Mo–Si–B based coating for oxidation protection of SiC–C composites

Silicon carbide conversion coated carbon/carbon composites (SiC–C) are promising materials for high temperature engine, aircraft and aerospace applications. However, the inherent coating cracks in SiC–C composites due to processing and rapid degradation of silica and SiC in corrosive combustion environments demand a protective external coating to preserve the integrity of the material during service. The Mo–Si–B based coating has been shown to be robust in oxidative environments at extreme temperature, making it an attractive candidate to protect SiC–C composites from oxidation. Isothermal oxidation behavior of Mo–Si–B coated SiC–C was studied and compared to uncoated SiC–C composites at 1073 K and 1773 K in ambient air. The Mo–Si–B based coating effectively protected the SiC–C composite with minimal weight lost due to the formation of a protective and continuous aluminoborosilica surface layer. Exposed carbon fibers in the uncoated SiC–C composites led to catastrophic oxidation. Thermogravimetric analysis of an uncoated SiC–C composite at 1588 K confirmed the accelerated rate at which the carbon fibers oxidized to leave a brittle SiO2 skeleton. The same analysis on Mo–Si–B coated SiC–C showed nearly negligible steady state weight loss. The initial evaluation provides clear evidence that the Mo–Si–B based coating serves as a robust and compatible coating for SiC–C composites over a wide temperature range.