Effect of Low-Density Fillers on the Properties of Ceramicizable Silicone Rubber Matrix Composites

In the field of aerospace thermal protection materials, the balance between lightweight properties and ablation resistance represents the core design challenge. This study utilized vinyl methylene silicone rubber (VMQ) as the matrix, incorporating mesophase pitch (MP) as a ceramic-promoting agent or blending it with poly(dicyandiamide) elastomer (PDPP). Fillers including fumed silica, MoSi2, boron-containing phenolic resin (BPR), silica aerogel (SA), and hollow glass microspheres (HGMs). Results indicate that both low-density fillers reduce the density of silicone rubber composites, with HGMs exhibiting a more pronounced effect. The density of HGMs/SPMB(VMQ/PDPP) composites reached 1.131 g/cm³, representing a 22.7% reduction compared to SPMB. The addition of SA enhances the tensile mechanical properties of the material. The tensile strength and elongation at break of SA/SPMB reached 6.34 MPa and 121.6%, respectively, representing increases of 23% and 27%. The tensile strength and elongation at break of SA/SMMB(VMQ/MP) reached 7.84 MPa and 218.6%, respectively, representing increases of 32% and 15%. Thermogravimetric analysis revealed that the thermal stability of the composites improved with the addition of SA and HGMs. SEM analysis indicated that pyrolytic composites incorporating low-density fillers exhibited superior ablation performance.