Magnetron Sputtered Mo(Six,Al1−x)2 Oxidation Protection Coatings for Mo–Si–B Alloys

Abstract

Double layer coatings consisting of a 2 µm thick diffusion barrier and a 5 µm thick oxidation protective topcoat were deposited on Mo–Si–B alloys using magnetron sputtering. During vacuum annealing of the amorphous as-deposited coatings, the tetragonal D8l–Mo5SiB2 phase evolved in the Mo–12Si–21B (at.%) interlayer; in the Mo–48Si–24Al and Mo–71Si–8Al (at.%) topcoats, the C40–Mo(Si,Al)2 and the C11b–MoSi2 phases formed, respectively. The oxidation behavior of the coated samples was investigated at 800 and 1000 °C under cyclic conditions in air. Compared to the bare substrate, the coated samples exhibited significantly reduced mass loss at both temperatures. A scale with a mixture of silica and mullite-like oxides formed on the coatings at 800 °C; in addition, outer aluminum borate needles grew on the Mo–48Si–24Al topcoat. At 1000 °C both coatings formed dense scales of SiO2 and a mullite-like phase, being protective for more than 100 h.