The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400°C

MoSi2 samples were prepared by hot uniaxial pressing from a 2 mu m grain-size powder of commercially available MoSi2. The oxidation behaviour of MoSi2 was systematically studied from 400 degrees C to 1400 degrees C, which includes the pest-oxidation temperature range. It was observed that the rate and mechanism for oxidation of MoSi2 change significantly with increasing temperature. Five temperature regimes have to be considered regarding both kinetic results and cross-sections: i) 400 < T < 550 degrees C; ii) 550 < T < 750 degrees C; iii) 750 <= T < 1000 degrees C; iv) 1000 <= T < 1400 degrees C; v) T >= 1400 degrees C. In the first range, pesting did not occur in samples that were free of cracks and residual stresses and the oxidation kinetics were governed by surface or phase boundary reactions. Above 550 degrees C, there was a change in the physical properties of the oxidation products due to the evaporation of MoO3. The formation of Mo5Si3 was observed above 800 degrees C showing that the thermodynamic previsions were satisfied above this temperature. At higher temperatures (>1000 degrees C), the oxide scale became very protective and transport in the silica scale (amorphous and beta cristobalite) governed the oxidation kinetics. The Mo5Si3 phase did not appear anymore at 1400 degrees C, indicating that another oxidation mechanism has to be proposed. (C) 2011 Elsevier Ltd. All rights reserved.