Time-dependent high-temperature compressive failure behavior of high-silica/boron-phenolic composites modified with boron carbide and talc

Polymer-based composites with high strength-to-weight ratio and excellent thermal insulation properties are widely used in aerospace thermal structures as thermal protection materials. In this work, high-silica/boron-phenolic composites modified with boron carbide (B4C) and talc are shown to exhibit improved thermal stability due to oxidation that occurs between 600 °C and 1000 °C. The oxidation reaction was verified and explained by the results of thermogravimetric analysis coupled with Fourier transform infrared spectroscopy. Temperature-dependent compression experiments were conducted for B4C-talc modified high-silica/boron-phenolic composites held for different holding times (ranging from 0.5 to 2 h) and loaded in two different directions (along the warp and along the weft). Digital microscopy and scanning electron microscopy were used to analyze the failure morphology of the tested specimens at different scales. It was found that the compressive strength of the composite specimen shows sensitivity against the fiber undulation, test temperature and the holding time in the high-temperature environment. Different trends were observed when varying the holding time at different temperatures: compressive strength increased at temperatures up to 130 °C, decreased between 340 and 560 °C, and slightly increased between 780 and 1000 °C. Three failure modes were observed with increasing temperature: shear failure of the fiber bundles at 25–340 °C, delamination at 560 °C, and fiber buckling at 780–1000 °C. These findings provide basic thermal and mechanical data for B4C-talc modified high-silica/boron-phenolic composites that can contribute to the design of thermal protection structures with greater integrity and higher reliability.