| As a kind of new carbon material, carbon/carbon composites are composition of carbon fiber as reinforcement and carbon or graphite as matrix. Carbon/carbon composites not only with some unique properties, such as low density, high strength and high modulus, but also could be used at high temperature. Nevertheless, carbon/carbon composites would be damaged seriously at high temperature and high velocity air because the reaction of carbon with oxgen at high temperature. This limits the application of carbon/carbon composites in milistary industry, aerospace industry and other fields. To solve this problem, The multilaye coating for C/C composites were investigate:1. Methyltrichlorosilane is the raw material with H2as carrier gas, Ar as dilution gas, SiC coating was prepared. The effects of reaction temperature, raw material feed rate, dilution gas ratios and other factors impact on the SiC coating thickness and microstructure were investigated. The results shows that, the large raw material feed rate would lead to the SiC coating surface loose crystalline and the density of SiC coating would increase with decreasing of material feed rate. More dense SiC coating could be made while the raw material feed rate is0.46L/min. The concentration of the diluention gas affects both of the efficiency of the reaction and the uniformity of reaction temperature. The best ratio of carrier gas and dilution gas is1:1. To enhance the reaction temperature could improve the methyltrichlorosilane’s conversion rate while the temperature below1300℃. A Optimal experimental condition was:temperature1300℃,0.46L/min raw material feed rate,1:1dilution gas rate and2h reaction time, a uniform and smooth SiC coating could be prepared.2. The density could be improved greatly after SiC coating preparation by Chemical Vapor Deposition (CVD) method on Low-density carbon/carbon composites. Densification effect is not obviously with the density increasing of carbon/carbon composite. SiC crystal has blocked the channels in carbon/carbon composites, which prevents the further Infiltration. This method could enhance the density of1.4g/cm2carbon/carbon composite obviously and the highest density could reach1.52g/cm3.3. With the side reaction of polymerization with methyltrichlorosilane and ethanol, a Cross-linked structure Polymethylhydrosiloxane sol was prepared. Tests showed that to Make the Polymethylhydrosiloxane sol with the molar ratio4:3of Methyltrichlorosilane and ethanol could easily attached to the surface of the SiC coating and uniformity. To exploit the sublimation of ZrCl4at370℃, a sol with zirconium coating was prepared on the surface of SiC coating. A SiC-MoSi2double coating was prepared by coated Mo and Si powder with1:2molar ratios at high temperature. After ablation static test, MoSi2coating was failed to fix the SiC coating defects. However, the ZrCl4crystal had a certain role to fix the SiC coating. The sol with Zirconium coating could cover the SiC coating cracks.4. A SiC out layer was prepared by Chemical Vapor Deposition method based on the SiC-ZrO2Double layer coating. Results showed that, both of SiC-ZrO2-SiC multilayer and SiC-SiO2/ZrO2-SiC multilayer could reduce the stress of expansion coefficient mismatch in the preparation process. It could prevent the cracks and improve the oxidation resistance. The sol-condensing vapor adsorption method to prepare SiC-SiO2/ZrO2-SiC composite coating could lead to SiC coating layer contains a lot of ZrO2crystals. After the ablation test, there is lot of SiO2was reduced by the leaking carbon from carbon/carbon composites on the inner layer surface, and producing abundance of Si crystal. The structure of Si crystal was cross-linked. This structure could not only block the oxidation but also to prevent carbon diffusion. Moreover, the cracks on the inner layer surface could be self-healing by the Si when the Si melted into molten state at1400℃. The mass loss rate of SiC-SiO2/ZrO2-SiC multilayer is0.4g/(m2·s) at1800℃for120s. |
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