Modification of Refractory Metals by CVD and Its High Temperatutre Oxidation Resistance

在能源相关行业，对高温使用的高强度材料的需求正在增长。金属材料的高温抗氧化性是其能否在氧化环境中使用的关键。钼、钨等高熔点难熔金属是高温金属材料的候选母材。然而，这些金属形成挥发性氧化物，并遭受严重的氧化。金属材料的抗氧化性是由氧化铝、氧化铬或二氧化硅形成的受保护的氧化膜提供的。通过渗铝和渗硅的方法，在氧化环境中对钼表面进行渗铝和渗硅，形成具有保护性的氧化铝或二氧化硅涂层，渗铝形成的Mo3Al8涂层由于涂层与钼基体的热膨胀差异和裂纹尖端氧化铝生成的应力而容易引起裂纹。氧化过程中裂纹的扩展降低了抗氧化性。钼表面硅铝共渗层是在1323K下由Mo_3Al_8(约含1At)组成。%Si中，Mo(Si，Al)2和Mo5(Si，Al)3分别在涂层表面以下和涂层/Mo界面析出。涂层钼在1520K的空气中氧化得到了薄的氧化铝膜，表现出良好的抗氧化性。涂层中很少有裂纹形成，用X射线衍射法绘制了1323K时Mo-Si-Al系相图，预测了涂层的组织。

Demands for high strength materials for high temperature use is growing in energy related industries. The high temperature oxidation resistance of metallic materials is a key for the usage in oxidative environment. Refractory metals with high melting points such as molybdenum and tungsten are candidates for the base metals of metallic materials for high temperature use. However, these metals forms volatile oxides and suffer from severe oxidation. The oxidation resistance of metallic materials is provided by the formation of protected oxide scales of alumina, chromia or silica. In this study, aluminizing and siliconizing of the surface of molybdenum were carried out by pack cementation to form the protective alumina or silica scales in the oxidative environment.Aluminizing provided the Mo_3 Al_8 coating layer in which cracks were easily introduced by the difference of thermal expansion between the coating and the Mo substrate and by the stress arising from the formation of alumina in the crack tips. The propagation of cracks during oxidation degraded the oxidation resistance. Silico-aluminized coating on molybdenum was per-formed by pack cementation at 1323 K. The coating was composed of Mo_3Al_8 containing about 1 at. % Si in which Mo (Si, Al)_2 and Mo_5 (Si, Al)_3 precipitated beneath the surface and at the coating/Mo interface, respectively. The oxidation of the coated molybdenum at 1520 K in air gave the thin alumina scale, exhibiting good oxidation resistance. The crack formation in the coating layer was seldom.The microstructure of the coating layer was predicted by the phase diagram of Mo-Si-Al system at 1323K which was made by X-ray diffraction.