Preparation And Anti-oxidation Property Investigation Of SiC/SiO₂Multilayer Coating

Carbon materials are promising candidate materials for high temperature. However, their applications are greatly limited because they will be oxidized at high temperature in the presence of oxygen. In order to improve the anti-oxidation property of the carbon materials within a wide temperature range, a SiC/SiO2multilayer coating was designed and prepared. The phase composition, microstructure, anti-oxidation property and oxidation mechanism were investigated by XRD, SEM, EDS and XPS.Microstructure and anti-oxidation property of single SiC coatings prepared by pack cementation, liquid silicon reaction and chemical vapor deposition (CVD) were studied. The results showed that as cracks and holes in the coating were inevitably, anti-oxidation property of single SiC coating was limited. Furthermore, an integral SiO2film could not be formed by the oxidation of SiC to block the oxygen diffusion in the temperature range800-1200℃. So, single SiC coating could not provide enough oxidation protection within a wide temperature range.Effects of Si sources, H2/CO2inputs and deposition temperature on the coating were investigated. The results showed when CH3SiCl3was used as Si source, H2/CO2input was0.78and temperature was1100℃, the coating was dense and uniform, and owned the best anti-oxidation property.SiC/SiO2multilayer coating was designed and prepared, and its oxidation property was also studied. The coating structure, from inner to outer, was SiC bond layer, CVD SiC barrier layer, CVD SiC-SiO2transition layer and CVD SiO2sealing layer. The layers were dense and had good compatibility with each other. After oxidation in ambient air at1000℃and1500℃for192h, the weight gains of the as-coated carbon materials were only0.133mg-cm-2and0.283mg·cm-2, respectively. After oxidation in ambient air from400℃to1500℃which consisted of a series of12one-hour treatments, the weight gain of the as-coated carbon materials was only0.067mg·cm-2, indicating good oxidation anti-oxidation property within a wide temperature range from400℃to 1500℃.Oxidation mechanism of SiC/SiO2multilayer coating was investigated. At the beginning stage of oxidation, oxygen migrated rapidly to the interface of SiC/SiO2through mircoflaws in the coating, which resulted in a liner growth law of the oxidation kinetics. As oxidation time prolonged, microflaws in the coating were filled, and oxygen could only diffuse through the SiO2coating, which led to a parabolic growth law of the oxidation kinetics.