Preparation and anti-oxidation mechanism of mullite/yttrium silicate coatings on C/SiC composites

Abstract

In order to enhance the oxidation resistance of C/SiC composites, mullite/yttrium silicate coatings were fabricated on C/SiC composites through dip-coating route. Al₂O₃-SiO₂ sol with high solid content was selected as the raw material for mullite and “silicone resin + Y₂O₃ powder” slurry was used to synthesize yttrium silicate. The microstructure and phase composition of coatings were characterized, and the investigation on oxidation resistance and anti-oxidation mechanism was emphasized. The as-fabricated coatings consisting of SiO₂-rich mullite phase and Y₂Si₂O₇ phase show high density and favorable bonding to C/SiC composites. After oxidized at 1 400 °C and 1 500 °C for 30min in static air, the coating-containing C/SiC composites possess 91.9% and 102.4% of the original flexural strength, respectively. The desirable thermal stability of coatings and the further densification of coatings due to viscous flow of rich SiO₂ and Y-Si-Al-O glass are responsible for the excellent oxidation resistance. In addition, the coating-containing composites retain 99.0% of the original flexural strength and the coatings exhibit no cracking and desquamation after 12 times of thermal shock from 1 400 °C to room temperature, which are ascribed to the combination of anti-oxidation mechanism and preferable physical and chemical compatibility among C/SiC composites, mullite and Y₂Si₂O₇. The carbothermal reaction at 1 600 °C between free carbon in C/SiC substrate and rich SiO₂ in mullite results in severe frothing and desquamation of coatings and obvious degradation in oxidation resistance.

Keywords

anti-oxidation / coatings / mullite / yttrium silicate / C/SiC composites

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