Structure and materials of a multi-layer coating for the oxidation protection of C C composites up to 1700°C

A four-layer coating for the oxidation protection of CIC composites was presented on the basis of investigating failure mechanism of a three-layer coating by XRD, SEM, TEM, Auger and Raman. The three-layer coating, which can be used for 30 hours at 1650 degrees C, consisted of a conversion SIC layer, a CVD SiC layer and a liquid-reaction SI-W layer. The failure process was essentially caused by interfacial reaction which produces gaseous pressure of higher than the ambient with nonhomogeneous nucleation, growth, bursting and sealing of bubbles. Holes were formed on the surface of coating when the bubbles are large enough that the burst bubbles can not be sealed in time by the viscous flow. W ion increases oxygen diffusion through the oxide film of the coating, and accelerates the failure process. The four-layer coating consists of a conversion SIC inner layer, a CVD SiC barrier layer, a Si-Zr layer for increasing the interfacial stability of CVD SIC and outer layer, a sol-gel silica outer layer for decreasing oxygen diffusion.