Improvement in Oxidation Resistance of a C/C Composite by Using CVD-SiC Coating with SiC Fiber-Composite Layer

Abstract

A newly developed SiC fiber-composite coating that was combined with SiC/SiC and CVD-SiC has been used to improve the oxidation resistance of CVD-SiC coated C/C composites at high temperatures. An SiC fiber-composite coating of about 300 gm in thickness was prepared by covering the 3D-C/C substrate with SiC felt, and by impregnating a slurry of finely dispersed C and Si powders. A CVD process was used for densification of the coating layer on the composite. In a CO2-H2O-N2 containing atmosphere (CO29%, N273%, H2O 18%), oxidation tests were applied at 1700°C for 5 h to the CVD-SiC coated C/C composites with and without the coating layer of SiC fiber-composite. The C/C composites with the SiC fiber-composite coating underwent much lower mass loss than those without the SiC fiber-composite. This SiC fiber-composite coating has a two-layer structure consisting of inner porous SiC/SiC layer and outer dense SiC layer. Since the SiC/SiC layer shows an intermediate coefficient of thermal expansion between those of C/C and CVD-SiC, this SiC/SiC layer played an important role in crack propagation of the coating to minimize the thermal stress caused by mismatching between C/C and CVD-SiC layers. The residual strengths of these samples after the oxidation test were measured with a modified small punch (MSP) test. The strengths of C/C composites measured by MSP test tend to be scattered depending on the fiber configuration of fracture part due to the presence of the fiber in the z direction. However, this method is suitable for testing a small size specimen. It was found that the residual strength after oxidation of the coated specimen developed in this study was kept high owing to its higher oxidation resistance.