A novel ablative material for thermal protection system: Carbon fiber/polysiloxane composites

Fiber reinforced composite materials are widely used in the aerospace industry due to their high strength to weight ratio. One of their applications is as an ablative material placed at the outermost layer of a thermal protection systems (TPS). A TPS requires the ablative material to have low density, low thermal conductivity, high temperature resistance, formation of a stable and high shear strength char. This paper introduces a carbon fiber (CF) reinforced polysiloxane (UHTR) composite material processed and fabricated in a laboratory environment. The fabrication method of this material is illustrated in detail. Thermal, ablation, flammability, and mechanical properties of the CF/UHTR material are characterized and compared to a commercial model ablative material, MX4926. MX4926 is a carbon fiber phenolic (CF/Ph) composite material manufactured by Solvay-Cytec. In this study, the carbon fiber used to make the CF/UHTR material is a PAN-based 8-harness fabric provided by Hexcel. The polymer matrix, UHTR, is a colorless semi-solid polysiloxane resin manufactured by Techneglas LLC. Raw materials are firstly made into CF/UHTR prepreg sheets through a hot-melt process and then compression molded into molding compound (MC) samples or two-dimensional (2D) laminates by a hot press. All samples for testing are post cured in a programmable oven at 350 °C for 2 hours. The density of the fully-cured material is measured by the water displacement method. Thermal stability, flammability, and ablation properties of the material (in the format of MC) are characterized using thermogravimetric analysis (TGA), microscale combustion calorimeter (MCC), and oxyacetylene test bed (OTB) with three different heat fluxes. Mechanical properties of the material (in 2D laminates) are measured by a universal testing machine (UTM) to the ASTM standards. Testing results of the CF/UHTR material are compared with the commercial model ablative material, MX4926. Microstructures of the CF/UHTR material before and after mechanical and ablation tests are investigated and compared by an optical microscope and scanning electron microscopy (SEM) to further study the failure mode of the material.