALYSIS


SURFACE

Figure 10.

ISS/SIMS data from adhesive matching areas to adherend shown in Fig. 9.


7.


BAUN


141

adhesive primer oxide adherend


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


Applications of Ion Beam Methods to Characterization of Adhesive

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