Protective Aluminide Coatings for Refractory Metals

A.S. Ulrich, M.C. Galetz

Refractory metals are promising materials for high-temperature applications. However, these materials exhibit low oxidation resistance at elevated temperatures. To overcome this problem, aluminum diffusion layers were applied to molybdenum, niobium, tantalum, and tungsten using a pack cementation process. The coated samples were characterized using EPMA, optical microscope, and XRD. Homogeneous diffusion layers of different intermetallic phases were identified. The observed phases were in agreement with phase predictions made using thermodynamic calculations. Oxidation tests at 1300 C for up to 100 h in synthetic air were carried out on uncoated as well as aluminized samples. The oxidation kinetics were analyzed via TGA measurements. After oxidation, the formed oxides and the remaining aluminum diffusion layers were characterized. It was found that an additional halogen treatment can significantly reduce oxidation attack of the substrate and support the formation of a continuous protective Al2O3 layer.

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