Comparing oxidation behaviors at 1773 K and 1973 K of HfB₂-MoSi₂/SiC-Si coating prepared by a combination method of pack cementation, slurry painting and in-situ synthesis

HfB₂-MoSi₂/SiC-Si coating was fabricated via pack cementation (PC) combined with slurry painting (SP) followed by heat treatment, where MoSi₂ was in-situ synthesized from Mo and Si. The microstructure evolution and high-temperature oxidation resistance at 1773 K and 1973 K of the HfB₂-MoSi₂/SiC-Si coated specimens were comparatively studied. The protection mechanism of the coating was investigated, based on the structure and thermodynamic transformation of the oxide scale. The results revealed that the HfB₂-MoSi₂/SiC-Si coating possessed better oxidation resistance at 1973 K than that at 1773 K, mainly due to the changed properties of the multiphase Hf-Si-O oxide scale. At 1973 K, the formed Hf-Si-O oxide scale has low oxygen permeability, superior high-temperature stability and self-healing ability. While the Hf-Si-O oxide scale formed at 1773 K is loose, which caused oxygen penetrating into the inner layer and C/C substrate, finally resulting in a high mass loss. This study can provide valuable information and induce facile method to develop high-temperature environmental barrier coatings for C/C composites.