Designed,Fabrication And Properties Of Anti-oxidation Coating For Carbon Fiber Reinforced Carbon-Ceramic Matrix Composites

Carbon fiber reinforced carbon(Cf/C)and carbon fiber reinforced silicon carbide(Cf/SiC)composites possess many excellent properties such as low density,high specificstrength and modulus,low coefficient of thermal expansion(CTE),high thermal conductivity(TC),and good resistance of ablation.It has become one of the most promising candidates for aviation,aerospace structural materials.However,Cf/C begins to oxidize at 4000C.The operating temperature of Cf/C coated with anti-oxidation coating is not more than1650?,The temperature limit for long time use of CVSiC in aerobic environment is only 1650?.The composites are easy to be oxidized and ablated in high and ultra-high temperature oxygen-containing environments,thus the carbon fibers in the matrix materials are oxidized and the mechanical properties are greatly reduced,which greatly limits the application of the composites in the oxidation environment at high temperature.Therefore,thermal protection must be carried out on composite materials.Oxidation resistance coating technology is the most widely used technology for thermal protection of carbon fiber reinforced carbon(Cf/C)and carbon fiber reinforced silicon carbide(Cf/SiC)composites materials at present.The composite materials protected by oxidation resistant coating technology still have good properties under 1800 aerobic environment.However,the anti-oxidation coating technology also has some defects such as low bonding strength and low oxidation resistance in the ultra-high temperature section(above 1800?).Therefore,this paper focuses on improving the bonding strength and ultra-high temperature oxidation resistance of the oxidation resistant coatings of composite materials by improving the coating preparation process to improve the coating's bonding strength thwn improve the oxidation resistance of the composite in ultra-high temperature environment.The main results are as follows;(1)Characterized the morphology of SiC/UHTC/SiC three-layer anti-oxidation coating prepared by CVD and slurry brush and high temperature heat treatment.(2)The whole structure of the coating is complete,the inner layer and the outer layer of SiC are dense,and the coating particles are uniform,However,the density of UHTC layer is poor and the coating particles are not uniform.The inner and outer layers of SiC are prepared by CVD process.The binding of the inner and outer layers to the substrate and the UHTC layer is chemically bonded,so the binding is good.However,the UHTC layer was prepared by slurry brush coating and high temperature heat treatment.The combination between UHTC layer and inner SiC layer was physical,and the binding property was poor.The binding strength of inner SiC layer and UHTC layer is 8.8 MPa.(3)In order to solve the problem of poor bonding strength between UHTC layer and SiC layer,the preparation process of oxidation resistant coating was improved and optimized.Two kinds of coating preparation methods,pre-oxidation treatment of inner SiC layer and oxidation treatment of UHTC layer,were explored.The binding strength of UHTC layer to inner SiC layer decreased after oxidation treatment,and did not reach the expected goal.The bonding strength between the inner SiC layer and the UHTC layer was increased to 10.5 MPa after pre-oxidation treatment,which was 19.3%higher than that of the SiC layer and UHTC layer without pre-oxidation treatment.The ablative performance evaluation and ablation behavior of SiC/UHTC/SiC composite oxidation resistant coatings prepared by improved and optimized process were studied.The SiC/UHTC/SiC composite anti-oxidation coating prepared by the improved postoptimization process can be subjected to oxyacetylene cycle assessment at 2050?,the accumulated ablation time is 60s,and the weight loss rate is only 1.35×10-4g/s,weight loss is 0.2%.The ablation rate of SiC/UHTC/SiC composite oxidation resistant coatings is controlled by different processes when during the ablation process from room temperature to 2050?.When the temperature is below 1650?,the ablation rate is mainly controlled by the oxidation of ZrB2and SiC,resulting in a large amount of ZrO2,B2O3 and SiO2,resulting in continuous weight gain.At temperatures above 1650?,the ablation rate of the material is mainly controlled by the volatilization of glass phase and mechanical erosion of the solid phase ZrO2,which causes the material to lose weight continuously.The above process will occur during each cycle of ablation.