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

能源效率的提高要求材料在氧化性气氛中高温使用。在材料表面涂覆抗氧化膜是在不降低其力学性能的情况下最有利的方法之一。在1000℃以上的氧化条件下，二氧化硅和氧化铝是潜在的抗氧化膜材料。(1)硅基二氧化硅薄膜的物理化学研究(A)硅热氧化机理研究利用热氧化法、化学气相沉积法和旋涂玻璃技术在硅基上制备了三种二氧化硅薄膜。这些样品在高温下被氧化，以评估氧在薄膜中的扩散性。由于…的不同，不同薄膜的扩散系数也不同更多的微观结构。氧压对氧化速率的依赖关系表明，氧在二氧化硅薄膜中的溶解度与氧压成正比，扩散系数与氧压无关。发展了一种测量薄膜热导率的新技术--“点-脉冲法”。在这种技术中，脉冲激光聚焦在薄膜上，测量薄膜表面温度的变化。(2)高温铝基铝化物涂层(A)铝基铝基复合渗层是通过复合渗层的方法在钼表面涂覆的。在Mo_3O_8和Mo之间形成了厚厚的Mo_3O_8膜和Mo_3Al薄膜。涂层中含有许多裂纹，这些裂纹是在冷却过程中形成的。裂纹垂直于表面扩展，部分裂纹延伸至钼衬底。虽然Mo_3O_8本身表现出较高的抗氧化性，但由于裂纹到达衬底，涂层的抗氧化性能较差，形成了连续的Al_2O_3膜层。(B)开发了硅铝两段涂层，在铝化物涂层之前先涂覆少量的硅。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。没有到达钼衬底的裂纹，提供了极强的抗高温氧化能力。较少

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