Ultra-high-temperature ceramic HfB₂-SiC coating for oxidation protection of SiC-coated carbon/carbon composites

To protect the carbon/carbon (C/C) composites from oxidation, an outer ultra-high-temperature ceramics (UHTCs) HfB₂-SiC coating was prepared on SiC-coated C/C composites by in situ reaction method. The outer HfB₂-SiC coating consists of HfB₂ and SiC, which are synchronously obtained. During the heat treatment process, the formed fluid silicon melt is responsible for the preparation of the outer HfB₂-SiC coating. The HfB₂-SiC/SiC coating could protect the C/C from oxidation for 265 h with only 0.41 × 10^-2 g/cm² weight loss at 1773 K in air. During the oxidation process, SiO₂ glass and HfO₂ are generated. SiO₂ glass has a self-sealing ability, which can cover the defects in the coating, thus blocking the penetration of oxygen and providing an effective protection for the C/C substrate. In addition, SiO₂ glass can react with the formed HfO₂, thus forming the HfSiO₄ phase. Owing to the "pinning effect" of HfSiO₄ phase, crack deflecting and crack termination are occurred, which will prevent the spread of cracks and effectively improve the oxidation resistance of the coating.