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HfB 2 -SiC coatings with 30–60 wt% HfB 2 additions were prepared by slurry-dipping on SiC-coated C/C composites for investigating their oxidation behavior at low (1173 K) and high (1773 K) temperatures. With the increase of HfB 2 content, more B 2 O 3 glass was produced in the coating and the coating became denser during oxidation at 1173 K. 50 wt% HfB 2 -SiC coating can protect C/C composites for 494 h against oxidation, which was attributed to the formation of the dense borosilicate glass with high thermal stability as an oxygen barrier layer. After oxidation at 1773 K, 30 wt% HfB 2 -SiC coating provided the longest protection time (460 h) for C/C composites among 30–60 wt% HfB 2 -SiC coatings. A few Hf-oxides inhibited the volatilization of SiO 2 glass as the main phase in the 30 wt% HfB 2 -SiC coating after oxidation. As for other coatings with higher content of HfB 2 , the violent oxidation of HfB 2 particles made more bubbles produced, resulting in the poor oxidation resistance at 1773 K. [ABSTRACT FROM AUTHOR]

HfB 2 -MoSi 2 /SiC-Si coating was fabricated via pack cementation (PC) combined with slurry painting (SP) followed by heat treatment, where MoSi 2 was in-situ synthesized from Mo and Si. The microstructure evolution and high-temperature oxidation resistance at 1773 K and 1973 K of the HfB 2 -MoSi 2 /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 HfB 2 -MoSi 2 /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. • HfB 2 -MoSi 2 /SiC-Si coating was made by PC, SP and in-situ synthesis of MoSi 2. • Varying Hf-Si-O film led to the changed oxidation behaviors at 1773 K and 1973 K. • Hf-Si-O with self-healing ability formed at 1973 K caused good oxidation resistance. [ABSTRACT FROM AUTHOR]