Effect of mullite on the microstructure and oxidation behavior of thermal-sprayed MoSi₂ coating at 1500 °C

MoSi₂ coatings with different mullite additions (0, 10 wt%, and 30 wt%) were prepared on siliconized Nb alloys and oxidation behaviors of the modified coatings were comparatively investigated at 1500 °C in static air. After oxidation for 140 h, the 10 wt% mullite modified coating presented an outstanding oxidation resistance with a mass loss of 4.06 mg/cm² in comparison to the unmodified coating and the 30 wt% modified case. A proper amount of mullite addition in MoSi₂ coating can restrain the crystallization of SiO₂ during the oxidation process, thus a crack-free oxide scale was obtained so as to prevent the inward diffusion of oxygen. However, excessive addition of mullite (30 wt%) in MoSi₂ coating can severely hinder the formation of the crack-free oxide scale due to the volume shrinkage of the amorphous mullite, which accelerated the failure of the protective coatings.