Abstract
The oxidation resistance of Hf(0.28)B(0.72) and Hf(0.11)Al(0.20)B(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 HfB(2) and AlB(2) compound targets. After oxidation at 700 °C for 8 h an oxide scale thickness of 31 ± 2 nm was formed on Hf(0.11)Al(0.20)B(0.69) which corresponds to 14% of the scale thickness measured on Hf(0.28)B(0.72). The improved oxidation resistance can be rationalized based on the chemical composition and the morphology of the formed oxide scales. On Hf(0.28)B(0.72) the formation of a porous, O, Hf, and B-containing scale and the formation of crystalline HfO(2) is observed. Whereas on Hf(0.11)Al(0.20)B(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(0.11)Al(0.20)B(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(0.12)Al(0.21)B(0.67) and Ti(0.27)Al(0.21)N(0.52), respectively. Contrary to Ti-Al-based diborides, Hf(0.11)Al(0.20)B(0.69) shows excellent oxidation behavior despite B-richness.