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

能源效率的提高需要在氧化气氛中高温使用的材料。在材料表面涂覆抗氧化膜是不降低材料力学性能的最有利方法之一。二氧化硅和氧化铝是在氧化条件下温度高于1000℃的抗氧化薄膜的潜在材料。本研究涉及在高温材料表面制备或形成二氧化硅和氧化铝的物理化学，以及抗氧化的保护性能。(1)硅基上硅膜的物理化学研究(a)硅的热氧化机理通过热氧化、化学气相沉积和自旋玻璃技术在硅基上形成了三种硅膜。这些样品在高温下被氧化，以评估氧在薄膜中的扩散率。由于膜的微观结构不同，膜间的扩散率也不同。氧化速率的氧压依赖性表明氧在氧化膜中的溶解度与氧压成正比，而扩散率与氧压无关。(b)研制了一种测量薄膜导热系数的新技术。研制了一种名为“点脉冲法”的新技术，该技术将脉冲激光束聚焦在薄膜上，测量表面温度的变化。结果表明，该技术适用于厚度大于25mm的薄膜。(2)高温用钼上的铝化物涂层(a)钼上的铝包覆层采用包覆胶结法将铝包覆在钼上。在Mo_3O_8和Mo之间形成了一层较厚的Mo_3O_8膜和一层较薄的Mo_3Al膜，涂层中含有许多在冷却过程中形成的裂纹。裂纹垂直于表面发育，部分裂纹到达Mo基体。涂层Mo的抗氧化性较差，因为裂纹到达基体，而Mo_3O_8本身具有较高的抗氧化性，形成连续的Al_2O_3层。(b) Si和Al的两段涂层，在铝化物涂层之前先涂少量的Si。两段涂层的显微组织与铝化物涂层基本相同，但明显抑制了裂纹的形成。无裂纹到达钼基板提供了极强的抗高温氧化能力。少

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