| Carbon/Carbon (C/C) composites exhibit excellent structural properties at elevated temperatures, and are considered as the most promising candidate materials for high temperature applications such as in aviation and space flight industries. As a key technology for the actual application in oxygen containing atmosphere of C/C composites, the oxidation resistance is the most important problem. But up to now, the long time oxidation protective technique at high temperature of the C/C in key part of the fight has not been made significant progress. In order to solve this problem, we have studied Si-SiC coating, yttrium silicates coating and (Y2SiO5+Y2Si2O7) coating prepared by pack cementation, hydrothermal electrolytic method and hydrothermal electrophoretic method, respectively. The as-prepared coatings are characterized by XRD, SEM, oxidation test and mechanical property test. The structures and oxidation properties are also investigated. The main contents and conclusions are as follows:SiC coating was prepared on C/C composites by pack cementation method with Si, C and additives (A, B1, B2, M1, M2, S3) as raw pack materials. The structure and failure mechanism of the (A+B1) coating were mainly studied. SiC coated C/C samples were also tested in high temperature wind tunnel. The results show that the isothermal oxidation resistant properties of the SiC coated C/C samples prepared by two-step pack cementation method are all better than those of by one-step packed samples. The (A+B1) coating could protect the C/C composites from oxidation for 200h at 1500℃, and the weight loss is only 4.42×10-4g.cm-2. The (A+B1) coating composed of granular SiC crystallite and melted Si can not be self-melted after long time in isothermal oxidation test at 1500℃, which results in the final failure of the coating. The SiC/glass coated C/C composites could be protected effectively for 58h in wind tunnel at 1500℃, and the weight loss is only 1.466%.Yttrium silicates coatings were prepared on C/C composites by hydrothermal electrolytic method. The influence of the temperature, ampere density and the pH value of the electrolyte on the structure of the coating were explored, and the bond strength of the obtained coating was also investigated. In addition, yttrium silicates coatings were also prepared preliminarily on C/C-SiC substrates with hydrothermal electrolytic method. The structure, bond strength and the isothermal oxidation resistant property of the coating were characterized. The results show that the coating prepared on C/C substrates is composed of Y2SiO5,Y2Si2O7 and Y4Si3O12 under the condition of the pH value of 1.52.5, ampere density of 0.51.5A/cm2 and temperature of 150250℃. And the bond strength of the coating is 5.89.3MPa. Acicular and flaky yttrium silicates coatings for C/C-SiC substrates could also be achieved with hydrothermal electrolytic method. The bond strength of the coating is up to about 15MPa and the coated C/C composites could be protected for 5h at 1500℃.Yttrium silicates powders were synthesized by sonochemical precipitation method. The influence of initial solution composition, temperature and the content of the flux on the structure of the coating were studied. (Y2SiO5+Y2Si2O7) coating for C/C-SiC substrate was achieved by hydrothermal electrophoretic method with yttrium silicates suspension of high conductivity and stability. The effects of electric-field intensity and hydrothermal temperature were primarily explored. The results show that different yttrium silicate precursors could be obtained with corresponding Y:Si:OH (4:2:8, 4:3:6 and 4:4:4 (by mol ratio)) and content of LiF (2.5%, 2.5% and 1.5%). And Y2SiO5 of 4055nm, Y4.67(SiO4)3O of 5065nm and Y2Si2O7 of 2540nm can be achieved after 2h thermal treatment of the precipitations at 1100℃,1000℃and 900℃, respectively. With the concentration of suspension of 20g/L, the content of iodine of 0.6g/L, the electric-field intensity of 7080V/cm and the hydrothermal temperature of 60150℃, (Y2Si2O7+Y2SiO5) coating for C/C-SiC substrates could be prepared after 10min depositing. The bond strength of the coating is up to about 20MPa, and the coated C/C-SiC samples could be protected effectively for 10h with weigh loss of 1.8%. The kinetic analyses indicate that the hydrothermal electrophoretic process is controlled by the diffusion velocity of the charged yttrium silicates particles, and the deposition activation energy is 17.8kJ/mol.
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