In this paper, a high-yield Hf-modified SiHfBOC ceramic precursor was developed, and a high-pressure assisted impregnation pyrolysis method was proposed to achieve the preparation of 3D PyC–C_f/SiHfBOC composites. This high-pressure assisted impregnation method significantly improves impregnation filling effect of the precursor in and between fiber bundles compared to dozens of traditional impregnation cycles. After undergoing just 9 precursor infiltration pyrolysis (PIP) cycles, the composites achieved relative density of approximately 90% and density of 1.64 g/cm³. The critical temperature difference of the 3D PyC–C_f/SiHfBOC composites after the shock of room temperature (RT)–1000 ℃ is as high as 650 ℃, which is twice that of traditional ceramic materials, showing good thermal shock resistance. Under the effect of Hf modification, a dense HfO₂–SiO₂ oxide layer (thickness of 93 μm) was formed in situ on the surface of the 3D PyC–C_f/SiHfBOC composites, effectively preventing further erosion of the composite matrix by high-temperature oxidation gas. Even in the ultra-high-temperature oxygen-containing environment at 1800 ℃, it still exhibits an excellent non-ablative result (with a linear ablation rate of 0.83×10⁻⁴ mm/s). This work not only enriches the basic research on lightweight ultra-high-temperature ceramic composites converted from Hf ceramic precursors, but also provides strong technical support for their applications in ultra-high-temperature non-ablative thermal protection materials for high-speed aircraft.

To further improve the oxidation resistance of polymer derived ceramic (PDC) composites in harsh environments, C_f/SiC/SiHfBOC composites were prepared by chemical vapor infiltration (CVI) and precursor impregnation pyrolysis (PIP) methods. The weight retention change, mechanical properties, and microstructure of C_f/SiC/SiHfBOC before and after oxidation in air were studied in details. Microscopic analyses showed that only the interface between the ceramics and fibers was oxidized to some extent, and hafnium had been enriched on the composite surface after oxidizing at different temperature. The main oxidation products of C_f/SiC/SiHfBOC composites were HfO₂ and HfSiO₄ after oxidation at 1500 ℃ for 60 min. Moreover, the weight retention ratio and compressive strength of the C_f/SiC/SiHfBOC composites are 83.97% and 23.88±3.11 MPa, respectively. It indicates that the C_f/SiC/SiHfBOC composites should be promising to be used for a short time in the oxidation environment at 1500 ℃.