Magnetron-sputtered oxidation protection coatings for Mo-Si-B-alloys

Three phase Mo-Si-B alloys were coated with intermetallic layers to impede low temperature pest-oxidation and to improve the oxidation resistance at temperatures above 1000 °C. The about 7 µm thick coatings of MoSi2 and Mo5Si3 phases were produced by magnetron sputtering and subsequent annealing in vacuum. Both molybdenum silicides were doped with 10 – 20 at. % boron to prevent pesting by formation of a protective borosilicate scale. Furthermore, an alumina forming Al8Mo3 layer was deposited. All coatings were dense and well adherent to the Mo-Si-B substrate materials. To suppress interdiffusion between coating and substrate, a 2 µm thick intermediate barrier layer of Mo5SiB2 was applied. The coated samples were exposed to laboratory air at temperatures between 750 and 1300°C for up to 100 h under isothermal conditions. Microstructural examinations were performed using GD-OES, XRD and SEM. The scope of this talk is on oxide formation, interdiffusion and phase evolution