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C/C-HfC composites with a density of 2.14 g/cm3 was successfully prepared using an inorganic hafnium solution as a precursor via chemical vapor infiltration (CVI) and precursor infiltration pyrolysis (PIP) methods, and its microstructure and composition were analyzed. The results show that the HfC ceramics are uniformly dispersed in the pores of the matrix and closely bonded to the matrix. The material's ablation resistance is significantly enhanced by introducing HfC ceramics. Under the same heat flux conditions, when the ablation time is 120 s, the linear and mass ablation rates of the C/C-HfC composite are 6.20×10-2 mm·s-1 and 2.03×10-2 g·s-1, respectively, which are reduced by 48.33% and 40.12%, respectively, compared to those of the C/C composite. During the ablation process, HfC would react with oxygen in the heat flux to generate molten HfO2, which uniformly covers the surface of the matrix to form a protective layer, isolates the heat flux, prevents oxidation of the matrix, and prevents heat transfer. The evaporation of molten HfO2 also takes away some of the surface's heat. With the increase of ablation time, the loss of HfO2 will gradually increase, the protective layer on the surface of the matrix will be gradually destroyed, and the ablation damage of the heat flux on the matrix would become more severe.

Carbon/carbon-polyimide composites were prepared by chemical vapor infiltration and vacuum-pressure impregnation curing process. Firstly, the carbon felt was deposited with pyrolytic carbon by chemical vapor infiltration method to prepare porous carbon/carbon materials with pyrolytic carbon contents of 7.33%(mass fraction, the same below), 14.55% and 22.00%, respectively. Then, through a specific impregnation curing process, the carbon/carbon-polyimide composites P1, P2 and P3 and the control group P0 without pyrolytic carbon were obtained in turn. Through SEM, linear expansion coefficient test and mechanical properties test, the effects of different pyrolytic carbon contents on the microstructure, average linear expansion coefficient and mechanical properties of carbon/carbon-polyimide composites were investigated. The results show that in the temperature range of 20-300℃, the XY-direction average linear expansion coefficients of P0, P1, P2 and P3 are between (0.67-0.79)×10-6℃-1, and the XY-direction average linear expansion coefficients of four groups are basically the same. The Z-direction average linear expansion coefficients of P1, P2 and P3 decrease by 41.6%, 41.8% and 24.1%, respectively compared with P0. Pyrolytic carbon significantly reduces the Z-direction average linear expansion coefficient of materials. The XY-direction compressive strengths of P0, P1, P2 and P3 are 243.91, 244.73, 216.65 MPa and 210.79 MPa, respectively, and the Z-direction compressive strengths are 269.22, 258.80, 246.68 MPa and 219.20 MPa, respectively. The compressive strength of P1 is equivalent to that of P0. The Z-direction bending strengths of P0, P1, P2 and P3 are 92.77, 77.11, 80.71 MPa and 86.06 MPa, respectively. Different pyrolytic carbon contents have different effects on the mechanical properties of materials. The lower pyrolytic carbon content can basically maintain compressive strength of materials, while the higher pyrolytic carbon content can basically maintain bending strength of materials.

Taking the three-directional orthogonal preform with different Z-direction fiber distance as the research object, the carbon/carbon (C/C) composites were prepared by the combination of chemical vapor infiltration and resin impregnation, and the effect of weaving parameters on the microstructure and bending performance of C/C composites were studied. The calculation model was established by taking the smallest repeated structure of three-directional orthogonal preform as a unit, and the relationship between fiber content and weaving parameters of three-directional preform was obtained and verified. The results show that the fiber content of preform is increased with the decrease of Z-direction fiber distance and X, Y-direction fiber layer distance; when the Z-direction fiber distance is increased, the twist deformation of the fiber interweave becomes larger, resulting in the change of the pore structure of the preform; under the same densification process, the change of pore structure affects the composition and distribution of carbon matrix in C/C composites, but has no effect on the morphology of carbon matrix; when the fiber content in X, Y-direction is increased and the distance between fibers in Z-direction is decreased, the flexural strength of the C/C composites is higher.