Abstract Ultra‐high‐temperature ceramics (UHTCs) are promising candidates for thermal protection systems in hypersonic vehicles operating under extreme environments. This study presents the successful synthesis of large‐sized, dense bulk hafnium carbonitride (HfC 0.3 N 0.7 ) UHTCs via hot‐pressing sintering. The hardness, fracture toughness, and high‐temperature flexural strength in a diluted oxygen environment were systematically investigated for the first time. Through comprehensive macro‐ and microstructural analyses, the underlying evolution mechanisms governing the high‐temperature mechanical properties of HfC 0.3 N 0.7 ceramics were revealed. It demonstrates a characteristic brittle fracture mode between 25°C and 1500°C, transitioning to a metal‐like ductile deformation behavior above 2000°C. The flexural strength decreases from 365 MPa at room temperature to 170 MPa at 1500°C, primarily attributed to oxidation effects and grain‐boundary degradation. A more pronounced strength reduction to 73.2 and 54.2 MPa occurs at 2000°C and 2100°C, respectively, resulting from grain boundary softening and slip deformation. Notably, the HfC 0.3 N 0.7 exhibits exceptional fracture toughness (7.3 MPa·m¹/ 2 ), attributed to valence electron concentration modification and densified homogeneous microstructure. These findings provide crucial insights into the high‐temperature mechanical properties, performance evaluation, and modification of HfCN UHTCs serving in a near‐space environment.
Deng et al. (Sat,) studied this question.