Improved Oxidation Behavior of HfAlB in Comparison to HfB Magnetron Sputtered Thin Films

The oxidation resistance of HfB and HfAlB thin films was investigated comparatively at 700 °C for up to 8 h. Single-phase solid solution thin films were co-sputtered from HfB and AlB compound targets. After oxidation at 700 °C for 8 h an oxide scale thickness of 31   2 nm was formed on HfAlB which corresponds to 14% of the scale thickness measured on HfB. The improved oxidation resistance can be rationalized based on the chemical composition and the morphology of the formed oxide scales. On HfB the formation of a porous, O, Hf, and B-containing scale and the formation of crystalline HfO is observed. Whereas on HfAlB a dense, primarily amorphous scale containing O, Al, B as well as approximately 3 at% of Hf forms, which reduces the oxidation kinetics significantly by passivation. Benchmarking HfAlB with Ti-Al-based boride and nitride thin films with similar Al concentrations reveals superior oxidation behavior of the Hf-Al-based thin film. The incorporation of few at% of Hf in the oxide scale decelerates oxidation kinetics at 700 °C and leads to a reduction in oxide scale thickness of 21% and 47% compared to TiAlB and TiAlN, respectively. Contrary to Ti-Al-based diborides, HfAlB shows excellent oxidation behavior despite B-richness.