To protect carbon fiber-reinforced SiC (C/SiC) composites against oxidation,mullite coating was prepared on C/SiC composites by dip-coating method with high solid content Al2O3-SiO2 sol as raw materials. X-ray diffraction and scanning electron microscopy were employed to analyze the phase and microstructure of the coating. The results show that the as-prepared coating is SiO2-rich, monolithic and well bonded with substrate without penetrating crack, giving rise to good oxidation-resistance. After soaked at 1400°C for 30min under static air, the coated C/SiC composites possess 87% of original flexural strength. As a result of sealing and filling of cracks and pores by viscous SiO2 in coating, the coated C/SiC composites exhibit improved oxidation resistance at 1500°C and 1600°C. There is no change in flexural strength after oxidized at 1500°C and 1600°C for 30min, respectively. Nevertheless, the carbothermal reduction between viscous SiO2 and free carbon in C/SiC substrate would occur obviously when oxidation temperature was elevated or oxidation time was prolonged, leading to local foaming in coating and decreasing in oxidation resistance.

Mullite matrix composites with laminated and stitched carbon fiber cloth preform as reinforcement were fabricated via the route of “infiltration-drying-heat treatment” using Al2O3-SiO2 sol as raw materials. Thermal properties from room temperature to 1673K of the composites were investigated. The coefficient of thermal expansion (CTE) increases first and then decreases, and reaches a maximum of 4.83×10-6K-1 at 1273K. As a result of the further sintering of matrix, the CTE is negative at above 1300°C. The specific heat capacity increases to the maximum of 1.547J·g-1·K-1 at 1473K and remains stable at above 1473K, with a minimum of 0.756J·g-1·K-1 at room temperature. The thermal diffusivity decreases from 1.1mm2·s-1 at room temperature to 0.707 mm2·s-1 at 973K as the temperature was elevated, and remains stable at above 973K. On the contrary, the thermal conductivity is improved with increasing temperature on the whole and varies from 1.859W·m-1·K-1 at room temperature to 2.325W·m-1·K-1 at 1473K.