Japan Aerospace Exploration Agency Institute of Space Technology and Aeronautics
Japan Aerospace Exploration Agency Institute of Space Technology and Aeronautics
Japan Aerospace Exploration Agency Institute of Space Technology and Aeronautics
Japan Aerospace Exploration Agency Institute of Space Technology and Aeronautics
Silicon carbide made by the chemical vapor deposition method (CVD-SiC) was studied to determine its potential as an anti-oxidation coating for carbon fiber-reinforced carbon (C/C) composites. Three specimens were prepared for evaluation: CVD-SiC thin plates, two-dimensional C/C (2D-C/C) composites with a CVD-SiC protective coating, and isotropic graphite with a CVD-SiC protective coating. Tests were conducted in high-temperature still air. The temperature was varied from 1,673 K to 1,973 K, the heating period was varied from 1 minute to 285 hours, and ambient pressure was maintained at 1 atm. Although the CVD-SiC thin plate was noticeably oxidized early in the heating period, the oxidation rate of the CVD-SiC decreased rapidly as a silica layer grew over the surface of the CVD-SiC thin plate. At 1,873 K, the mean regression rate of the CVD-SiC surface was estimated to be about 0.6 micrometer/hr at 20 minutes after commencement of heating, but the rate subsequently decreased to about 0.08 micrometer/hr. The oxidation rate of the CVD-SiC increased with increasing temperature. When the CVD-SiC specimen touched alumina at temperatures over 1,923 K for over 1 hour, the oxidation rate increased owing to melting and bubbling of the silica layer on the CVD-SiC surface. Two-dimensional C/C composites with a CVD-SiC protective coating showed a high mass loss rate early in the heating period, because the SiC layer crazed during the coating process due to the difference in thermal expansion coefficients between the CVD-SiC coating and the 2D-C/C substrate. In these cases, the rate reached about 1.2 x 10(exp 4)(g m(sup -2) hr(sup -1)) at 1,973 K. As the heating progressed, the rate decreased to about 5 x 10(exp 2)(g m(sup -2) hr(sup -1)) due to crack-sealing action by the silica around the crack in the CVD-SiC coating. After that, severe mass loss due to a chemical reaction accompanied by gasification of the specimen occurred, because the silica came in contact with the C/C substrate through the cracks of the CVD-SiC coating. Furthermore, the mass loss of the C/C substrate accelerated because the silica layer partially disappeared due to the bubbling and gasification related to the above-mentioned chemical reaction. Isotropic graphite with a CVD-SiC protective coating showed almost no mass loss early in the heating period, because the SiC protective coating had no defects such as cracks. However, as the heating progressed, several pinholes appeared in the CVD-SiC protective coating. Once the protective coating was damaged, the mass loss of the specimen was remarkable, occurring by the same mechanism as in the 2D-C/C composites. The free carbon generated in the CVD process or in the passive oxidation process of the CVD-SiC was thought to be the cause of pinhole formation. CVD-SiC showed good oxidation resistance up to 1,973 K under atmospheric pressure. This antioxidation characteristic resulted from the generation of a silica layer in the passive oxidation process of the CVD-SiC. Contact of silica with distinct materials such as carbon or alumina brought about noticeable exhaustion of the silica, and therefore the working temperature of the CVD-SiC was limited to a maximum of about 1,773 K in such cases. There are two important factors for maintaining the antioxidation capability of a CVD-SiC coating on C/C composites. One is avoiding defects such as cracks or pinholes in the CVD-SiC protective coating. The other is isolating the silica from distinct materials such as carbon or alumina.
C/C複合材用CVD-SiC耐酸化膜の酸化消耗特性の研究
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