Enhancement of high-temperature oxidation resistance and thermal stability of hard and optically transparent Hf–B–Si–C–N films by Y or Ho addition

•Oxidation resistant and thermally stable Hf–B–Si(–Y/Ho)–C–N films above 1000 °C.•Stabilization of t-HfO2 phase upon oxidation to 1500 °C by addition of Y or Ho.•Lowest oxide thickness upon annealing in air to 1500 °C for Hf6B12Si29Y2C2N45 film.•No mass changes upon annealing in He to 1350 °C for Hf6B12Si29Y2C2N45 film.•Optically transparent Hf6B12Si29Y2C2N45 film after annealing in He to 1400 °C.