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

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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.