Atomic simulation and experimental: Oxidation protective SiC coating produced by chemical vapor deposition and pack cementation for carbon/ carbon composites

In this study, the oxidation processes of SiC coatings with different crystal structures were systematically investigated by experimental and density functional theory (DFT) calculations. The SiC coatings were prepared on carbon/carbon (C/C) composites through pack cementation (PC) and chemical vapor deposition (CVD), respectively. The SiC coating prepared by PC method has superior oxidation resistance to that prepared by CVD. It was ascribed to a strong adhesion of PC -coating and substrate. And, the addition of Al, which improves the strength of Si -O bonding, inhibiting the diffusion of O in the SiO2. Additionally, it is found that alpha-SiC, prepared by PC, has better intrinsic oxidation resistance than beta-SiC. The reason is that alpha-SiC has high adsorption energy of the O atom on the C-terminated surface and low adsorption energy of the O atom on the Si-terminated surface. Moreover, the diffusion of O on the surface of alpha-SiC at 1773 K was more difficult due to the more stable Si -O bonding. These findings demonstrate that the oxidation resistance of alpha-SiC is superior to beta-SiC and can help guide the structural design of the high-temperature coatings.