Observation of plastic deformation in sapphire base ceramics
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Observation of Plastic Deformation in Sapphire Base Ceramics GEORGE F. H U R L E Y
BOTH s a p p h i r e
and r u b y a r e r e f r a c t o r y i n o r g a n i c o x i d e s whose s t i f f n e s s and potential s t r e n g t h m a k e t h e m a t t r a c t i v e s t r u c t u r a l m a t e r i a l s . In addition, r e c e n t p r o g r e s s in the growth of t h e s e and s i m i l a r m a t e r i a l s in f i l a m e n t a r y f o r m has m a d e the d e t a i l e d nat u r e of t h e i r m e c h a n i c a l b e h a v i o r all the m o r e p e r tinent. 1-3 In p a r t i c u l a r , the r o l e of p l a s t i c d e f o r m a t i o n in the low to m i l d l y e l e v a t e d (600~ t e m p e r a t u r e int e r v a l is of c o n s i d e r a b l e i n t e r e s t , since q u e s t i o n s of s t a t i c fatigue and t e m p e r a t u r e dependence of f r a c t u r e p h e n o m e n a a r e r e l a t e d to such d e f o r m a t i o n . The c a p a c i t y of s a p p h i r e to undergo p l a s t i c d e f o r m a t i o n at r o o m t e m p e r a t u r e is a s u b j e c t of c o n s i d e r able c o n t r o v e r s y . P l a s t i c flow has b e e n e s t a b l i s h e d 4 in m e c h a n i c a l t e s t s above 900~ and can be u s e d to e x p l a i n the i n c r e a s e in s t r e n g t h s s e e n for s a p p h i r e and ruby b e t w e e n 600 ~ and 1000~ B e l o w 600~ t h e r e is a d r a m a t i c , i n v e r s e e f f e c t of t e m p e r a t u r e on the f r a c t u r e s t r e n g t h which m a y be e x p l a i n e d by d i s l o c a tion m o v e m e n t p h e n o m e n a , 6 but which is too g r e a t to be e x p l a i n e d by t e m p e r a t u r e e f f e c t s on s u r f a c e e n e r g y or e l a s t i c modulus. ~ Two r e c e n t i n v e s t i g a t i o n s of f r a c t u r e p h e n o m e n a in s a p p h i r e r e a c h e d opposite c o n c l u s i o n s r e g a r d i n g the r o l e of slip. Congleton and P e t c h 7,s concluded that d i s l o c a t i o n motion was r e s p o n s i b l e for the t e m p e r a t u r e dependence of f r a c t u r e in s a p p h i r e at t e m p e r a t u r e s n e a r a m b i e n t . T h i s a r g u m e n t followed t h e i r o b s e r v a t i o n s in c o n d i t i o n s not p e r m i t t i n g e n v i r o n m e n t a l c a u s e s of s t a t i c fatigue. W i e d e r h o r n , 9 on the o t h e r hand, concluded that p l a s t i c flow did not o c c u r d u r i n g f r a c t u r e , m a i n l y s i n c e he did not o b s e r v e c o n c e n t r a t i o n s of d i s l o c a t i o n s along fracture surfaces. The c r e a t i o n of i n d e n t a t i o n s (such as those f o r m e d in m i c r o h a r d n e s s t e s t i n g ) without the o c c u r r e n c e of chipping is a c l e a r i n d i c a t i o n of p l a s t i c flow. Two r e c e n t s t u d i e s of m i c r o h a r d n e s s indentations in g e r m a n i u m have s u g g e s t e d that d i s l o c a t i o n motion o c c u r s in this m a t e r i a l at r o o m t e m p e r a t u r e , r~ Germ a n i u m , in fact, s
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