Catalytic effect of cobalt on the carburization kinetics of tungsten
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I ~ E C E N T L Y s t u d i e s have b e e n done d e a l i n g with the g a s - p h a s e c a r b u r i z a t i o n of t u n g s t e n f r o m the s t a n d point of i n i t i a l r e a c t i o n k i n e t i c s by D a v i d s o n e t a l . ~-2 F r o m t h e s e s t u d i e s a model was developed that adequately p r e d i c t s the r a t e of r e a c t i o n for a v a r i e t y of r e a c t i o n conditions. To f u r t h e r develop such a model for d e s c r i b i n g the total c a r b u r i z a t i o n p r o c e s s a b e t t e r u n d e r s t a n d i n g of the effect of the e x p e r i m e n t a l conditions on the total r e a c t i o n s e q u e n c e is n e c e s s a r y . The effect of c a t a l y s i s by t r a n s i t i o n e l e m e n t s on the r e a c tion m u s t a l s o be d e t e r m i n e d before the k i n e t i c s of c a r b u r i z a t i o n can be u n d e r s t o o d . L a t e l y m u c h work has b e e n done on the g a s - p h a s e c a r b u r i z a t i o n p r o c e s s in a n a t t e m p t to u n d e r s t a n d the r e a c t i o n s involved and the n a t u r e of the compounds f o r m e d . ~-9 R e p o r t e d l y the compounds f o r m e d d u r i n g c a r b u r i z a t i o n of t u n g s t e n have been d o c u m e n t e d by S t o r i n 3 and H a r a . 4 T h e s e i n v e s t i g a t o r s studied the f o r m a t i o n of compounds u s i n g X - r a y d i f f r a c t i o n following c a r b u r i z a t i o n of c a r b o n b l a c k - t u n g s t e n m i x t u r e s . They both o b s e r v e d that i n c o m p l e t e c a r b u r i z a t i o n produced c o m b i n a t i o n s of W, W~C and WC. F u r t h e r c a r b u r i z a t i o n yielded only compounds of W and WC, and upon complete r e a c t i o n only WC was p r e s e n t . H a r a also noted that t h e r e was a definite p a r t i c l e size d i f f e r e n c e b e t w e e n his s t a r t i n g t u n g s t e n and the r e a c t e d t u n g s t e n c a r b i d e . R o s s e t a l 5 have o b s e r v e d that d u r i n g the g a s - p h a s e c a r b u r i z a t i o n of t u n g s t e n t h e r e is a s e r i e s of r e a c t i o n s p r e s e n t which involve the f o r m a t i o n of c a r b o n on tungs t e n and the d i s s o l u t i o n - d i f f u s i o n of c a r b o n into the t u n g s t e n m e t a l where it r e a c t s to f o r m t u n g s t e n c a r b i d e . The c a r b u r i z a t i o n k i n e t i c s of t u n g s t e n - c o b a l t a l l o y s by m e t h a n e - h y d r o g e n have b e e n studied by B o n d a r e n k o e t a l . 6 T h e y noted that c a r b u r i z a t i o n was most efficient b e t w e e n the t e m p e r a t u r e s of 1000 to 1050~ but that the r e a c t i o n c o m m e n c e d at t e m p e r a t u r e s as low as 800~ T h e y a l s o noted that the cobalt compounds were r e d u c e d m o r e c o m p l e t e l y if p r e l i m i n a r y h e a t i n g to 500~ was p e r f o r m e d . The e t a - p h a s e (Co~W3C) was always p r e sent in the s a m p l e s which were u n d e r c a r b u r i
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