Fabrication and oxidation resistance of mullite/yttrium silicate multilayer coatings on C/SiC composites

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.