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Thermal protection system (TPS) is of great significance to launch hypersonic flight and landing process of hypersonic vehicles, which can effectively shield the hypersonic vehicle from severe aerodynamic heating encountered. Phenolic aerogels play an important role in TPS due to their characteristics of low density, high porosity, and low thermal conductivity. However, phenolic aerogel is easy to be oxidized at elevated temperatures under oxidizing environments, which severely limits its large-scale application as thermal insulation materials in TPS. In this study, a novel TiB2–B4C/carbon (TB/C) aerogel composite was synthesized by introducing TiB2 and B4C particles into phenolic aerogels through quick-gelation and mechanical mixing. The developed aerogel composites were characterized by scanning electron microscopy, Fourier transform infrared, thermal analysis, etc., to evaluate their microstructure, oxidation resistance, and mechanical properties. Experimental evidence showed that TiB2 and B4C particles reacted with the oxygen-containing molecules to form TiO2–B2O3 layer, which effectively improved oxidation resistance and mechanical properties of phenolic aerogel composites.

Carbon fiber fabric-reinforced phenolic resin composites are widely used as thermal protection materials for thermal protection systems in hypersonic vehicles and capsules. In this work, carbon fiber fabric-reinforced boron phenolic resin composites modified with MoSi2 and B4C were prepared via a compression molding technique. The high-temperature performance of the composites as well as the oxidation behavior of the carbon fibers was studied. The results indicate that the incorporation of B4C improves the performance of composites at high temperatures. The residual weight rate of composites with 15 phr B4C (BP-15) sufficiently increased from 23.03% to 32.91% compared with the composites without B4C (BP-0). After being treated at 1400 °C for 15 min, the flexural strength of BP-15 increased by 17.79% compared with BP-0. Compared with BP-0, the line ablation rate and mass ablation rate of BP-15 were reduced by 53.96% and 1.56%, respectively. In addition, MoSi2 and B4C particles had a positive effect on the oxidation of carbon fibers in the composites. After treatment at 1400 °C, the diameter of the as-received carbon fiber was reduced by 31.68%, while the diameter of the carbon fiber in BP-0 and BP-15 decreased by 15.12% and 6.14%, respectively. At high temperatures, the liquid B2O3 from B4C and MoSi2-derived complex-phase ceramics (MoB, MoB2, Mo2C, Mo4.8Si3C0.6) acted as an oxygen barrier, effectively mitigating the oxidation degree of the carbon fibers.

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