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. Al_2O_3-SiO_2 sol with high solid content was selected as the raw material for mullite and ＂silicone resin ＋ Y_2O_3 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 SiO_2-rich mullite phase and Y_2Si_2O_7 phase 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 SiO_2 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 ℃ 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 Y_2Si_2O_7. The carbothermal reaction at 1 600 ℃ between free carbon in C/Si C substrate and rich SiO_2 in mullite results in severe frothing and desquamation of coatings and obvious degradation in oxidation resistance.

Fabrication of Y_2Si_2O_7 coating and its oxidation protection for C/SiC composites

Yttrium silicate （Y2Si2O7） coating was fabricated on C/SiC composites through dip-coating with silicone resin ＋ Y2O3 powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y2Si2O7 coating were investigated. Y2Si2O7 can be synthesized by the pyrolysis of Y2O3 powder filled silicone resin at mass ratio of 54.2：45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and favorable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%-140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y-Si-Al-O glassy phase between Y2Si2O7 and Al2O3 sample bracket at 1500 °C is discovered.

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

A tri-layer coating of mullite/Y_(2)Si_(2)O_(7)/(70wt%Y_(2)Si_(2)O_(7)+30wt%Y_(2)SiO_(5))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 Al_(2)O_(3)-SiO_(2) sol with high solid content was selected as raw material for mullite,and a slurry of Y_(2)O_(3) 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 SiO_(2) in mullite resulted in severe frothing and desquamation of coating and obvious degradation in oxidation resistance.

C/SiC复合材料用莫来石/硅酸钇双层涂层的抗氧化性能研究

针对C/SiC复合材料的防氧化要求,在材料表面通过等离子喷涂法制备了莫来石/硅酸钇的双层涂层,对涂层的形貌、组成和结构及其与基底的结合强度进行了表征,开展了1500℃、1h静态空气氧化实验,对抗氧化涂层的结构演变进行了分析,并对C/SiC复合材料氧化实验前后的质量和力学性能变化进行了研究。结果表明,莫来石/硅酸钇双层涂层抗氧化作用较好,涂层C/SiC复合材料的强度保留率达95.3%。