Mechanical and structural characterization of sapphire ribbons and continuous filaments
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AND
J. T. A. POLLOCK
The m e c h a n i c a l and s t r u c t u r a l c h a r a c t e r i z a t i o n of s a p p h i r e r i b b o n s and f i l a m e n t s is p r e s e n t e d . Ribbons w e r e grown in s h o r t (12 in.) lengths which w e r e cut and t e s t e d in q u a r t e r point loading. The r i b b o n s had t h i c k n e s s e s in the r a n g e 0.007 to 0.024 in., and widths i n the r a n g e 0.027 to 0.260 in. and d i s p l a y e d a m a x i m u m a v e r a g e f a i l u r e strength of 300 ksi. F i l a m e n t s w e r e grown in e s s e n t i a l l y continuous l e n g t h s with a c i r c u l a r c r o s s - s e c t i o n n o m inally 0.010 in. in d i a m . The a v e r a g e t e n s i l e s t r e n g t h was 400 k s i . Both r i b b o n s and f i l a m e n t s w e r e a n n e a l e d at 1860~ for v a r i o u s t i m e s . Strength e n h a n c e m e n t s a r e m e a s u r e d in a n n e a l e d r i b b o n s and the i n c r e a s e s a r e c o m p a r e d with c a l c u l a t e d v a l u e s obtained by a n a l y s i s b a s e d on the edge c u r v a t u r e of split a s - g r o w n r i b b o n s . No evidence of s t r e n g t h e n h a n c e m e n t was noted in a n n e a l e d f i l a m e n t . I n s t e a d , s t r e n g t h d e g r a d a t i o n s w e r e m e a s u r e d in a n n e a l e d f i l a m e n t s and a r e r e l a t e d to the i n t r o d u c t i o n of s t r e s s c o n c e n t r a t o r s through t h e r m a l etching. T h e s t r e n g t h - c o n t r o l l i n g f e a t u r e of f i l a m e n t is d i s c u s s e d in t e r m s of a v o i d - f r e e shell at the s u r f a c e .
MANYr e f r a c t o r y
m a t e r i a l s have r e c e i v e d e x t e n s i v e attention due to t h e i r p r o j e c t e d i m p o r t a n c e a s high s t r e n g t h , high t e m p e r a t u r e s t r u c t u r a l r e i n f o r c e m e n t s . S e v e r a l of t h e s e m a t e r i a l s such as SiC, B, and C a r e a l r e a d y finding use in p l a s t i c and a l u m i n u m m a t r i x c o m p o s i t e s . However, the l a t t e r m a t e r i a l s a r e t h e r m o d y n a m i c a l l y u n s t a b l e in m o s t m e t a l m a t r i c e s at h i g h e r t e m p e r a t u r e s (>500~ w h e r e the m o s t i m p o r t a n t a p p l i c a t i o n s for t h e i r high t e m p e r a t u r e / h i g h s t r e n g t h c h a r a c t e r i s t i c s a r e to be found. Sapphire, a-A1203, m a y also be i n c l u d e d ' a m o n g the r e f r a c t o r y m a t e r i a l s with e x c e l l e n t high t e m p e r a t u r e / h i g h s t r e n g t h p r o p e r t i e s and, m o s t i m p o r t a n t , it is t h e r m o d y n a m i c a l l y stable in many m e t a l s and alloys at high t e m p e r a t u r e s . Sapphire has room t e m p e r a t u r e v a l u e s of 2000 kg p e r sq m m and 67.5 • 106 p s i for its h a r d n e s s and Y o u n g ' s modulus, respectively. The m e c h a n i c a l p r o p e r t i e s have b e e n i n v e s t i g a t e d for s i n t e r e d p o l y c r y s t a l l i n e a l u m
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