Preparation and Anti-oxidation Mechanism of Mullite/Yttrium Silicate Coatings on C/SiC Composites

In order to enhance the oxidation resistance of C/Si C composites, mullite/yttrium silicate coatings were fabricated on C/Si C composites through dip-coating route. AlO-SiOsol with high solid content was selected as the raw material for mullite and "silicone resin + YOpowder" 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 SiO-rich mullite phase and YSiOphase show high density and favorable bonding to C/Si C composites. After oxidized at 1 400 ℃ and 1 500 ℃ for 30 min in static air, the coating-containing C/Si C 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 SiOand 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 ℃ to room temperature, which are ascribed to the combination of anti-oxidation mechanism and preferable physical and chemical compatibility among C/Si C composites, mullite and YSiO. The carbothermal reaction at 1 600 ℃ between free carbon in C/Si C substrate and rich SiOin mullite results in severe frothing and desquamation of coatings and obvious degradation in oxidation resistance.