Preparation and Oxidation Behaviors of C/SiC-BN Composites
As thermal structural materials operating at high temperatures,C/SiC composites have extensive applications as thermal protection systems in aerospace vehicles.However,their limited antioxidant performance at low and medium temperatures restricts their utilization in diverse fields.In this work,an improved slurry impregnation technique was employed to introduce antioxidant component BN at different contents into the carbon fiber felts by adjusting the ceramic content in the slurry,and then densified the felts to prepare C/SiC-BN composites using reaction melt infiltration method.Effects of BN content on the composition,microstructure and oxidation behaviors of the composites were systematically studied,and relevant oxidation mechanisms were explored.Experimental results indicate that introduction of BN particles significantly reduces the open porosity of C/SiC and enhances the initial oxidation temperature of C/SiC.The C/SiC-BN composite with 3％BN(mass fraction,sample B3)has optimal antioxidant performance,and the corresponding mass loss rates in static air at 900,1200 and 1500 ℃ for 1 h are 0.009％,-0.301％,and-0.596％,respectively.After oxidation at 1500 ℃,the strength retention rate of sample B3 is up to 52％.At 900 ℃,the mass change is dominated by slow weight loss controlled by O2 diffusion and oxidation of C and BN.At 1200 ℃,the oxidation rates of C and BN phases increase while SiC begins to undergo significant oxidation.New formed oxidation products,such as B2O3,borosilicate and SiO2,gradually slow the diffusion of O2 and reduce the damage of carbon.At 1500 ℃,the oxidation rate of SiC is accelerated obviously.Finally,a continuous oxidation film formed by generated SiO2 retards the inward diffusion of O2,and overflow of gas products(such as B2O3 and CO)and continuous formation of SiO2 dominate the oxidation process.
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