C/C-SiC-(Zr_xHf_1−x)C composites were prepared by the reactive melt infiltration method. The microstructure of the porous C/C skeleton and the C/C-SiC-(Zr_xHf_1−x)C composites, as well as the structural evolution and ablation behavior of the C/C-SiC-(Zr_xHf_1−x)C composites, were systematically investigated. The results show that the C/C-SiC-(Zr_xHf_1−x)C composites were mainly composed of carbon fiber, carbon matrix, SiC ceramic, (Zr_xHf_1−x)C and (Zr_xHf_1−x)Si₂ solid solutions. The refinement of the pore structure is beneficial to promote the formation of (Zr_xHf_1−x)C ceramic. The C/C-SiC-(Zr_xHf_1−x)C composites exhibited outstanding ablation resistance under an air–plasma environment at around 2000 °C. After ablation for 60 s, CMC-1 appeared to possess the minimum mass and linear ablation rates of only 2.696 mg/s and −0.814 µm/s, respectively, which are lower than those of CMC-2 and CMC-3. During the ablation process, a Bi-liquid phase and a liquid–solid two-phase structure were formed on the ablation surface which could act as an oxygen diffusion barrier to retard further ablation, which is responsible for the excellent ablation resistance of the C/C-SiC-(Zr_xHf_1−x)C composites.

In this study, two types of porous C_f/C_m composites were obtained by introducing pyrolytic carbon (PyC) and pyrolytic carbon/furan resin carbon (PyC/FRC). Subsequently, C_f/C_m-SiC-(Zr_xHf_1−x)C composites with different carbon matrices were prepared by introducing SiC and (Zr_xHf_1−x)C matrices into the porous C_f/C_m composites via the reactive melt infiltration method, specifically termed as C_f/PyC-SiC-(Zr_xHf_1−x)C and C_f/PyC/FRC-SiC-(Zr_xHf_1−x)C composites. The microstructures of the porous C_f/C_m and C_f/C_m-SiC-(Zr_xHf_1−x)C composites with different carbon matrices were examined, and a comprehensive analysis was conducted on microstructural evolution and mechanical behaviors of the C_f/C_m-SiC-(Zr_xHf_1−x)C composites. The results indicate that both C_f/C_m-SiC-(Zr_xHf_1−x)C composites underwent similar microstructural evolution processes, differing only in terms of evolution kinetics and final microstructure. Differences in the pore structures of porous C_f/C_m composites, as well as in the reactivities of carbon matrices, were identified as primary influencing factors. Additionally, both C_f/C_m-SiC-(Zr_xHf_1−x)C composites exhibited “pseudo-ductile” fracture characteristics, with flexural strengths of 214.1 ± 8.8 MPa and 149.6 ± 12.2 MPa, respectively. In the C_f/PyC-SiC-(Zr_xHf_1−x)C composite, crack initiation during loading primarily originated from the ceramic matrix, while in the C_f/PyC/FRC-SiC-(Zr_xHf_1−x)C composite, failure initially arose from the residual FRC matrix. Excessive fiber corrosion and the presence of residual low-modulus FRC matrix resulted in lower mechanical performance.