C/C-SiC-(ZrxHf1−x)C composites were prepared by the reactive melt infiltration method. The microstructure of the porous C/C skeleton and the C/C-SiC-(ZrxHf1−x)C composites, as well as the structural evolution and ablation behavior of the C/C-SiC-(ZrxHf1−x)C composites, were systematically investigated. The results show that the C/C-SiC-(ZrxHf1−x)C composites were mainly composed of carbon fiber, carbon matrix, SiC ceramic, (ZrxHf1−x)C and (ZrxHf1−x)Si2 solid solutions. The refinement of the pore structure is beneficial to promote the formation of (ZrxHf1−x)C ceramic. The C/C-SiC-(ZrxHf1−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-(ZrxHf1−x)C composites.

Twill multidirectional carbon-fiber-reinforced carbon and silicon carbide composites (i.e., C/C-SiC) were prepared via chemical vapor infiltration combined with reactive melt infiltration process. The effect of heat treatment (HT) on the microstructure and mechanical properties of C/C-SiC composites obtained by C/C preforms with different densities was thoroughly investigated. The results show that as the bulk density of C/C preforms increases, the thickness of the pyrolytic carbon (PyC) layer increases and open pore size distribution narrows, making the bulk density and residual silicon content of C/C-SiC composites decrease. Moreover, the flexural strength and tensile strength of the C/C-SiC composites were improved, which can be attributed to the increased thickness of the PyC layer. The compressive strength reduces due to the decrease of the ceramic phase content. HT improves the graphitization degree of PyC, which reduces the silicon-carbon reaction rate and thereby the content of the SiC phase. HT induces microcracks and porosity but not obviously affects the mechanical properties of C/C-SiC composites. However, the negative impact of HT can be compensated by the increased density of the C/C preforms.

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