| Carbon/carbon composites(C/C composites)have many excellent mechanical and thermal properties,especially with the increase of temperature(up to 2200 C),the strength of C/C composites will not decrease,but will increase,so it is considered as an ideal thermal structural material in advanced fields such as aerospace.However,C/C composites are prone to oxidation in an aerobic environment with the temperature exceeding 673 K,and the oxidation rate increases significantly with the temperature rising,which seriously affects the mechanical properties of C/C composites and limits their applications at high temperature.In order to solve this problem,researchers put forward the solution of preparing antioxidant coating on the surface of C/C composites.At present,plenty of researches focus on how to develop high-temperature oxidation-resistant coatings,but less on low-temperature oxidation-resistant coatings(below 1273 K).However,in the process of practical application of C/C composites,the working temperature of many components is concentrated below 1273 K,and the high-temperature components will inevitably experience low temperature during the process of heating up.Therefore,it is very necessary to explore the low temperature anti-oxidation coating.B2O3 with low melting point can be used in the preparation of low temperature antioxidant coatings,but B2O3 is very volatile at high temperature.SiO2 glass is a common self-healing coating material,but the viscosity of SiO2 glass is larger when it is below 1273 K,and the ability of flow filling defects is greatly reduced.Based on this,the idea of preparing B2O3@SiO2 core-shell particles and fabricating them as low-temperature oxidation-resistant coatings is proposed in this study.On the one hand,the volatilization of B2O3 can be minimized by encapsulating SiO2;on the other hand,when critical conditions are reached,B2O3@SiO2 core-shell particles in the coating will form borosilicate glass with good fluidity and stability,so as to achieve the purpose of self-healing and oxidation resistance.(1)In this thesis,B2O3@SiO2 core-shell powders were prepared by a sol-gel method.By changing the amount of TEOS(2 mL,4 mL,6 mL),the composition and structure of B2O3@SiO2 particles were altered,and they were named BS2,BS4 and BS6 respectively.(2)B2O3@SiO2 coatings were prepared on the surface of C/C-SiC samples by a hot dipping method,and three kinds of coating samples were obtained:C/C-SiC-BS2,C/C-SiC-BS4 and C/C-SiC-BS6.The isothermal oxidation resistance tests were performed at 973 K,1073 K and 1173 K.The results showed that SiC-BS4 coating samples exhibited the best isothermal oxidation resistance at all temperatures.The mass loss rate was 2.30%at 973K for 1500 h,2.35%at 1073 K for 2600 h,and 2.55%at 1173 K for 400 h.In the temperature range from room temperature to 1173 K,the thermal cycling tests of C/C-SiC-BS2,C/C-SiC-BS4 and C/C-SiC-BS6 samples were carried out.The results showed that C/C-SiC-BS4 samples had the most outstanding thermal cycling performance,and the mass loss rate was 2.36%after 270 thermal cycles.Finally,the anti-oxidation mechanism of C/C-SiC-BS4 coating samples was explored.In the temperature range of 873 K to 1073 K,the oxidation activation energy was 104.25 kJ,and oxygen mainly contacted the substrate through slow diffusion within the coating.In the temperature range of 1073 K to 1273 K,the calculated oxidation activation energy was about 74.88 kJ,and the majority oxygen passed through the defects in the coating and reacted with the matrix.(3)In order to further improve the oxidation resistance of C/C-SiC-BS4 sample,glass coating was prepared on the surface of C/C-SiC-BS4 sample by a hot dipping method with borosilicate glass powders and a small amount of MoSi2 as raw materials.The isothermal oxidation resistance of C/C-SiC-BS4-Glass coating samples were tested at 973 K,1073 K and 1173 K.At 973 K oxidation for 1500 h,the weight loss rate of the sample was only 0.35%.The weight loss rate was 0.66%at 1073 K antioxidant for 2600 h.When the sample was oxidized at 1173 K for 400 h,the mass increased by 0.05%.Compared with C/C-SiC-BS4,the isothermal oxidation resistance of the sample was significantly improved.The thermal cycling test of C/C-SiC-BS4-Glass was carried out at room temperature to 1173 K.The results showed that the mass loss rate of C/C-SiC-BS4 was 0.56%after 270 thermal cycles,which was 1.80%higher than that of C/C-SiC-BS4 under the same thermal cycling environment and times.After that,the oxidation resistance mechanism of SiC-BS4-Glass composite coating was analyzed and explored,and the oxidation activation energy of C/C-SiC-BS4-Glass sample in the range of 973 K to 1273 K was calculated to be 105.58 kJ,which was closer to the theoretical value of 112 kJ,indicating that the way of oxygen passing was mainly through diffusion in the coating system.Compared with the C/C-SiC-BS4 sample,the activation energy of C/C-SiC-BS4-Glass at high temperature was obviously increased,which indicated that oxygen needed more energy to penetrate through the SiC-BS4-Glass coating,and the anti-oxygen performance of the coating system was improved. |
PDF Full Text Request