Stability of single crystal sapphire in nickel and nickel-alloy matrices
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u s e of m e t a l - m a t r i x c o m p o s i t e s for e l e v a t e d temperature applications is recognized as a potential m a t e r i a l s solution for s o m e s t r u c t u r a l c o m p o n e n t s that r e q u i r e high s t r e n g t h and s t i f f n e s s c o m b i n e d with low weight. N i c k e l - b a s e c o m p o s i t e s a r e p a r t i c u l a r l y a t t r a c t i v e b e c a u s e n i c k e l i s the b a s e of m a n y of the c u r r e n t oxidation r e s i s t a n t s u p e r a l l o y s , which have useful p r o p e r t i e s up to a p p r o x i m a t e l y 1050~ Such a l l o y s have b e e n r e i n f o r c e d with r e f r a c t o r y m e t a l w i r e f i b e r s such a s t u n g s t e n and m o l y b d e n u m , and have shown exc e l l e n t high t e m p e r a t u r e p r o p e r t i e s . 1'2 However, such c o m p o s i t e s a r e r e l a t i v e l y heavy and r e a c t with n i c k e l b a s e a l l o y s to v a r y i n g d e g r e e s , depending on m a t r i x c o m p o s i t i o n and t e m p e r a t u r e . A s u b s t a n t i a l i m p r o v e m e n t could r e s u l t if l o w - d e n s i t y c e r a m i c f i l a m e n t s w e r e u s e d in place of r e f r a c t o r y m e t a l f i l a m e n t s . F r o m a c h e m i c a l s t a b i l i t y viewpoint, oxide f i l a m e n t s a r e v e r y p r o m i s i n g , but b e c a u s e of t h e i r p o l y c r y s t a l l i n e n a t u r e o r t h e i r fine size and d i a m e t e r (such a s w h i s k e r s ) they have not in the past s a t i s f i e d all the d e s i r e d m e c h a nical or chemical requirements. Recently, however, s e m i c o n t i n u o u s s i n g l e - c r y s t a l a l u m i n u m oxide f i l a m e n t s having both s t r e n g t h and a high d e g r e e of c h e m ical s t a b i l i t y have b e c o m e a v a i l a b l e . 3 The p r o b l e m is not e n t i r e l y solved b e c a u s e c e r t a i n c h e m i c a l r e a c t i o n s at t e m p e r a t u r e s in e x c e s s of 1000~ even though they s e e m m i n o r in extent, can g r e a t l y r e d u c e the s t r e n g t h of oxide f i l a m e n t s such a s s a p p h i r e by a s m u c h a s a f a c t o r of t h r e e . 4 Such r e a c t i o n s m u s t be avoided (or at l e a s t m i n i m i z e d ) in o r d e r to u t i l i z e fully the p o t e n t i a l of the r e i n f o r c i n g f i l a m e n t . T h e r e a r e at l e a s t two a p p r o a c h e s which m a y be t a k e n to m i n i m i z e the s t r e n g t h - d e g r a d i n g effects of such c h e m i c a l r e a c t i o n s . One a p p r o a c h , which has b e e n s u c c e s s f u l l y u s e d i n t h i s l a b o r a t o r y , i s to p r o tect the f i l a m e n t s u r f a c e with a thin stable coating. 4'~ T h e other a p p r o a c h i n v o l v e s studying the c o m p a t i b i l i t y of the f i l a m e n t s and m a t r i x , and d e t e r m i n i n g the effect of m a t r i x alloy e l e m e n t s on the d e g r e e of f i l a m e n t m a t r i x r e a c t i o n . 6 T h i
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