Boron and boron carbide coatings by vapor deposition
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A. COCHRAN
AND
JAMES
B. STEPHENSON
The B u r e a u of Mines i n v e s t i g a t e d the f o r m a t i o n of b o r o n and b o r o n - c a r b i d e c o a t i n g s by v a p o r p h a s e r e a c t i o n s . O p t i m u m p a r a m e t e r s w e r e d e t e r m i n e d f o r h y d r o g e n r e d u c t i o n of b o r o n t r i c h l o r i d e and f o r the f o r m a t i o n of b o r o n - c a r b i d e c o a t i n g s on g r a p h i t e by r e a c t i o n with the d e p o s i t i n g b o r o n . At 1300~ about 85 pct of the b o r o n was d e p o s i t e d . Tungsten s u b s t r a t e s did not r e a c t with the b o r o n d e p o s i t ; o t h e r s u b s t r a t e s r e a c t e d to v a r i o u s e x t e n t s . The h y d r o g e n r e duction of b o r o n t r i b r o m i d e was b r i e f l y i n v e s t i g a t e d . Boron c a r b i d e w a s d e p o s i t e d at 1300~ b y adding methane to the b o r o n t r i c h l o r i d e - h y d r o g e n feed gas. The c h e m i c a l c o m p o s i t i o n of the v a p o r - d e p o s i t e d b o r o n c a r b i d e a p p r o x i m a t e d B4C. A m e t h o d of e t c h i n g B4C was d e v e l o p e d to study i t s m i c r o s t r u c t u r e . When b o r o n w a s d e p o s i t e d on g r a p h i t e at 1500~ v e r y h a r d , u n i f o r m , s t r o n g l y a d h e r e n t c o a t i n g s of B4C w e r e f o r m e d that might be useful in a p p l i c a t i o n s such a s r o c k e t n o z z l e s and c h e m i c a l r e a c t i o n and p r o c e s s i n g v e s s e l s .
ALTHOUGH
b o r o n m i n e r a l s h a v e long b e e n u s e d a s fluxes, w a t e r s o f t e n e r s , and c o m p o n e n t s of l o w - e x p a n sion g l a s s , m e t a l l u r g i c a l r e s e a r c h on the e l e m e n t i t s e l f i s r e l a t i v e l y r e c e n t . The r e a s o n f o r t h i s d e l a y i s p r o b a b l y the high t e m p e r a t u r e r e q u i r e d f o r the p r o d u c tion of the e l e m e n t f r o m i t s c o m p o u n d s , and the g r e a t r e a c t i v i t y of b o r o n at t h e s e t e m p e r a t u r e s . During the twentieth c e n t u r y , a n u m b e r of i n v e s t i g a t o r s p r o d u c e d both b o r o n and b o r o n c a r b i d e (B4C), often by v a p o r d e position.1 The use of B4C b e c a m e m o r e w i d e s p r e a d a s i t s p h y s i c a l p r o p e r t i e s b e c a m e b e t t e r known. B o r o n a l l o y s and compounds a r e useful in n u c l e a r r e a c t o r a p p l i c a t i o n s due to b o r o n ' s high c a p a c i t y f o r a b s o r b i n g slow n e u t r o n s . 2,3 Boron and B4C w h i s k e r s and f i l a m e n t s have b e e n w i d e l y u s e d r e c e n t l y due to t h e i r g r e a t s t r e n g t h and light weight. 4,5 B4C i s u s e d i n d u s t r i a l l y f o r d r i l l i n g tungsten die nibs, f o r a b r a s i v e - b l a s t i n g n o z z l e s , f o r r e p l a c e m e n t of s a p p h i r e - j e w e l e d b e a r i n g s , for w i r e - d r a w i n g d i e s , and s i m i l a r a p p l i c a t
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