Abstract: C/C-HfC-SiC composites are considered promising candidate materials for thermal protection systems in hypersonic vehicles. Under extreme service conditions, structural surfaces are simultaneously exposed to intense aerodynamic heating and high-velocity gas flow, creating complex localized thermo-mechanical environments that strongly influence the formation and stability of protective oxide layers. To clarify how local ablation environments regulate the dynamic evolution and failure behavior of surface oxide layers, annular specimens were designed and fabricated to simulate the throat-liner structure of propulsion systems. Oxy-acetylene torch tests were performed at stand-off distances of 10 mm (Sample D10) and 15 mm (Sample D15), creating two representative thermo-mechanical environments characterized by high heat flux/high aerodynamic shear and relatively lower heat flux/lower aerodynamic shear , respectively. The resulting effects on the oxide-layer evolution and ablation resistance across different characteristic surfaces of the annular specimens were systematically investigated. Distinct oxide-layer behaviors were observed in different surface regions. In the stagnation region (α surface), oxide evolution is primarily governed by temperature. Under the higher temperature condition at D10, a dense HfO₂-SiO₂ composite oxide layer forms and exhibits pronounced self-healing capability. In contrast, the lower temperature at D15leads to a slower oxide formation rate, making it difficult for the oxide layer to form promptly and re-cover the surface after local damage or spallation. . In the parallel-flow region (β surface), the oxide morphology is mainly governed by aerodynamic shear. Under high aerodynamic shear (D10), HfO₂ is dragged and spread into a continuous thin film. Under lower aerodynamic shear (D15) gaseous by-products accumulate to form discrete spherical-shell structures, which are mechanically fragile and thus less protective. These differences are ultimately reflected in the overall ablation performance. After five ablation cycles (600 s), the average mass ablation rates of D10 and D15 are 0.038 and 0.067 mg/s, respectively. The results reveal that oxide formation is primarily driven by temperature, whereas oxide migration and morphological evolution are regulated by aerodynamic shear.

Key words: C/C-HfC-SiC composite, matrix modification, reactive melt infiltration, polymer impregnation and pyrolysis, ablation