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 ℃.

A tri-layer coating of mullite/Y₂Si₂O₇/(70wt%Y₂Si₂O₇+30wt%Y₂SiO₅) was prepared on carbon fiber reinforced silicon carbide (C/SiC) composite substrate through dip-coating route for the sake of improving oxidation resistance of C/SiC composites. An Al₂O₃-SiO₂ sol with high solid content was selected as raw material for mullite, and a slurry of Y₂O₃ powder filled silicone resin was used to synthesize yttrium silicate. The microstructure, phase composition, and oxidation resistance of the coating were investigated. The as-fabricated coating shows high density and favorable bonding to C/SiC substrate. After oxidation at 1400 and 1500 ℃ for 30 min under static air, the flexural strengths of coated C/SiC composite were both increased by ~30%. The desirable thermal stability and the further densification are responsible for excellent oxidation resistance. With the additional help of compatible thermal expansion coefficients among substrate and sub-layers in coating, the coated composite retained 111.2% of original flexural strength after 12 times of thermal shock in air from 1400 ℃ to room temperature. The carbothermal reaction at 1600 ℃ between free carbon in C/SiC substrate and rich SiO₂ in mullite resulted in severe frothing and desquamation of coating and obvious degradation in oxidation resistance.