Synthesis of protective Mo–Si–B coatings in molten salts and their oxidation behavior in an air–water mixture

The formation of various phases during boronizing of silicided molybdenum substrates (MoSi2/Mo) was investigated. Boronizing treatments were conducted in molten salts under an inert gas atmosphere in the 700–1000 °C temperature range for 3–7 h. Depending on the process type (non-current or electrochemical) and molten salt temperature, the formation of different boride phases (MoB, Mo2B5, MoB2, MoB4) was observed. At the same time, substantial oxidation of the bulk molybdenum disilicide phase (MoSi2) to the Mo5Si3 phase was observed in non-current boronizing. The oxidation resistance of the coatings was investigated by the weight change in an air–water (2.3 vol.%) mixture at a temperature of 500 °C for a period up to 700 h. Results indicated that a two-phase microstructure consisting of the MoSi2 matrix phase with 12–15 wt.% of the MoB4 phase greatly improved the oxidation resistance of the molybdenum substrates. The weight gain rate observed was 6.5 · 10− 4 mg/cm2 h.