Investigation on the oxidation behavior of Mo–Ru hard coatings

Mo–Ru coatings are refractory metal alloy coatings that are used as protective coatings on the top surface of glass molding dies to prolong the lifetime of the die material. In a realistic molding environment, the atmosphere consists of steadily purged nitrogen and residual oxygen. The oxidation resistance of surface coatings under thermal cycling in mass production is a critical issue. In this study, the diffusion of oxygen into the Mo38.1Ru61.9 coatings annealed at 600 °C in an oxygen containing atmosphere was investigated. Three stages of oxidation behavior were verified. Solid solution strengthening and recrystallization due to the introduction of oxygen raised the surface hardness of the Mo38.1Ru61.9 coatings from 13 GPa to 19 GPa after 4 h annealing. The formation of oxide scale and Mo-depleted zone due to the preferential oxidation of Mo and high volatility of MoO3 were revealed after 8 h annealing. Cracks on the surface and Mo-depleted zone were observed after 20 h annealing. Chemical composition and structure variations and mechanical properties of the Mo38.1Ru61.9 coatings were intensively investigated.