Mechanical properties and thermal stability of carbon fiber cloth reinforced sol-derived mullite composites

For the wide application as thermal protection materials, it is very necessary for mullite ceramics to improve fracture toughness. In this paper, the laminated and stitched carbon fiber cloth preform reinforced mullite (C/mullite) composites were prepared through the route of sol impregnation and heat treatment using the Al₂O₃-SiO₂ sol with a high solid content as raw materials. The C/mullite composites showed a flexural strength of 228.9 MPa that was comparable to that of dense monolithic mullite although the total porosity reached 13.4%. Especially, a fracture toughness of 11.2 MPa·m^1/2 that was 4-5 times that of dense monolithic mullite was obtained. Strength deterioration due to the carbothermal reduction between carbon fiber and the residual SiO₂ in matrix was found above 1200 ℃. A pyrolytic C (PyC) coating was deposited on carbon fibers as interfacial coating. The chemical damage to carbon fibers was obviously alleviated by the sacrifice of PyC coating. Accordingly, the C/PyC/mullite composites kept strength unchanged up to 1500 ℃, and showed much higher strength retention ratio than C/mullite composites after annealing at 1600 ℃.