The combined reactor core and reflector of the very high temperature reactor (VHTR) is supported by a graphite support column composed of several types of graphitic material. Graphite in the VHTR reacts readily with O2, thereby forming CO2. Increasing the resistance of graphite to oxidation is therefore essential to its use as a high-temperature structural material. Conventional silicon carbide (SiC) is widely used as a carbon coating material that is resistant to oxidation. SiC was deposited by E-beam evaporation coating as a functionally gradient material. However, SiC was susceptible to the formation of cracks during thermal shock. These cracks acted as a pathway for the transport of O2 to the graphite substrate. Therefore, this study aimed to heal the cracks via chemical vapor deposition (CVD) and thereby prevent oxidation resulting from the abovementioned cracks. Crack healing was investigated at thermodynamically calculated methane ratios of 0.35-0.55. Micrographs obtained at a methane ratio of 0.55 revealed homogeneously faceted surface structures. A dense SiC coating layer appeared necessary for effective crack healing. Therefore, crack healing was performed at a methane ratio of 0.55 by CVD. In that case, cracks were completely healed by the ~ 0.1-mm-thick chemical-vapor-deposited coating. In contrast, cracks persisted on the surface after crack healing by chemical vapor reaction (CVR). The SiC coating produced via CVD constitutes a promising method for crack healing stemming from thermal shock; this coating is well-suited for use in the VHTR and various semiconductor industries.

Keywords: SiC, Crack healing, Chemical vapor deposition, Chemical vapor reaction.