Microstructure evolution and oxidation mechanism of HfB₂-SiC coating on SiC-coated C/C composites at 1173 K and 1773 K

HfB₂-SiC coatings with 30–60 wt% HfB₂ 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 HfB₂ content, more B₂O₃ glass was produced in the coating and the coating became denser during oxidation at 1173 K. 50 wt% HfB₂-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% HfB₂-SiC coating provided the longest protection time (460 h) for C/C composites among 30–60 wt% HfB₂-SiC coatings. A few Hf-oxides inhibited the volatilization of SiO₂ glass as the main phase in the 30 wt% HfB₂-SiC coating after oxidation. As for other coatings with higher content of HfB₂, the violent oxidation of HfB₂ particles made more bubbles produced, resulting in the poor oxidation resistance at 1773 K.