Oxidation Behaviors And Mechanisms Of Boron-Containing CVD SiC Complex Coating

Oxidation resistance of C/SiC composites should be improved to meet the demand of high performance aero engines up to 1650℃ for a long time. CVD SiC coating is the appropriate oxidation protective coating, but the oxidation protective property of CVD SiC coating was far below predicted value because of the coating defects. In this thesis, modification of SiC coating was carried out with Chemical Vapor Deposition (CVD). The oxidation behaviors and mechanisms were analyzed and the effects of the modified coatings by the two different methods were compared. The main contents and results are as follows:(l)The oxidation behaviors, mechanisms and oxidation resistance of SiC/B/SiC were investigated. The results showed that after oxidation at 700℃ for 10h, boron oxidation can seal cracks well;the kinetics of oxidation was controlled by diffusion of oxygen through microcracks. After oxidation at 1000℃ for 10h, borosilicate glass volatilized seriously. The glass volatilization rate increased quickly with temperature, which led to the composites weight losses rate increases continuously. The outside SiC coating spalled seriously after oxidized at 1300°C for 10h, the viscousity of the borosilicate glass was very low and can not block the diffusion of oxygen effectively due to volatilized seriously, so it can not improve the oxidation resistance of composites efficiently.(2) The oxidation behaviors, mechanisms and oxidation resistance of SiC/B_xC_1-x/SiC were investigated. The results showed that CVD boron carbide decreased the composites weight losses at 700℃, 1000℃ and 1300℃ remarkably, even showed weight gain at 1000℃, 1300°C, the weight gain rate was 0.015%, 0.027% separately, the relation of residual strength to oxidization temperature of CVD boron carbide modified composites was similar to that of SiC-C/SiC. CVD boron carbide improved the oxidation resistance of composites efficiencyly from 700℃ to l300℃.(3)The effect of CVD boron, CVD boron carbide and boron ion implantation were compared. The CVD boron carbide modified coating can be used at higher temperature up to 1300℃ and the modification effect was the best;boron ion implantation modification effect was inferior to that of the CVD boron carbide above 1000℃. The CVD boron modification decreased the composites weight losses at 1000℃.