Effects of SiO2-coated CNTs on the directional formation of SiC whiskers and improvement in the ablative resistance of polymer-matrix composites

As the development of hypersonic aerospace technology progresses, greater challenges are presented for solid rocket motors (SRMs) thermal protection, and the ablation performance of insulation materials needs to be further improved. Carbon nanotubes (CNTs) as a new type of reinforcing nano-filler, readily react with the oxidative components in the working gas during SRMs operation, limiting their excellent performance. In this study, we propose to coat the commonly used reinforcing filler, SiO₂, on the surface of CNTs to suppress their susceptibility to oxidation and investigate the effects of adding CNTs, SiO₂, and CNTs@SiO₂ to the matrix on material properties. The results show that the addition of CNTs@SiO₂ significantly improves the ablation resistance of the insulation material, with the linear ablation rate of M-@SiO₂-2 being 56 % lower than that of M-SiO₂-2. Based on the analysis of the material's antioxidation performance and the strength of the resulting char layer after ablation, the reasons for the improvement of ablation performance are discussed. By conducting high-temperature tube furnace tests, the composition and structure of the char layer at different temperatures are studied, and it is found that CNTs in the CNTs@SiO₂ formulation can directly provide the carbon source required for the carbon thermal reduction reaction, promoting the directional growth of SiC whiskers. Based on these findings, an ablation mechanism is proposed.