Oxidation behaviour of the Mo–Si–B and Mo–Si–B–Al alloys in the temperature range of 700–1300 °C

The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo₃Si, Mo₅SiB₂ and SiO₂ or α-Al₂O₃ phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO₃ as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO₂ and MoO₃, while those formed at 900 °C or above contain Mo, MoO₂ and SiO₂. In addition, α-Al₂O₃ or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B₂O₃–SiO₂.