Ultra‐High‐Temperature Ceramic HfB ₂ ‐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 ( UHTC s) HfB ₂ ‐SiC coating was prepared on SiC‐coated C/C composites by in situ reaction method. The outer HfB ₂ ‐SiC coating consists of HfB ₂ and SiC, which are synchronously obtained. During the heat treatment process, the formed fluid silicon melt is responsible for the preparation of the outer HfB ₂ ‐SiC coating. The HfB ₂ ‐SiC/SiC coating could protect the C/C from oxidation for 265 h with only 0.41 × 10 ⁻² g/cm ² weight loss at 1773 K in air. During the oxidation process, SiO ₂ glass and HfO ₂ are generated. SiO ₂ 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, SiO ₂ glass can react with the formed HfO ₂ , thus forming the HfSiO ₄ phase. Owing to the “pinning effect” of HfSiO ₄ phase, crack deflecting and crack termination are occurred, which will prevent the spread of cracks and effectively improve the oxidation resistance of the coating.