Carbon/Carbon composites, consist of a fibrous carbon in a carbonaceous matrix, are considered as the most promising candidate materials for high-temperature applications in aeronautical and aerospace fields because of their excellent properties at high temperatures. It exhibits many excellent properties, such as low density, high strength-to-weight ratio, good thermal shock resistance, and retention of excellent mechanical properties especially at elevated temperature. These applications require the C/C composites to operate in an oxidizing atmosphere. However, the oxidation of these composites at temperatures above 723K limits their use to inert atmosphere.Anti-oxidation coating is considered a good choice to prevent C/C from oxidizing at elevated temperature. However, most of the coatings are made up of silica-based ceramic, silicide in coatings will react with oxygen to form silicon dioxide glass phase at high temperature. The SiO2 glass phase with low viscosity can flow into the cracks of the coating and play a seal role. But at intermediate temperatures, due to the high viscosity and poor fluidity, the SiO2 glass phase can not provide protection any more. How to solve this problem has become the key to broaden the protective range of the anti-oxidation coatings. And in order to solve the problem, this paper mainly studies the use of boron oxide glass with lower melting point, by adding boron oxide and boron into silicate coating, to improve the coating oxidation resistance at intermediate temperature stage. The B2O3-ZrSiO4 coating and MoSi2/SiO2-B2O3-Al2O3 coating were prepared on the SiC-C/C sample successfully by pulse arc discharge deposition (PADD). The B-Y2SiO5/Y2Si2O7 coating was prepared on the SiC-C/C sample successfully by hot dipping method. The phase composition, microstructure of the surface and cross-section of the coating were characterized by XRD, SEM and EDS. The oxidation resistance of the coated sample was also investigated. The results are as follows:The B2O3-ZrSiO4 coating was prepared by PADD. The influence on deposition voltage and hydrothermal temperatures on the microstructure and oxidation resistance of the B2O3-ZrSiO4/SiC-C/C coating were investigated. The influence of the content of boron oxide on oxidation resistance of the coating also was investigated. The best process conditions are:pulse voltage of 400 V and hydrothermal temperature of 100 "C. And the optimal content of boron oxide is 10 wt%. The thickness of the B2O3-ZrSiO4 coating is 180 μm, the coating can effectively protect C/C composites from oxidation in air at 1500℃ for 170 h with a weight loss was 1.42% and at 800℃ for 170 h with a weight loss was 0.98%. The main failure reason of the coating is the volatilization of B2O3. In order to solve the problem, the MoSi2/SiO2-B203-Al2O3 coating was prepared by PADD. The thickness of the coating is 150 μm. The coating can effectively protect C/C composites from oxidation in air at 1073 K for 200 h with none weight loss or 1773 K for 200 h with a weight loss of 0.7% and endure the thermal cycling for 50 cycles between 1773 K and room temperature. The formation of B2O3 glass phase at low temperature and borosilicate glass at high temperature are the main reasons for the good oxidation resistance of the coating.The B-Y2SiO5/Y2Si2O7 coating was prepared by hot dipping method. The main factors of the preparation process were studied (sample temperature and volume ratio of silica solution and water). The best process conditions are:hot dipping temperature is 250℃ and ratio of silica solution and water is 1:3. The B-Y2SiO5/Y2Si2O7 coating prepared by hot dipping method can effectively protect C/C composites from oxidation in air at 1500℃ for 200 h with a weight loss of 1.55% and at 800℃ for 200 h with a weight loss of 0.55%. And the thickness of the coating is 180 μm.Add a certain amount of boron oxide or elemental boron in coatings can protect C/C matrix at intermediate temperatures. The coatings with boron in them also have good protection at high temperatures. And broaden the application temperature range of the coaings. |