Physicochemical Study on Preparation of Oxidation-Resistant Films on High Temperature Materials and Their Oxidation Rate.

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Improvement of energy efficiency requires materials for high temperature use in oxidizing atmospheres. The coating of oxidation-resistant films on materials is one of the most favorable ways without the degradation of their mechanical properties. Silica and alumina are the potential materials for oxidation-resistant films at temperatures higher than 1000 C in oxidizing conditions. This research dealt with the physical chemistry involving in the preparation or formation of silica and alumina on the surface of high temperature materials, and the protective property against oxidation.(1) Physicochemical Study on Silica Films on Silicon (a)Mechanism of thermal oxidation of silicon Three kind of silica films were formed on silicon substrates by thermal oxidation, chemical vapor deposition and spin-on glass techniques. These samples were oxidized at elevated temperatures to evaluate the diffusivity of oxygen in the films. The diffusivity varied among the films because of the difference in th … More e microstructure. Oxygen pressure dependence of the oxidation rate indicated that the solubility of oxygen in the silica film was proportional to the oxygen pressure and that the diffusivity was independent of the pressure. (b)Development of a new technique for measurement of thermal conductivity of thin films A new technique named "Spot-Pulse Method" was developed, in which technique a pulsed laser beam was focused on the film and the change in surface temperature was measured. This technique proved to be applied the films with thickness more than 25mum.(2) Aluminide Coatings on Molybdenum for High Temperature Use (a)Pack-cementation coating of aluminum on molybdenum Aluminum was coated on molybdenum by pack-cementation. A thick film of Mo_3O_8 was formed and thin film of Mo_3Al existed between Mo_3O_8 and Mo. The coating layer contained many cracks which formed during cooling. The cracks were developed perpendicularly to the surface, and some of them reached the Mo substrate. The oxidation resistance of the coated Mo was poor because of the crack reaching the substrate although Mo_3O_8 itself exhibited the high resistance to oxidation forming a continuous Al_2O_3 fayer. (b)Two-stage coating of Si and Al Two-stage coating of Si and Al was developed, in which a small amount of Si was coated prior to the aluminide coating. Mirostructure of the two-stage coating was basically same as that of the aluminide coating but the marked suppression of the formation of cracks. The absence of cracks reaching the Mo substrate provided the extremely strong resistance to high temperature oxidation. Less