Abstract: To unveil the prolonged ablation resistance mechanisms of SiC/PyC bilayer-coated C/C-HfC-SiC composites (noted as SC-HSC) under extreme high-heat flux conditions. The composite structure is composed of a C/C-HfC-SiC substrate with an in-situ HfC/HfSi₂/SiC coating and a protective SiC/PyC bilayer coating, which was fabricated through a synergistic approach combining reactive melt infiltration and chemical vapor deposition techniques. The bilayer coating provides effective high-temperature ablation protection, while the ceramic matrix plays a critical role in resisting ultra-high temperature ablation. The synergistic coupling of these two components enables the material to demonstrate exceptional ablation resistance performance under high heat flux environments. Following sequential ablation testing during prolonged plasma wind tunnel exposures, the linear ablation rate of SC-HSC was measured at 0.41 µm/s. Morphological and structural analyses of the ablated oxides revealed three stages of phase transition after HfC/SiC oxidation: i) Initial formation of solid HfO₂ particulates and molten SiO₂ phase; ii) The synthesis of HfSiO₄ through SiO₂ infiltration into HfO₂; iii) Volatilization of low melting point components of HfSiO₄ and SiO₂ matrix components at high temperatures, resulting in residual molten HfO₂. Complementary laser ablation experiments conducted in argon-purged environments (low oxygen environment) elucidated the thermal protection mechanisms under oxygen-deficient conditions. Results demonstrate that the oxidation of cellular SiC in a low oxygen environment at ultrahigh temperatures undergoes a crystalline transition to form α-SiC. These results demonstrate the good ablation resistance of HfC/SiC modified C/C composites at ultrahigh temperatures and establish a scientific basis for designing future thermal protection systems capable of withstanding combined extreme thermal in high-speed applications.

Key words: C/C composites, Ultra-high temperature ceramics, Ablation behavior, Reactive melt infiltration