| Carbon materials are widely used in high temperature due to exhibiting high-temperature strength, corrosion resistance, heat shock resistance, electrical and heat conductivity, etc. However, the applications of carbon materials in aerospace field are limited because of the oxidation at 350～500℃in atmosphere.In the present study, we focused on investigating the self-healing oxidation resistance of carbon/ceramic material.Based on simulated calculations by a finite element software(COSMOS Floworks),SiC, B4C,TiB2 and Cflake were chosen as raw materials.The raw powders were first mixed by two methods, respectively, one was uniform mixing, the other was coating mixing. Cflake-SiC-B4C-TiB2 composite was obtained by hot-pressed sintering, and the preparation, mechanical property, oxidation resistance and microstructure of the specimens were investigated. The simulated and experimental results were as the following.1.Taking the reentry module as an example, the three-dimensional flow field was simulated by COSMOS Floworks at the speed of 6 Ma. It was found that the temperature of the windward of the reentry module was 1500-1650℃,the highest temperature was 1679℃,the maximum of the heat flux was 2.67×106W/m2.2.According to the simulations of oxidation process of C-SiC-B4C-TiB2 composite with Cflake,Cfiber and Cparticle, respectively, it was confirmed that the specimen with Cflake showed the best oxidation resistance.The distribution of temperature field in the specimen with Cflake was obviously affected by temperature, composition, relative density and Cflake with different degrees of orderliness. When the surface heat flux was 2.67×106 W/m2 and the surface temperature was 1650℃,the specimen showed the lowest ablation ratio of 0.135mm/s implying the highest oxidation resistance. Meanwhile the optimum composition of the specimen was the Cflake in 65 mass% and 5:1 for the mass ratio of SiC to B4C in matrix.3.The experimental results of the Cflake-SiC-B4C-TiB2 composites prepared by uniform mixing method showed that Cflake and SiC were densified, moreover, the bending strength and the fracture toughness were obviously improved with increasing the hot-pressed temperature. On the other hand, the fracture toughness was increased while the bending strength was decreased with increasing the Cflake content. At 2000℃,when the Cflake content was 20wt% the density, porosity, bending strength and fracture toughness were 2.81g/cm3,2.4%,279MPa and 5.3MPa-m1/2, respectively, showing the best mechanical properties of the-SiC-B4C-TiB2 composite.The thermal stress caused by the thermal expansion mismatching between the Cflake and the ceramic matrix resulted in delamination of the Cflake and ceramic phases. That could probably improve the fracture toughness of the Cflake-SiC-B4C-TiB2 composite.4. For the specimens prepared by uniform mixing method, the oxidation kinetic curves of the specimen at 600℃,800℃,1000℃,1200℃and 1400℃all showed parabola-shape implying an inactivating oxidation. At 600℃the specimen showed no oxidation behavior. At 800～1000℃,the flaws of the material surface could not completely sealed by the glass phase resulted in forming the discontinuous film. At 1200～1400℃,the material was coated with densified film of B2O3·SiO2 consisted of three areas. When the hot-pressing temperature was 2000℃and the Cflake was 20wt%, the oxidized depth and the lost weight of the oxidation were 400μm and 2.50mg/cm2 at 1400℃for 6 hours, having the best oxidation resistance.5.The oxidation behavior of Cflake-SiC-B4C-TiB2 composites prepared by uniform mixing method was investigated. At the temperature range of 530～960℃,it belonged to the oxidation controlled by several chemical reactions of C-O2, TiB2-O2 and B4C-O2.At the temperature range of 960～1100℃the oxidation behavior was controlled by oxygen diffusion through the micro-cracks. With increasing the oxidation temperature, the bubbles in the oxidation films increased due to the volatilization of B2O3.At the temperature range of 1100～1300℃,the oxidation behavior was controlled by oxygen diffusion among bubbles. With increasing the temperature, the self-healing oxidation resistance was obviously developed due to the formation of SiO2-B2O3 continuous glass film that effectively prevented oxygen diffusion. At the temperature range of 1300～1400℃the oxidation was controlled by the SiC-O2 and TiB2-O2 reactions.The kinetics equation and model deduced from the diffusion oxidation were shown in the following.6.Cflake-SiC-B4C-TiB2 composites with 50wt.% Cflake content were prepared by the coating and hot-pressing technology. The carbon phase was consisted of two morphologies, olivary ball with 40μm in diameter of the long axes and 0.5～1mm striate structure in the matrix. The density, bending strength and fracture toughness were improved with increasing the hot-pressing temperature. When the hot-pressing temperature was 2000℃,the density, porosity, bending strength and fracture toughness were 2.41g/cm3,3.42%,176.5MPa and 6.1MPa·m1/2,respectively.In the present study, Cflake-SiC-B4C-TiB2 composite was characteristic of low density, high strength and high oxidation resistance.Furthermore, the self-sealing oxidation film could be successfully controlled at the temperature range of 600～1400℃.The results of this study significantly developed the applications of the carbon/ceramic material.
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