Oxidation of Carbon Fiber‐Reinforced Silicon Carbide Matrix Composites at Reduced Oxygen Partial Pressures

Abstract

Carbon fibers (Polyacrylonitrile‐derived T‐300) and T‐300 carbon fiber‐reinforced silicon carbide composites (C/SiC) were oxidized in flowing 0.1 MPa reduced oxygen partial pressure environments (50% O2, 5% O2, 0.5% O2, and 0.1% O2 in argon). Experiments were conducted at temperatures of 816°, 1149°, 1343°, and 1538°C. The oxidation kinetics were monitored using thermogravimetric analysis. T‐300 fibers were completely oxidized for times between 0.6 and 325 h. C/SiC coupons were oxidized for either 25 or 100 h. Fiber oxidation rates had an oxygen partial pressure dependence with a power‐law exponent close to one but were only weakly dependent on temperature. Fiber oxidation kinetics were consistent with gas‐phase diffusion control at these temperatures. The C/SiC coupon oxidation kinetics showed some variability, attributed to differences in the number and width of cracks in the SiC seal coat. Oxidation of the carbon fibers dominated the coupon oxidation behavior. For C/SiC coupons, low temperatures and high oxygen pressures resulted in the most rapid consumption of the carbon fibers. At higher temperatures, the lower oxidation rates were attributed to crack closure due to SiC thermal expansion, rather than oxidation of SiC. At the highest temperature and lowest oxygen partial pressure, active oxidation of SiC was observed.