Microstructure evolution of HfB₂-SiC/SiC coating for C/C composites during long-term oxidation at 1700 °C

A rigorous understanding of the structural evolution of oxidation-resistant coatings in service can pave the way for the development of coatings for carbon/carbon composites, and therefore a typical HfB₂-SiC/SiC coating was investigated here. The initial oxidation of the coating resulted in a SiO₂/HfO₂ scale, and the subsequent structural evolution was dominated by the growth of HfSiO₄, forming a SiO₂/HfSiO₄/HfO₂ protective layer that was stable at 1700 °C with long-term exposure to air. Ultimately, the SiC transition layer gradually degraded due to the active oxidation of SiC. Based on these findings, feasible strategies to improve coating longevity are proposed.