Fabrication And Anti-oxidation Mechanism Of Mullite & Yttrium Silicate Coatings On C/SiC Composites

So far, carbon fiber reinforced silicon carbide matrix?C /SiC? composites are considered as a kind of continuous fiber-reinforced ceramic matrix composites?CFCMCs? with the best general properties and the most maturity in develepment & application. However, anti-oxidation has been being a key factor to the wide application of C/SiC composites for a long time in the future. Based on the review of progress on anti-oxidation techniques of C/SiC co mposites and yttrium silicate coatings, the research project consisting of “mullite coatings derived from Al₂O₃-SiO₂ diphasic sol with high solid content” and “yttrium silicate coatings derived from silicone resin and Y2O3 powders” were proposed in this dissertation. The research work focused on the fabrication, microstructure evolution, anti-oxidation properties and mechanism of coatings.Compared with the sol technology with inorganic or organic salts as raw materials, it is more efficient for Al₂O₃-SiO₂ sol with high solid content to fabricate densified mullite coatings with favorable bonding to substrate via dip-coating. The mullite coatings show desirable oxidation resistance. After soaked at 1400? for 30 min under static air, the coating-contained C/SiC composites possess 87.07±7.7% of original flexural strength. As a result of sealing and filling of cracks and pores by viscous SiO 2 in coatings, the coating-contained C/SiC composites exhibit improved oxidation resistance at 1500? and 1600?. There is no change in flexural strength after oxidized at 1500? and 1600? for 30 min and 109.77±8.5%?98.72±2.8% of original flexural strength, respectively. Nevertheless, the carbothermal reduction between viscous S i O₂ and free carbon in C/SiC composites substrate would occur obviously when oxidation temperature was elevated or oxidation time was prolonged, leading to local foaming in coatings and decreasing in oxidation resistance.Yttrium silicate can be synthesized from Y2O3 powders and silicone resin after pyrolysis of silicone resin at 800? under static air and then heat treatment at 1400? under inert atmosphere. The phase composition of yttrium silicate coatings can be easily adjusted between Y2Si2O7 and Y2 SiO5 by changing the ratio of Y2O3 powders to silicone resin. Densified Y2Si2O7 coatings with good bonding to substrate were fabricated through dip-coating method. Benefiting from the further densification during high-temperatue oxidation, the Y2Si2O7 coatings demonstrate excellent anti-oxidation property. After soaked at 1400?¹600? for 30 min under static air, the coated C/SiC composites exhibit higher flexural strengths which are 130%¹40% of original flexural strength. In addition, it has been found that eutectic Y-Si-Al-O glass phase begins to form as a result of the reaction between Y2Si2O7 and alumina underprop when oxidation temperature exceeded 1500?, weakening oxidation resistance.Based on the research of monolayer coatings, densified mullite/yttrium silicate multilayer coatings with favorable bonding to substrate were prepared through dip-coating route. Due to the cooperative anti-oxidation mechanism of viscous SiO₂ and Y-Si-Al-O phase, the multilayer coatings present oxidation resistance at 1400? and 1500? prior to mullite coatings and equal to Y2Si2O7 coatings. The YS1 possess 91.92±5%?102.38±4.8% of original flexural strength and The YS2 possess 127.78±5.2%?130.13±4.9% of original flexural strength, respectively. Furthermore, the multilayer coatings show excellent thermal shock enduration. After 12 times for 10 min every time of thermal shock from RT to 1400? under air, there are no crack or desquamation in coatings and little change in weight and flexural strength of the coated C/SiC composites, which is 99.02%?111.20% of original flexural strength, respectively. However, the carbothermal reduction between the viscous flow phase and substrate become obvious at 1600 ?, resulting in remarkable weight loss and reduction in oxidation resistance of the coated C/SiC composites, which is 84.19±4.5%?84.91±4.2% of original flexural strength, respectively.