Kinetics of interaction of tantalum disilicide with tantalum and other metals
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H I G H - S P E E D ai.rcraft may r e q u i r e m a t e r i a l s for a i r b r e a t h i n g turbines capable of o p e r a t i n g at t e m p e r a t u r e s approaching 1370~ (2500~ Although a l l o y s based on tantalum p o s s e s s the r e q u i s i t e high t e m p e r a t u r e s t r e n g t h , they a r e r e a d i l y o x i d i z e d . U n f o r t u n a t e l y , alloying to r e d u c e oxidation has not been s u c c e s s f u l . H o w e v e r , c o n v e r s i o n c o a t i n g s f o r m e d by s i t tciding the a l l o y s have been e m p i r i c a l l y d e v e l o p e d and have shown p r o m i s e . Although t h e s e c o a t i n g s contain e l e m e n t s added to i m p r o v e t h e i r b e h a v i o r , they a r e c o m p o s e d m a i n l y of the d i s i l i c i d e s of the s u b s t r a t e . Oxidation p r o t e c t i o n depends upon the ability of the coating to g e n e r a t e a s i l i c a - b a s e g l a s s at the i n t e r f a c e with the oxidizing e n v i r o n m e n t . T h i s g l a s s y film is s e l f - h e a l i n g as long as it is supplied with s i l i c o n f r o m the bulk of the d i s i l t e t d e coating. T a n t a l u m is capable of f o r m i n g s e v e r a l s i l i c ides : 'ram,., TasSia, Ta2Si, and Ta4.sSi have been r e p o r t e d ) Oxide f i l m s f o r m e d by the tower s i t i c i d e s do not inhibit f u r t h e r oxidation. The d i s i l t c i d e is m o s t p r o t e c tive and is the d e s i r e d coating. H o w e v e r , a d i s i t i c t d e c o a t i n g on tantalum is slowly c o n v e r t e d to a l o w e r s t l t c i d e a s s i l i c o n d i f f u s e s into the s u b s t r a t e . When c o n v e r s i o n to the tower s i t i c i d e is c o m p l e t e , the r a t e of oxidation of a coated s p e c i m e n will r i s e s h a r p l y . H e n c e , the s e r v i c e l i f e t i m e of a coating may be l i m i t e d not only by its r a t e of oxidation, but also by the r a t e at which it is c o n v e r t e d to a l o w e r s i l i c i d e through i n t e r a c t i o n with s u b s t r a t e . In the investigation r e p o r t e d h e r e , the r a t e of f o r mation of the lower s i t t c i d e TasSia between TaSta and tantalum was m e a s u r e d at t e m p e r a t u r e s f r o m 1150~ to 1370~ (2100 ~ to 2500~ Observations are also r e p o r t e d on the p h a s e s g e n e r a t e d between TaSi2 and p o s s i b l e diffusion b a r r i e r s : W, Mo, Nb, Z r , T i , and Re. It has p r e v i o u s l y been r e p o r t e d ~''a that as tong as L. I. SCHWARTZ, formerly Doctoral Student at The City College of The City Umversity of New York, New York, N. Y., is now Research Scientist at Pitney Bowes, lnc., Stamford, Conn. M. KOLODNEY and R. A. GRAFF are Professor and Associate Professor, respectively, in the Department of Chemical Engineering of The City College. Manuscript submitted September 16, 1970, M E'I" A k L.URG ICA L T R A N S A C T I O N S
TaSia is p r e s e n t , only the t r i s i t i c t d e , TasSts, g r o w s in d i s c e r n i b l e
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