The current research in high temperature materials mainly focuses on increasing operation temperatures beyond the capacity of commonly used Ni-based single crystal superalloys in order to increase process efficiencies. Refractory metals are interesting new materials due to their significantly higher melting points and their promising mechanical properties at high temperatures. The greatest challenge in using these materials is their low oxidation resistance at elevated temperatures. Aluminum diffusion layers offer an opportunity to meet this challenge by forming protective oxide scales on the substrate surface during exposure in oxidizing atmosphere. In this study such aluminum diffusion layers were manufactured on molybdenum, niobium, tantalum, and tungsten using a pack cementation process at 1000°C for 8 h. The produced coatings were analyzed by optical microscope, EPMA and XRD. It is shown that the results are in agreement with phase predictions based on thermodynamic activity calculations. On all substrates homogeneous single intermetallic phases with high aluminum contents were identified revealing a sufficient aluminum reservoir to form a closed and protective Al2O3 layer. Along with the phase prediction the partial diffusion coefficients of aluminum in the four different refractory metals were determined via Boltzmann-Matano analysis at 900°C, 950°C, 1000°C and 1050°C, allowing the prediction of future interdiffusion layer thicknesses.
Investigations on the oxidation resistance of the samples were performed via thermogravimetric analysis at 1300°C for up to 100 h in synthetic air. The experiments were conducted on samples that were uncoated, coated, and coated with an additional halogen treatment. It was shown that the halogen treatment highly supports the formation of a dense Al2O3 layer. Using mass change curves, oxide growth kinetics were analyzed and compared. The formed oxide layers were investigated using, again, optical microscopy, EPMA, and XRD analysis.