Improved oxidation behavior of Hf_0.11Al_0.20B_0.69 in comparison to Hf_0.28B_0.72 magnetron sputtered thin films

The oxidation resistance of Hf_0.28B_0.72 and Hf_0.11Al_0.20B_0.69 thin films was investigated comparatively at 700 °C for up to 8 h. Single-phase solid solution thin films were co-sputtered from HfB₂ and AlB₂ compound targets. After oxidation at 700 °C for 8 h an oxide scale thickness of 31  $$  2 nm was formed on Hf_0.11Al_0.20B_0.69 which corresponds to 14% of the scale thickness measured on Hf_0.28B_0.72. The improved oxidation resistance can be rationalized based on the chemical composition and the morphology of the formed oxide scales. On Hf_0.28B_0.72 the formation of a porous, O, Hf, and B-containing scale and the formation of crystalline HfO₂ is observed. Whereas on Hf_0.11Al_0.20B_0.69 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 Hf_0.11Al_0.20B_0.69 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 Ti_0.12Al_0.21B_0.67 and Ti_0.27Al_0.21N_0.52, respectively. Contrary to Ti-Al-based diborides, Hf_0.11Al_0.20B_0.69 shows excellent oxidation behavior despite B-richness.