Oxidation properties of WC3015
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AN i m p o r t a n t
technology area affecting development of t h e S p a c e S h u t t l e i s t h e T h e r m a l P r o t e c t i o n S y s t e m (TPS). Preliminary designs show that aerodynamic h e a t i n g d u r i n g r e e n t r y to e a r t h a t m o s p h e r e w i l l r a i s e t h e T P S t e m p e r a t u r e in s o m e s u r f a c e a r e a s to a b o u t 2 4 0 0 ~ (1316~ 1 U n f o r t u n a t e l y , n o n e of t h e p r e s e n t primary columbium(Nb) alloy candidates (C129Y, C b 7 5 2 , a n d FS85) a r e s u f f i c i e n t l y o x i d a t i o n r e s i s t a n t a t t h i s t e m p e r a t u r e to b e u t i l i z e d w i t h o u t p r o t e c t i v e coatings. WC3015 is a newly developed columbium alloy a n d h a s b e e n r e p o r t e d to h a v e a s u p e r i o r o x i d a t i o n r e s i s t a n c e to t h e c a n d i d a t e a l l o y s . 2 O t h e r m a t e r i a l s selection properties such as ductility, ductile-to-brittle t r a n s i t i o n t e m p e r a t u r e , a n d f a b r i c a b i l i t y of W C 3 0 1 5 a t t h i s s t a g e in t h e a l l o y d e v e l o p m e n t a r e n o t a s g o o d a s t h o s e of the p r i m a r y c a n d i d a t e s . H o w e v e r , if t h i s a l l o y h a s a b e t t e r o x i d a t i o n r e s i s t a n c e in t h e b a r e c o n d i t i o n s o t h a t it c a n r e l i a b l y w i t h s t a n d s h o r t - t i m e e x p o s u r e , it c o u l d l e a d to a s a f e r T P S , s i n c e o t h e r a l l o y s d e g r a d e rapidly when coatings are damaged. A 4 h r e x p o s u r e of s e v e r a l W C 3 0 1 5 " a n d Cb752~ s a m *Heat 590216, Cb-28.5Hf-14.4W-4.6Ti-lZr-0.1C-O.05Y.
Cb752
WC3015
Fig. 1--Effect of 4 hr exposure at 2400~ (1316~ and Cb752 samples, with 0.021 in. thickness.
Table I. Effect of 30 Min Exposure to 2400~ (1316~ in Still Air on Properties of Sheet WC3015 and Cb752
tHeat 86DI 185, Cb-10.45W-2.62Zr-0.43Ta
ples, 2.5 by 0.5 by 0.021 in., to 2400~ (1316~ in still air showed that neither of the alloys can survive this exposure as shown in Fig. 1. WC3015 will also need protective coatings. However, the space shuttle thermal protection system (TPS) will operate at 2400~ (1316~ at very high altitudes and correspondingly low pressures. At lower pressures obviously, oxidation is less severe than 1 aim pressure used here. Nevertheless, under reentry conditions, where a combination of aerodynamic heating with temperature, stress, and pressure changes occur, oxidation results are not expected to be significantly less severe than the above test in still air. A 30 rain exposure of both alloys to 2400~ (1316~ in still air was carried out to simulate the maximum oxidation effect of one reentry in case of a coating failure. The effect of such exposure on tensile properties, hardness, and weight change per unit area is shown in Table I. The samples were cooled from 2400~ to 1600~ (871~ in about 15 rain and then slowly cooled to room temperature in another 15 min. Cb752 samples severely fractured on faster cooling rates such as a i r cooling.
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