Oxidation and ablation performance of densified Cf/HfB2-SiC composites at different heat fluxes and prolonged exposure above 2500°C

Developing non-ablative ultra-high-temperature ceramic matrix composites (UHTCMCs) capable of enduring long-term severe aerodynamic heating remains a critical challenge for hypersonic vehicle thermal protection systems. In this work, a dense C f /HfB 2 -SiC composite was successfully fabricated via an integrated strategy combining pressure slurry impregnation (PSI) and liquid polycarbosilane (LPCS) infiltration, achieving a low open porosity of 1.87%. The composite exhibited exceptional ablation and oxidation resistance under simulated aerodynamic heating conditions (2–4 MW/m 2 ) and prolonged oxy-acetylene torch exposure at 2527±37 °C for 1000 s, maintaining remarkably low mass and linear ablation rates of 0.78 mg/s and 0.115 μm/s, respectively. Microstructural characterization coupled with thermodynamic and kinetic analyses revealed that this superior performance originates from the formation of a hierarchical, self-adaptive oxide layer regulated jointly by local oxygen partial pressure ( ) and temperature gradients. At the outer surface, high oxygen availability promotes the passive oxidation of SiC to form a continuous SiO 2 -assisted sealing phase, which accelerates the liquid-phase densification of the protective HfO 2 framework. Conversely, the inner oxygen-depleted sub-surface region undergoes active SiC oxidation, yielding a porous intermediate buffer layer that effectively accommodates thermal stress and regulates mass transport. A formidable kinetic barrier against aggressive oxidation was confirmed by a high apparent activation energy of 475 kJ/mol. This work demonstrates that the long-term structural integrity of the composite at temperatures above 2500 °C does not solely rely on a single continuous SiO 2 glass layer but rather arises from the synergistic interplay among the refractory HfO 2 skeleton, residual Si-containing sealing phases, and the stable fiber reinforcement. These findings establish a clear correlation between microstructure and property, offering key insights for the design of near-non-ablative UHTCMCs for extreme aerospace applications.