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.
The authors are grateful to the financial supports from the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Science Innovation Foundation of Shanghai Academy of Spaceflight Technology (No. SAST2015043).
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