A Fracture Mechanics Approach to the Adhesion of

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The tensile adhesion test (A.S.T.M., 1969; BBhme,. 1979) is used widely in industry for ..... Specification C633-69. BASCOM, W.D. and BITNER, J.L. (1977).
A Fracture Mechanics Approach to the Adhesion of Flame and Plasma Sprayed Coatings C. C. BERNDT and R. McPHERSON*

SUMMARY A f r a c t u r e mechanics approach i s a p p l i e d t o measuring t h e a d h e s i o n of plasma and flame sprayed c o a t i n g s of mild s t e e l , nickel- aluminium composite and alumina a p p l i e d t o a mild s t e e l s u b s t r a t e . The e x p e r i m e n t a l method e s s e n t i a l l y i n v o l v e s i n c o r p o r a t i n g t h e c o a t i n g i n t o e i t h e r a double t o r s i o n o r double c a n t i l e v e r beam geometry. The r e s u l t s show t h a t t h e c o a t i n g adhesion can be q u a n t i t a t i v e l y determined and s e v e r a l t r e n d s a r e d i s c e r n e d and r e l a t e d t o t h e f r a c t u r e morphology. The m e t a l c o a t i n g s e x h i b i t a h i g h a d h e s i o n when compared t o t h e ceramic c o a t i n g and t h i s may be r e l a t e d t o p l a s t i c deformation of t h e i n d i v i d u a l l a m e l l a e , of which t h e c o a t i n g i s made up, d u r i n g f a i l u r e . I n p a r t i c u l a r i t h a s been found t h a t t h e s i g n i f i c a n t l y g r e a t e r a d h e s i o n of ceramic c o a t i n g s a p p l i e d over N i - A 1 bond c o a t i n g s on s t e e l , compared w i t h t h e ceramic c o a t e d d i r e c t l y o n t o s t e e l , may be a t t r i b u t e d t o t h e deformation of t h e m e t a l l i c c o a t i n g i n t h e r e g i o n of t h e bond coating- ceramic i n t e r f a c e . The t e s t i n g t e c h n i q u e d e s c r i b e d i s a p p l i c a b l e t o a d h e s i o n of measurements on o t h e r metal- ceramic c o a t i n g s . LIST OF SYMBOLS compliance ( m K 1 ) c r i t i c a l f o r c e (N) c r i t i c a l s t r a i n energy r e l e a s e r a t e ( ~ m - ~ ) s t r e s s i n t e n s i t y f a c t o r (Nm-3/2) c r a c k l e n g t h (m) specimen w i d t h (m) d i s t a n c e between t h e i n s i d e and o u t s i d e r o l l e r s f o r t h e double t o r s i o n t e s t (m) specimen t h i c k n e s s (m) t h i c k n e s s of specimen a t notched r e g i o n (m) Poisson's r a t i o INTRODUCTION The a d h e s i o n of a plasma o r flame s p r a y e d c o a t i n g t o a s u b s t r a t e i s a fundamental p r o p e r t y o f t h e c o a t i n g t h a t determines i t s use and a p p l i c a t i o n . The t e n s i l e a d h e s i o n t e s t (A.S.T.M., 1969; BBhme, 1979) i s used w i d e l y i n i n d u s t r y f o r r o u t i n e q u a l i t y c o n t r o l . The mode of f r a c t u r e can be c h a r a c t e r i s e d a s e i t h e r cohesive (within t h e coating) o r a d h e s i v e (between t h e c o a t i n g and s u b s t r a t e ) dependi n g on t h e i n t e r p a r t i c l e and p a r t i c l e - s u b s t r a t e i n t e r a c t i o n s . True a d h e s i v e f a i l u r e a t t h e i n t e r f a c e v e r y r a r e l y o c c u r s because of t h e v e r y c o a r s e n a t u r e of t h e s u b s t r a t e s u r f a c e and a d h e s i v e f a i l u r e i n t h i s case is defined a s taking place near t o the i n t e r f a c e where t h e f r a c t u r e s u r f a c e e x h i b i t s a r e a s devoid of t h e c o a t i n g . The problems a s s o c i a t e d w i t h t h e t e n s i l e a d h e s i o n t e s t s fundamentally a r i s e from u s i n g an o r g a n i c adhesive t o j o i n a plug t o t h e coating s o t h a t t h e c o a t i n g can s u b s e q u e n t l y be d e t a c h e d . The a d h e s i v e used must have a r e l a t i v e l y h i g h t e n s i l e s t r e n g t h and s u i t a b l e m a t e r i a l s may have a c u r i n g s c h e d u l e

*

M r Berndt i s a P o s t g r a d u a t e S t u d e n t and M r McPherson an A s s o c i a t e P r o f e s s o r o f M a t e r i a l s Engineering a t Monash U n i v e r s i t y . (Paper M1079 s u b m i t t e d 9 May 1980).

Mechanicnl Engineering Transactions, 1981.

t h a t can a f f e c t t h e r e s i d u a l s t r e s s d i s t r i b u t i o n of t h e c o a t i n g . The a d h e s i v e may a l s o p e n e t r a t e t h e c o a t i n g ( C a t h e r a l l and K o r t e g a s , 1972; Hermanek, 1978) and i f f a i l u r e t a k e s p l a c e through t h e comp o s i t e c o a t i n g - a d h e s i v e zone t h e n t h e t r u e t e n s i l e s t r e n g t h of t h e c o a t i n g w i l l n o t be measured. In many c a s e s t h i s t e s t produces mixed a d h e s i v e and c o h e s i v e f a i l u r e and t h e r e f o r e t h e t e n s i l e s t r e n g t h i s an a v e r a g e of b o t h of t h e s e modes of c r a c k i n g . A c o a t i n g i s inhomogeneous w i t h many flaws i n t h e form of m i c r o c r a c k s , p o r o s i t y , p a r t i c l e b o u n d a r i e s and o x i d e i n c l u s i o n s and t h e e f f e c t of t h e s e micros t r u c t u r a l c o n s t i t u e n t s on t h e mechanical p r o p e r t i e s of t h e c o a t i n g i s n o t r e a d i l y a s c e r t a i n e d from a t e n s i l e a d h e s i o n t e s t . However t h i s t e s t h a s t h e advantage of b e i n g r e l a t i v e l y s i m p l e t o perform and i s e x t e n s i v e l y used i n q u a l i t y c o n t r o l t o p r o v i d e a r a n k i n g s c a l e f o r c o a t i n g s . A f u r t h e r problem w i t h t h e t e n s i l e t e s t s i s t h a t t h e v a l u e s o b t a i n e d cannot e a s i l y be r e l a t e d , i n a fundamental way, t o t h e n a t u r e of t h e bond between c o a t i n g and s u b s t r a t e o r t o t h a t between i n d i v i d u a l p a r t i c l e s w i t h i n t h e coating.

3

FRACTURE MECHANICS

Another approach t h a t h a s been s u c c e s s f u l l y a p p l i e d t o t h e g e n e r a l problem of c o a t i n g - s u b s t r a t e i n t e r a c t i o n i s t h a t of f r a c t u r e mechanics. F r a c t u r e mechanics t e s t s , u n l i k e t h e t e n s i l e a d h e s i o n t e s t where t h e s t r e s s a t f a i l u r e i s found, measures t h e energy r e q u i r e d t o e x t e n d a c r a c k . The parameter measured i s termed t h e c r i t i c a l s t r a i n energy rel e a s e r a t e ( G 1 ~ )o r t h e f r a c t u r e toughness. The s u f f i x I r e f e r s t o t h e opening mode of l o a d i n g , r a t h e r t h a n s h e a r i n g (11) o r t e a r i n g ( I I I ) , and i s t h e deformation mechanism under t e n s i l e c o n d i t i o n s . There a r e many t y p e s of f r a c t u r e toughness t e s t s (Evans, 1974; Wachtman, 1974) and t h e s e d i f f e r r e g a r d i n g t h e specimen shape and mode of l o a d a p p l i cation. E s s e n t i a l l y a l l of t h e t e s t s r e l a t e t h e e l a s t i c p r o p e r t i e s of t h e m a t e r i a l t o t h e e x t e n s i o n of a c r a c k . I n some c a s e s , such a s t h e s i n g l e edge notched specimen, f a i l u r e i s c a t a s t r o p h i c and t h e t o t a l c r a c k p r o p a g a t i o n d i s t a n c e i s used i n conjunct i o n with the force a t f a i l u r e t o c a l c u l a t e the c r i t i c a l s t r a i n energy r e l e a s e r a t e . Crack growth '

A Fracture Mechanics Approach to the Adhesion of Flame and Plasma Sprayed Coatings C. C. BERNDT and R. McPHERSON*

A f r a c t u r e mechanics approach i s a p p l i e d t o measuring t h e adhesion of plasma and flame sprayed SUMMARY c o a t i n g s of mild s t e e l , nickel- aluminium composite and alumina a p p l i e d t o a mild s t e e l s u b s t r a t e . The experimental method e s s e n t i a l l y i n v o l v e s i n c o r p o r a t i n g t h e c o a t i n g i n t o e i t h e r a double t o r s i o n o r double c a n t i l e v e r beam geometry. The r e s u l t s show t h a t t h e c o a t i n g adhesion can be q u a n t i t a t i v e l y determined and s e v e r a l t r e n d s a r e d i s c e r n e d and r e l a t e d t o t h e f r a c t u r e morphology. The m e t a l c o a t i n g s e x h i b i t a high adhesion when compared t o t h e ceramic c o a t i n g and t h i s may be r e l a t e d t o p l a s t i c deformation of t h e i n d i v i d u a l l a m e l l a e , of which t h e c o a t i n g is made up, d u r i n g f a i l u r e . I n p a r t i c u l a r i t h a s been found t h a t t h e s i g n i f i c a n t l y g r e a t e r adhesion of ceramic c o a t i n g s a p p l i e d over Ni- A1 bond c o a t i n g s on s t e e l , compared w i t h t h e ceramic coated d i r e c t l y onto s t e e l , may be a t t r i b u t e d t o t h e deformation of t h e m e t a l l i c c o a t i n g i n t h e region of t h e bond coating- ceramic i n t e r f a c e . The t e s t i n g technique d e s c r i b e d is a p p l i c a b l e t o adhesion of measurements on o t h e r metal- ceramic c o a t i n g s . LIST OF SYMBOLS compliance (m~' l ) c r i t i c a l f o r c e (N) c r i t i c a l s t r a i n energy r e l e a s e r a t e ( ~ m - ~ ) s t r e s s i n t e n s i t y f a c t o r (Nm- 3 / 2 ) crack l e n g t h (m) specimen width (m) d i s t a n c e between t h e i n s i d e and o u t s i d e r o l l e r s f o r t h e double t o r s i o n t e s t (m) specimen t h i c k n e s s (m) t h i c k n e s s of specimen a t notched region (m) Poisson's r a t i o INTRODUCTION The adhesion of a plasma o r flame sprayed c o a t i n g t o a s u b s t r a t e i s a fundamental p r o p e r t y of t h e c o a t i n g t h a t determines i t s use and a p p l i c a t i o n . The t e n s i l e adhesion t e s t (A.S.T.M., 1969; Bbhme, 1979) i s used widely i n i n d u s t r y f o r r o u t i n e q u a l i t y c o n t r o l . The mode of f r a c t u r e can be c h a r a c t e r i s e d a s e i t h e r cohesive ( w i t h i n t h e c o a t i n g ) o r adhesive (between t h e c o a t i n g and s u b s t r a t e ) dependi n g on t h e i n t e r p a r t i c l e and p a r t i c l e - s u b s t r a t e i n t e r a c t i o n s . True adhesive f a i l u r e a t t h e i n t e r f a c e very r a r e l y occurs because of t h e very c o a r s e n a t u r e of t h e s u b s t r a t e s u r f a c e and adhesive f a i l u r e i n t h i s c a s e is d e f i n e d a s t a k i n g p l a c e n e a r t o t h e i n t e r f a c e where t h e f r a c t u r e s u r f a c e e x h i b i t s a r e a s devoid of t h e coating. The problems a s s o c i a t e d w i t h t h e t e n s i l e adhesion t e s t s fundamentally a r i s e from u s i n g an o r g a n i c adhesive t o j o i n a plug t o t h e c o a t i n g s o t h a t t h e c o a t i n g can subsequently be detached. The adhesive used must have a r e l a t i v e l y h i g h t e n s i l e s t r e n g t h and s u i t a b l e m a t e r i a l s may have a c u r i n g schedule

*

M r Berndt i s a Postgraduate Student and M r McPherson an A s s o c i a t e P r o f e s s o r o f M a t e r i a l s Engineering a t Monash University. (Paper M1079 submitted 9 May 1980).

Mechanical Engrngrneering lkansactionr, 1981.

t h a t can a f f e c t t h e r e s i d u a l s t r e s s d i s t r i b u t i o n of t h e coating. The adhesive may a l s o p e n e t r a t e t h e c o a t i n g ( C a t h e r a l l and Kortegas, 1972; Hermanek, 1978) and i f f a i l u r e t a k e s p l a c e through t h e comp o s i t e coating- adhesive zone then t h e t r u e t e n s i l e s t r e n g t h of t h e c o a t i n g w i l l n o t be measured. I n many c a s e s t h i s t e s t produces mixed adhesive and cohesive f a i l u r e and t h e r e f o r e t h e t e n s i l e s t r e n g t h is an average of b o t h of t h e s e modes of cracking. A c o a t i n g i s inhomogeneous w i t h many flaws i n t h e form of microcracks, p o r o s i t y , p a r t i c l e boundaries and oxide i n c l u s i o n s and t h e e f f e c t of t h e s e micros t r u c t u r a l c o n s t i t u e n t s on t h e mechanical p r o p e r t i e s of t h e c o a t i n g i s n o t r e a d i l y a s c e r t a i n e d from a t e n s i l e adhesion t e s t . However t h i s t e s t has t h e advantage of b e i n g r e l a t i v e l y simple t o perform and i s e x t e n s i v e l y used i n q u a l i t y c o n t r o l t o provide a ranking s c a l e f o r c o a t i n g s . A f u r t h e r problem w i t h t h e t e n s i l e t e s t s is t h a t t h e values o b t a i n e d cannot e a s i l y be r e l a t e d , i n a fundamental way, t o t h e n a t u r e of t h e bond between c o a t i n g and s u b s t r a t e o r t o t h a t between i n d i v i d u a l p a r t i c l e s w i t h i n t h e coating

.

3

FRACTURE MECHANICS

Another approach t h a t h a s been s u c c e s s f u l l y a p p l i e d t o t h e g e n e r a l problem of c o a t i n g - s u b s t r a t e i n t e r a c t i o n is t h a t of f r a c t u r e mechanics. F r a c t u r e mechanics t e s t s , u n l i k e t h e t e n s i l e adhesion t e s t where t h e s t r e s s a t f a i l u r e i s found, measures t h e energy r e q u i r e d t o extend a crack. The parameter measured i s termed t h e c r i t i c a l s t r a i n energy rel e a s e r a t e ( G 1 ~ )o r t h e f r a c t u r e toughness. The s u f f i x I r e f e r s t o t h e opening mode of loading, r a t h e r than s h e a r i n g (11) o r t e a r i n g (111), and i s t h e deformation mechanism under t e n s i l e conditions. There a r e many t y p e s o f f r a c t u r e toughness t e s t s (Evans, 1974; Wachtman, 1974) and t h e s e d i f f e r r e g a r d i n g t h e specimen shape and mode of l o a d a p p l i cation. Essentially a l l of t h e t e s t s r e l a t e the e l a s t i c p r o p e r t i e s of t h e m a t e r i a l t o t h e e x t e n s i o n of a crack. I n some c a s e s , such a s t h e s i n g l e edge notched specimen, f a i l u r e is c a t a s t r o p h i c and t h e t o t a l c r a c k propagation d i s t a n c e is used i n conjunct i o n with the force a t f a i l u r e t o calculate the c r i t i c a l s t r a i n energy r e l e a s e r a t e . Crack growth '

ADHESION OF FLAME AND PLASMA SPRAYED COATINGS - Berndt & McPherson i s s t a b l e f o r o t h e r t e s t i n g c o n f i g u r a t i o n s such a s double c a n t i l e v e r beam (DCB) and allows many f r a c t u r e toughness readings t o be taken from a s i n g l e specimen. To c a r r y o u t t h i s t e s t procedure t h e crack l e n g t h must be measured o r , i f t h e m a t e r i a l i s opaque, t h e crack l e n g t h may be i n f e r r e d from t h e measured specimen compliance u s i n g a c a l i b r a t i o n curve prepared u s i n g a specimen w i t h known c r a c k l e n g t h s . For t h e double t o r s i o n -(DT), contoured DCB and c o n s t a n t moment DCB specimens t h e crack l e n g t h i s not required t o carry out the calculation t o find t h e c r i t i c a l s t r a i n energy r e l e a s e r a t e and t h e specimen f a i l s a t a c o n s t a n t s t r e s s i n t e n s i t y factor. The adhesion of v a r i o u s t y p e s of j o i n t s and c o a t i n g s have been measured by t h e s e methods. E l s s n e r e t a2 (1975a, 1975b) used t h e s i n g l e edge notched geometry under f o u r p o i n t l o a d i n g t o f i n d t h e f r a c t u r e toughn e s s of metal- ceramic j o i n t s produced by h i g h tempe r a t u r e (up t o 1600°C) vacuum s i n t e r i n g processes. The contoured DCB and a p p l i e d moment DCB specimens have a l s o been used (Becher and Newell, 1977; Becher and Murday, 1977; Bascom and B i t n e r , 1977) t o i n v e s t i g a t e t h e adhesion of t h i c k f i l m m e t a l l i s a t i o n s t o ceramic s u b s t r a t e s . For b o t h t y p e s of t e s t t h e adhesion o f c o a t i n g s was expressed i n terms of f r a c t u r e toughness parameters ( t h e c r i t i c a l s t r a i n energy r e l e a s e r a t e , o r t h e c r i t i c a l s t r e s s i n t e n s i t y f a c t o r ) ; and t h e s e in t u r n could be r e l a t e d t o t h e f r a c t u r e morphology and mechanism of adhesion. 4

EXPERIMENTAL

4.1

Specimen P r e p a r a t i o n

a l o n g t h e specimen s i n c e t h i s would t e n d t o produce a c o a t i n g formed from p a r t i c l e s w i t h i n t h e j e t having h i g h e s t temperatures and v e l o c i t i e s and t h e r e f o r e w i t h s u p e r i o r p r o p e r t i e s than i n d u s t r i a l coatings '

.

A support b a r w i t h t h e same dimensions a s t h e s p e c i -

mens w i t h i t s edge g r i t b l a s t e d and then degreased was a t t a c h e d t o t h e c o a t i n g u s i n g an epoxy adhesive. The b a r was spaced 1-2 mm from t h e specimen and a mould formed around t h e j o i n t w i t h e i t h e r aluminium f o i l o r adhesive t a p e . The adhesive ( A r a l d i t e D w i t h hardener 951) was c a r e f u l l y prepared t o avoid a i r o r moisture entrapment d u r i n g t h e subsequent h a n d l i n g s o t h a t a d e f e c t f r e e , tough bond l a y e r could b e c a s t . The adhesive was mixed and degassed under vacuum f o r 1 5 minutes p r i o r t o pouring i n t o t h e mould; and then degassed f o r a f u r t h e r 1 5 minutes. A f t e r a 5-7 hour c u r e a t 40°C t h e excess adhesive was trimmed away and both s i d e s machine-ground without l u b r i c a n t . It should be r e p e a t e d t h a t t h e DT, compliance and DCB specimens were prepared i n e x a c t l y t h e same manner. There were s l i g h t v a r i a t i o n s i n t h e s p e c i men width (about 5 mm f o r t h e DT specimens and 6 mm f o r both t h e compliance and DCB specimens) s i n c e t h e alignment of t h e sample i n t h e mould when forming t h e adhesive j o i n t was more p r e c i s e i n t h e l a t t e r t e s t s . Therefore i t was n o t n e c e s s a r y t o e x t e n s i v e l y machine t h e specimens i n o r d e r t o make t h e 150 mm x 40 mm s u r f a c e s f l a t . Grooving of t h e specimens ( f i g u r e s 1 and 2) by g r i n d i n g i n t h e v i c i n j t y of t h e i n t e r f a c e was used t o promote e i t h e r adhesive o r cohesive f a i l u r e

.

The f r a c t u r e toughness t e s t s u t i l i s e d i n t h i s i n v e s t i g a t i o n were t h e DT and DCB. The specimens f o r both t e s t s were prepared i n t h e same manner from 150 mm x 6 mm mild s t e e l s u b s t r a t e s 20 mm t h i c k . The s u b s t r a t e edge was g r i t b l a s t e d w i t h alumina immediately p r i o r t o spraying, e i t h e r by t h e flame o r plasma p r o c e s s , c a r r i e d o u t under t h e c o n d i t i o n s d e t a i l e d i n Table I. The powder f e e d r a t e i n t o t h e plasma t a i l - f l a m e was c o n s t a n t f o r a l l samples of a s p e c i f i c c o a t i n g type. Generally f i v e samples were coated a t t h e same time by t r a v e r s i n g t h e t o r c h a c r o s s t h e specimen and working a l o n g t h e l e n g t h of t h e specimen so t h a t t h e d e p o s i t was l a i d down i n a r a s t e r p a t t e r n o n t o t h e s u b s t r a t e . I n t h i s way i t was expected t h a t t h e c o a t i n g p r o p e r t i e s would be t y p i c a l of those prepared i n d u s t r i a l l y . This method was used r a t h e r than scanning t h e t o r c h c e n t r e l i n e

4.2

Double Torsion T e s t s

For t h e DT t e s t used i n t h i s i n v e s t i g a t i o n t h e i n i t i a l 10 mm l e n g t h of t h e c o a t i n g was t a p e r e d s o t h a t t h e crack, which was s t a r t e d from a sawcut (0.2 mm t h i c k ) a t t h e l e a d i n g edge of t h e c o a t i n g t a p e r , g r a d u a l l y grew t o t h e f u l l width (about 5 nun) o f t h e specimen. For t h e DCB specimen t h e precrack was a sawcut (0.2 mm t h i c k ) which ,extended about 20 mm from t h e l o a d i n g a x i s . I n t h i s case t h e f i r s t e x t e n s i o n of t h e precrack r e s u l t s i n a s h a r p crack which can then be used i n following measurements. It should be k e p t i n mind though t h a t t h e n a t u r e of t h e precrack t i p r a d i u s may n o t be t o o s i g n i f i c a n t because ( i ) n a t u r a l micro- cracks a l r e a d y e x i s t w i t h i n t h e c o a t i n g o r ( i i ) t h e crack t i p r a d i u s may be c o n t r o l l e d by t h e presence of p o r o s i t y .

TABLE I SPRAYING CONDITIONS Coating M a t e r i a l

Deposit Thickness

Gas Flow Rate

(mm)

E l e c t r i c a l Input Voltage Current Power (volt) (amp) (kW

Estimated* Powder Feed Rate (kg hr' l )

Metco 450 (Ni-A1) Metco 1 0 1 (A1203-Ti02) Metco 91 (Mild s t e e l ) ( a ) Flame s p r a y i n g of Metco 101 powder (A1203-Ti02) c a r r i e d o u t w i t h a Metco 5P gun. Torch t o s u b s t r a t e d i s t a n c e (cm) Oxygen flow r a t e ( 8 min'l)

5-8 28

Acetylene flow r a t e (R min'l) Powder f e e d r a t e (kg hr'l)

16 1.4

(b) Plasma s p r a y i n g c a r r i e d o u t w i t h a Plasmadyne SG-lB(S1) gun u s i n g t h e c o n d i t i o n s s e t o u t above. *These v a l u e s have been e s t i m a t e d from t h e Metco d a t a s h e e t s of t h e r e l e v a n t powder. However t h e Metco equipment operated a t h i g h e r power l e v e l s s o t h e s e v a l u e s may be s l i g h t l y g r e a t e r than t h e t r u e powder flow r a t e s . l7te Institution of Engineers, Australioio

ADHESION OF FLAME AND PLASMA SPRAYED COATINGS - Bemdt & McPherson The DT t e s t s ( f i g u r e 1 ) were c a r r i e d o u t i n an I n s t r o n Universal t e s t i n g machine a t a cross- head speed of 2.5 x 10-1 mm min-' and t h e l o a d vs cross(Only flame head displacement curve recorded. sprayed alumina c o a t i n g s were t e s t e d i n t h i s way.) The c r i t i c a l s t r e s s i n t e n s i t y f a c t o r i s given by Evans (1974) :

where

5c

=

c r i t i a1 stress intensity factor ( ~ m -2,~

7

C = 2.108 x 10'~R 1 * 9 8 9 w i t h a c o e f f i c i e n t of determination of 0.989; where

C = compliance ( m K 1 ) R = c r a c k l e n g t h (m)

.

The c r i t i c a l s t r a i n energy r e l e a s e r a t e (GIG) may be determined from e q u a t i o n ( 3 ) :

where

PC = c r i t i c a l f o r c e (N) W = specimen width (m)

maximum f o r c e (N) d i s t a n c e between t h e i n s i d e and outside r o l l e r s t h i c k n e s s of specimen a t notched region specimen width specimen t h i c k n e s s Poisson's r a t i o

(e

0.25 f o r A1203)

The c r i t i c a l s t r a i n energy r e l e a s e r a t e was c a l c u l a t e d from t h e KIC d a t a u s i n g t h e r e l a t i o n s h i p GIC = K~~~ ( 1 - v 2 ) / ~ ;where t h e Young's Modulus (E) was e s t i m a t e d from t h e bulk p r o p e r t i e s of alumina. 4.3

Double C a n t i l e v e r Beam T e s t s

The plasma sprayed DCB specimens were loaded i n t e n s i o n (cross- head speed of 1 . 0 x 10-I mm min-l through h o l e s placed i n t h e c a n t i l e v e r arms (figure2). The r e l a t i v e displacement of t h e arms was measured w i t h a 25.4 mm gauge l e n g t h , 0 t o 10% extensometer. The crack l e n g t h could n o t be measured d u r i n g a t e s t s o t h e compliance of t h e specimen was measured f o r a r t i f i c i a l crack l e n g t h s t o c o n s t r u c t a c a l i b r a t i o n curve. The compliance d a t a f o r j o i n t t h i c k n e s s e s ranging from 0.5 t o 1 . 5 mm and f o r grooved and ungrooved specimens a l l f e l l on one curve ( f i g u r e 3) and t h i s f i t t e d an e m p i r i c a l expression:

DOUBLE TORSION DIMENSIONS IN mn

Equations (2) and (3) may be combined t o g i v e an e x p r e s s i o n t o f i n d t h e c r i t i c a l s t r a i n energy r e l e a s e r a t e from t h e c r i t i c a l f o r c e and c r a c k l e n g t h It has a l r e a d y (determined from t h e compliance). been e s t a b l i s h e d from f i g u r e 3 t h a t grooving does n o t s i g n i f i c a n t l y a f f e c t t h e compliance, however, t h e reduced width of t h e c o a t i n g must be used i n equation ( 3 ) t o f i n d t h e f r a c t u r e toughness. Thus i f t h e notched width i s 5 mm then:

The f r a c t u r e morphologies of t h e DT and DCB specimens were examined by scanning e l e c t r o n microscopy. In g e n e r a l t h e f r a c t u r e s u r f a c e s were vacuum cgated w i t h gold p r i o r t o examination. It was observed t h a t m e t a l l i c c o a t i n g s became charged i n t h e SEM i f t h e s e p r e c a u t i o n s were n o t taken and t h i s i n d i c a t e d t h a t some of t h e p a r t i c l e s were o x i d i s e d i n t h e plasma t a i l - f l a m e . Coatings of mild s t e e l , nickel- aluminium composite and alumina were t e s t e d . The Ni- A1 composite coati n g h a s been found t o adhere very s t r o n g l y (Longo, 1966) r e g a r d l e s s of t h e s u b s t r a t e p r e p a r a t i o n . The d e p o s i t a l s o h a s a n i d e a l s u r f a c e f i n i s h f o r subsequent c o a t i n g s and a m u l t i l a y e r system can be b u i l t up. Since t h e Ni- A1 i n t e r l a y e r c o a t i n g c o n f e r s both high adhesion t o t h e s u b s t r a t e and a l s o t o any overl a y c o a t i n g i t i s o f t e n r e f e r r e d t o a s a 'bond coating'.

5

RESULTS

GROOVING SPECIMEN

MODES OF FAILURE

The r e s u l t s f o r flame sprayed c o a t i n g s of alumina t e s t e d u s i n g t h e DT method a r e summarised i n Table 11. Generally two modes of f a i l u r e were observed. Mode I f a i l u r e occurred a t t h e h i g h e r f r a c t u r e toughness v a l u e and corresponded t o f a i l u r e w i t h i n t h e adhesive. F a i l u r e d i d n o t occur w i t h i n t h e adhesive f o r mode I1 b u t t h e c r a c k followed a mixed

Figure 1 Double t o r s i o n specimen c o n f i g u r a t i o n ALL DIMENSIONS IN mm GROOVE TO PROMOTE COHESIVE FAILURE

GROOVE TO PROMOTE ADHESIVE FAILURE

Figure 2

Double c a n t i l e v e r beam specimen configuration

.

Mechanical Engineering TPansactions.1981.

Figure 3

Compliance-crack l e n g t h c a l i b r a t i o n curve f o r c a n t i l e v e r beam specimen.

ADHESION OF FLAME AND PLASMA SPRAYED COATINGS - Berndt & McPherson

Specimen No.

TABLE I1

TABLE I11

RESULTS OF DOUBLE TORSION TESTS

RESULTS OF DOUBLE CANTILEVER BEAM TESTS

I F a i l u r e I S t r a i n EnergyG~~ Release Rate

Coating

Failure Mode

I

Cohesive Adhesive Adhesive Cohesive Cohesive

Alumina Alumina/Steel Ni-~l/A1203 Ni-A1 Mild S t e e l Mild S t e e l / Steel

adhesive- cohesive morphology w i t h t h e cohesive mode dominating where t h e r e were compressive f o r c e s i n t h e specimen. The f r a c t u r e toughness v a l u e thus i n c o r p o r a t e s both an adhesive and cohesive component and i s of l i m i t e d u s e f u l n e s s . The f r a c t o g r a p h ( f i g u r e 4) shows t h a t t h e alumina p a r t i c l e s a r e almost s p h e r i c a l and have n o t undergone much deformation. This h a s l e d t o a high p o r o s i t y c o a t i n g w i t h t h e consequence t h a t t h e o r g a n i c adhesive has p e n e t r a t e d t o t h e l o c u s of f a i l u r e and t h e t e s t i s n o t r e p r e s e n t a t i v e of t h e coating properties. The DCB t e s t s were c a r r i e d o u t on plasma sprayed c o a t i n g s and t h e r e s u l t s a r e summarised i n Table 111. Only t e s t s i n which f a i l u r e occurred e n t i r e l y w i t h i n t h e c o a t i n g have been r e p o r t e d . I n t h e c a s e s where t h i s d i d n o t occur then f a i l u r e w i t h i n t h e epoxy was a t t r i b u t e d t o e i t h e r poor pre- crack p r e p a r a t i o n o r flaws, such a s p o r o s i t y o r micro- cracks, i n t h e adhesive l a y e r . Grooving i n t h e v i c i n i t y of t h e i n t e r f a c e was used t o promote e i t h e r adhesive o r cohesive f a i l u r e . The force- displacement curves ( f i g u r e 5 ) show t h e o n s e t of cracking when t h e i n i t i a l l i n e a r region of t h e t r a c e l e v e l s out. When t h e cross- head o f t h e t e s t i n g machine is stopped a t t h i s p o i n t t h e crack grows - a s is e v i d e n t by t h e compliance change of t h e specimens. This compliance change can be found i n a number of ways which a r e i l l u s t r a t e d i n t h e i n s e t diagram of f i g u r e 5. The unloading curve (no. 3) was used t o f i n d t h e crack l e n g t h f o r t h e n e x t t e s t and i n t h i s manner any anomalies a s s o c i a t e d w i t h t h e i n i t i a l l o a d i n g of t h e specimen (such a s misalignment of t h e g r i p s ) could be accounted f o r . Once t h e c r a c k l e n g t h and f o r c e is known e q u a t i o n (4) can be a p p l i e d t o f i n d GI=.

(J m-2)

Type

Adhes i v e

= mean; S = s t a n d a r d d e v i a t i o n The d i s t r i b u t i o n of t h e GIG v a l u e s f o r s i m i l a r coati n g s was l a r g e s o t h e i n d i v i d u a l r e s u l t s were ranked and p r o b a b i l i t y p l o t s c o n s t r u c t e d ( f i g u r e 6) t o observe t r e n d s . A bimodal d i s t r i b u t i o n was e v i d e n t f o r alumina c o a t i n g s . High %C v a l u e s corresponded t o f a i l u r e a t t h e bond l a y e r - c o a t i n g i n t e r f a c e and lower v a l u e s t o cohesive f a i l u r e w i t h i n t h e alumina. Adhesive f a i l u r e f o r alumina on s t e e l occurred w i t h a lower GIC v a l u e than cohesive f a i l u r e .

The f r a c t u r e s u r f a c e s o f cohesive and adhesive f a i l u r e s f o r ceramic and metal c o a t i n g s appeared s i m i l a r ( f i g u r e s 7,8). Cohesive f a i l u r e always occurred between t h e l a m e l l a e of i n d i v i d u a l p a r t i c l e s except t h a t p l a s t i c deformation was observed f o r metal c o a t i n g s whereas ceramic c o a t i n g s ( f i g u r e 7) e x h i b i t e d c r a c k i n g through t h e edges of p a r t i c l e s . The processes involved d u r i n g adhesive f a i l u r e were most e a s i l y observed f o r metal c o a t i n g s which revealed p a r t i c l e s completely s t r i p p e d from t h e grit b l a s t e d s u b s t r a t e o r p a r t i c l e s which had been ext e n s i v e l y deformed. Adhesive f a i l u r e i n t h e case of t h e ceramic c o a t i n g s t i l l took p l a c e a l o n g t h e i n t e r f a c i a l boundary b u t t h e f e a t u r e s were n o t a s w e l l d e f i n e d a s i n t h e c a s e of metal coatings. Some a r e a s of t h e s u b s t r a t e were devoid of t h e c o a t i n g m a t e r i a l whereas o t h e r s e x h i b i t e d f a i l u r e between t h e l a m e l l a e of t h e coating. The adhesive f r a c t u r e s u r f a c e between t h e bond c o a t i n g and alumina e x h i b i t e d a r e a s c h a r a c t e r i s t i c of f a i l u r e through both metal and ceramic r e g i o n s ; and t h e s e a r e i n d i c a t e d on f i g u r e 8. '

6

DISCUSSION

It i s important t o d i s t i n g u i s h t h e d i f f e r e n t r o l e s t h a t t h e o r g a n i c adhesive h a s i n t h e t e n s i l e adhesion specimens and t h e f r a c t u r e toughness specimens. For t h e t e n s i l e adhesion t e s t specimens t h e adhesive

RUN 52

0

1

2

3

L

S

6

7

8

9

Ni-AI

1

0

1

1

1

2

1

3

DISPLACEMENT (remetres)

Figure 5 Figure 4

F r a c t u r e s u r f a c e of s u b s t r a t e f o r a double t o r s i o n specimen.

Typical force- displacement curves f o r DCB t e s t s a t v a r i o u s crack l e n g t h s . I n s e t : force-displacement l i n e s f o r compliance measurement.

The Institution ofEngineers. Auatraiirr.

ADHESION OF FLAME AND PLASMA SPRAYED COATINGS - Berndt 6'McPherson

10

30

Figure 6

Figure 7

50

70

90

110 4 0 0 5 0 0 6 0 0 7 W 8 0 0 G- STRAIN ENERGY RELEASE RATE ( J m-2)

1 0 0 2 a 0 3 0 0 W X ) 5 0 0

P r o b a b i l i t y p l o t s of DCB r e s u l t s f o r alumina, nickel- aluminium and m i l d .s t e e 1 c o a t i n g s

Cohesive f a i l u r e s u r f a c e o f alumina coating

must have a g r e a t e r t e n s i l e s t r e n g t h than t h e c o a t i n g t h a t is t e s t e d and f a i l u r e of t h e specimen always occurs a t t h e weakest s e c t i o n of t h e c o a t i n g ( o r f o r h i g h s t r e n g t h c o a t i n g s w i t h i n t h e adhesive). Usually t h e c o a t i n g f a i l u r e is complex w i t h c o n t r i b u t i o n s from both t h e adhesive and cohesive types. However i t is nonually expected t h a t one f a i l u r e type i s t y p i c a l of a c o a t i n g and t h e l i k e l i h o o d of e x c l u s i v e l y o b t a i n i n g t h e o t h e r f a i l u r e type i s s l i g h t ; o r i f t h e o t h e r f a i l u r e type does occur i t does n o t r e p r e s e n t optimum c o a t i n g p r o p e r t i e s . Thus f o r a N i - A 1 c o a t i n g , which h a s a high adhesive s t r e n g t h , t h e t e s t i n g c o n d i t i o n s may n o t b e appropr i a t e t o observe p u r e l y adhesive f a i l u r e and t h i s limits a'study into factors controlling a l l the a s p e c t s of c o a t i n g adhesion. On t h e o t h e r hand t h e f r a c t u r e mechanics approach does noe s u f f e r from t h i s l i m i t a t i o n because t h e t e n s i l e s t r e n g t h of t h e o r g a n i c a d h e s i v e is r e l a t i v e l y unimportant compared t o its own f r a c t u r e toughness p r o p e r t i e s . The o r g a n i c adhesives t h a t were used had a GIC of up t o 1000 ~ m and - ~ thus c r a c k i n g was r e s t r i c t e d t o t h e coating. The DCB geometry h a s been used w i t h s u c c e s s t o measure f r a c t u r e toughness parameters i n both adhesive and cohesive f a i l u r e modes and t h e s t r a i n energy r e l e a s e r a t e v a l u e s o b t a i n e d (GIG) appear t o provide a measure of t h e t r u e adhesion of t h e coating. A number of i n d i v i d u a l determinations may be made on one specimen ( t h e number of readings i n c r e a s e s w i t h t h e v a l u e of GIG), a l l o w i n g s t a t i s t i c a l a n a l y s i s o f t h e r e s u l t . Although t h e crack l e n g t h needs t o be known, i t i s r e a d i l y o b t a i n a b l e

Mechanical Engineering 7hmactions, 1981.

Figure 8

Adhesive f a i l u r e s u r f a c e o f alumina c o a t i n g on nickel- aluminium. Various r e g i o n s i n d i c a t e d .

from a c a l i b r a t i o n t e s t . The c a l i b r a t i o n curve is independent of t h e c o a t i n g type and t h i c k n e s s s o t h a t only one c a l i b r a t i o n i s n e c e s s a r y f 0 r . a given specimen geometry. The DT tests o f flame sprayed alumina c o a t i n g s were l i m i t e d by t h e p e n e t r a t i o n o f adhesive because of t h e high c o a t i n g p o r o s i t y s o t h a t t h e cohesive f a i l u r e r e s u l t s were f o r a composite f r a c t u r e p a t h through adhesive and. coating. This e x p l a i n s t h e h i g h e r GIG v a l u e s o b t a i n e d t h a n f o r DCB t e s t s pa plasma sprayed c o a t i n g s . The DT t e s t . a l s o has. t h e p e c u l i a r i t y OE producing an o b l i q u e f r a c t u r e aGross t h e c o a t i n g t h i c k n e s s a s a r e s u l t of t h e s t r e s s p a t t e r n s a r i s i n g from t h e f o u r p o i n t l o a d i n g arrangement. I n t h e compressive region t h e c o a t i n g f a i l e d cohesively whereas f a i l u r e was adhesive i n t h e t e n s i l e region. I n c r e a s e d s e n s i t i v i t y w i t h more c o n t r o l o v e r t h e c r a c k path may be o b t a i n e d by a l t e r i n g t h e geometrical arrangement of t h e l o a d i n g p o i n t s and t h e specimen design. Thus t h e r e l a t i v e angle a t f r a c t u r e between t h e specimens could be decreased and t h i s would change t h e s t r e s s d i s t r i b u t i o n i n such a way t h a t e i t h e r wholly cohesive o r adhesive f a i l u r e occurred. It should b e remembered t h a t t h e a n a l y s i s o f t h e DT c o n f i g u r a t i o n assumes t h a t t h e end of t h e specimen is b u i l t i n and t h i s is c e r t a i n l y n o t t h e c a s e f o r t h e adhesive j o i n t desc r i b e d . The t r u e DT mode of l o a d i n g can b e o b t a i n e d by r i g i d l y clamping t h e f r e e end of t h e specimen b u t t h i s c o n f i g u r a t i o n h a s n o t been r i g o r o u s l y analysed (Outwater and Gerry, 1969). Although more s a t i s f a c t o r y r e s u l t s may perhaps be o b t a i n e d w i t h low

ADHESION OF FLAME AND PLASMA SPRAY ED COATINGS - Berndt & McPherson

58

p o r o s i t y c o a t i n g s and a change i n specimen geometry, t h e main advantage of t h e DT t e s t , t h a t t h e crack l e n g t h need n o t be measured, is n o t s e e n t o be s u f f i c i e n t t o outweight the c o n s i d e r a b l e advantages of t h e DCB geometry. The f r a c t u r e toughness p r o p e r t i e s of t h e c o a t i n g s a r e considerably lower than those of t h e b u l k m a t e r i a l s ; 40-90 ~ m f" o r~ alumina (Wiederhorn, 1968) and 600-6000 ~ m (Hahn - ~ e t aZ. , 1972) f o r t h e b r i t t l e f r a c t u r e of s t e e l . These comparatively poor mechanical p r o p e r t i e s o f t h e c o a t i n g s a r e c o n s i s t e n t with t h e l a m e l l a r m i c r o s t r u c t u r e of thermally sprayed c o a t i n g s which a r i s e s from t h e process of c o a t i n g formatiron. Molten d r o p l e t s s t r i k i n g t h e s u b s t r a t e a t h i g h v e l o c i t y (100-400 m.sec-l) f l a t t e n and s o l i d i f y i n microseconds, t h e m i c r o s t r u c t u r e theref o r e c o n s i s t s of l a y e r s of t h e s e i n d i v i d u a l l y s o l i d i f i e d l a m e l l a e . The p a r t i c l e temperature and v e l o c i t y d i s t r i b u t i o n s a r e q u i t e l a r g e and t h e t h i c k n e s s of t h e l i m e l l a e formed, t h e degree of p o r o s i t y because of imperfect c o n t a c t between a n impacting p a r t i c l e and p r e v i o u s l y s o l i d i f i e d m a t e r i a l , and t h e n a t u r e of t h e i n t e r f a c e formed between t h e d r o p l e t and t h e s u r f a c e i t s t r i k e s w i l l vary from p o i n t t o p o i n t . I n t h e case of m e t a l l i c c o a t i n g s t h e p a r t i c l e s may p a r t i a l l y o x i d i s e d u r i n g f l i g h t r e s u l t i n g i n t h e presence of i n t e r l a m e l l a r oxide l a y e r s . The morphology of t h e cohesive f r a c t u r e s u r f a c e s of both m e t a l l i c and ceramic c o a t i n g s were s i m i l a r w i t h f a i l u r e o c c u r r i n g predominantly between lamellae. The much h i g h e r f r a c t u r e energy observed f o r t h e m e t a l l i c c o a t i n g s may be accounted f o r however by t h e f a c t t h a t ceramic c o a t i n g s f a i l e d p a r t l y by b r i t t l e f r a c t u r e of l a m e l l a e whereas p l a s t i c deformation occurred i n t h e m e t a l l i c lamellae. S i m i l a r l y adhesive f a i l u r e of m e t a l l i c c o a t i n g s on a s t e e l s u b s t r a t e involved p l a s t i c deformation of t h e l a m e l l a e g i v i n g a h i g h f r a c t u r e s u r f a c e energy a s s o c i a t e d w i t h good adhesion. I n t h e case of f r a c t u r e a l o n g t h e i n t e r f a c e between N i - A 1 bond coati n g and aluminium oxide f a i l u r e occurred between t h e lamellae of each c o a t i n g r e s u l t i n g i n a r e l a t i v e l y h i g h f r a c t u r e s u r f a c e energy because o f -p l a s t i c deformation of t h e m e t a l l i c lamellae. This probably e x p l a i n s t h e e f f e c t i v e n e s s of a m e t a l l i c bond c o a t on t h e adhesion of ceramic c o a t i n g s t o metals; s i n c e f a i l u r e along t h e i n t e r f a c e between b o t h t h e substrate- bond c o a t and bond coat- ceramic involve p l a s t i c deformation and t h u s a c o n s i d e r a b l e i n c r e a s e i n f r a c t u r e s u r f a c e energy compared w i t h a ceramic c o a t i n g sprayed d i r e c t l y onto s t e e l . 7

CONCLUSIONS

The adhesion of plasma and flame sprayed c o a t i n g s can be q u a n t i t a t i v e l y expressed i n terms of f r a c t u r e toughness parameters. A modified DCB specimen, i n c o r p o r a t i n g r i g i d arms, h a s been used t o measure t h e adhesive and cohesive f r a c t u r e p r o p e r t i e s of metal and ceramic c o a t i n g s . The adhesion of t h e c o a t i n g i s fundamentally c o n t r o l l e d by t h e deformat i o n mechanism of t h e p a r t i c l e s d u r i n g f r a c t u r e . For metal c o a t i n g s where t h e p a r t i c l e s e x t e n s i v e l y deform a l a r g e amount of energy i s n e c e s s a r y t o make a crack grow and t h u s t h e adhesion of t h e c o a t i n g s , a s measured by i t s f r a c t u r e toughness, i s a l s o high. The r o l e of t h e bond l a y e r i n promoting adhesion of a ceramic c o a t i n g can b e seen a s allowing d u c t i l e deformation a t t h e i n t e r f a c e between t h e two c o a t i n g s .

8

ACKNOWLEDGEMENTS

This work i s supported by t h e A u s t r a l i a n Welding Research Association under Contract No. 52 " S t r u c t u r a l Adhesion of Plasma and Flame Sprayed Coatings"

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REFERENCES

AMERICAN SOCIETY FOR TESTING MTERIALS (1969)Adhesive o r cohesive s t r e n g t h of flame- sprayed c o a t i n g s . S p e c i f i c a t i o n C633-69. BASCOM, W.D. and BITNER, J.L. (1977). A f r a c t u r e approach t o t h i c k f i l m adhesion measurements. J.Mater.Sci., Vol. 12, pp 1401-1410. BECHER, P.F. and MURDAY, J.S. (1977). Thick f i l m adherence f r a c t u r e energy: i n f l u e n c e of alumina s u b s t r a t e s . J.Mater.Sci., Vol. 12, pp 1088-1094. BECHER, P.F. and NEWELL, W.L. (1977). Adherencef r a c t u r e energy of a glass- bonded t h i c k f i l m conductor; e f f e c t of f i r i n g c o n d i t i o n s . J.Mater.Sci., Vol. 12, pp 90-96. B O W , D. (1979). Modification of a known and development of a new t e s t i n g method f o r t h e determination of t h e adhesive s t r e n g t h of thermally sprayed coatings. Advances i n Surface Coating Technology, I n t e r n a t i o n a l Conference,.London, 13-15 Feb. 1978. Pub. The Welding I n s t i t u t e .

CATHERALL, J.A. and KORTEGAS, K.E. (1972). Measurement of bond s t r e n g t h o f flame sprayed d e p o s i t s . Metal Construction and Br.Weld.J., Vol. 4, pp 11-13. ELSSNER, G. and PABST, R. (1975a). Adhesive j o i n t s of ceramic m a t e r i a l s and r e f r a c t o r y metals. Proc.Br.Ceram.Soc., Vol. 25, pp 179-186. ELLSNER, G., RIEDEL, S. and PABST, R. (1975b). Fractography and f r a c t u r e p a t h s i n ceramic- metal composites. Pract.Metallogr., Vol. 12, pp 234-243. EVANS, A.G. (1974). F r a c t u r e mechanics determinations. ~ r a c t b r eMechanics of Ceramics, Ed. R.C. Bradt, D.P.H. Hasselman and F .F. Lange, Vol. 1, pp 17-48. HAHN, G.T., KANNINEN, M.F. and ROSENFIELD, A.R. (1972). F r a c t u r e toughness of m a t e r i a l s . Annu.Rev. Mater.Sci., Vol. 2, pp 381-404.

HERMANEK, E. J. (1978). Determining t h e adhesive and cohesive s t r e n g t h s of t h i n thermally sprayed d e p o s i t s . Welding J., Vol. 57, pp 31-35. LONGO, F.N. (1966.). Metallurgy of flame sprayed n i c k l e aluminide c o a t i n g s . Weld.J.Supplement, Vol. 45, pp 66s-69s. OUTWATER, J . O . and GERRY, D . J . (1969). On t h e f r a c t u r e energy, r e h e a l i n g v e l o c i t y and r e f r a c t u r e energy of c a s t epoxy r e s i n . J.Adhes., Vol. 1, pp 290-298. WACHTMAN, J.B. Jr. (1974). Htighlights of progress i n t h e s c i e n c e of f r a c t u r e of ceramics and g l a s s . J.Am.Ceram.Soc., Vol. 57, pp 509-519. WIEDERHORN, S.M. (1968). F r a c t u r e of ceramics. ~ e c h a n i c a land Thermal P r o p e r t i e s of Ceramics, Proc. Symposium, Gaithersburg, Maryland, A p r i l 1-2, (1968), Ed. J.B. Wachtman, J r . , Pub. United S t a t e s National Bureau of Standards, SP 303.

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