The oxidation behaviour of uniaxial hot pressed MoSi 2 in air from 400 to 1400 °C

Abstract

MoSi 2 samples were prepared by hot uniaxial pressing from a 2 μm grain-size powder of commercially available MoSi 2. The oxidation behaviour of MoSi 2 was systematically studied from 400 °C to 1400 °C, which includes the pest-oxidation temperature range. It was observed that the rate and mechanism for oxidation of MoSi 2 change significantly with increasing temperature. Five temperature regimes have to be considered regarding both kinetic results and cross-sections: i) 400 < T < 550 °C; ii) 550 ≤ T < 750 °C; iii) 750 ≤ T < 1000 °C; iv) 1000 ≤ T < 1400 °C; v) T ≥ 1400 °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 °C, there was a change in the physical properties of the oxidation products due to the evaporation of MoO 3. The formation of Mo 5Si 3 was observed above 800 °C showing that the thermodynamic previsions were satisfied above this temperature. At higher temperatures (>1000 °C), the oxide scale became very protective and transport in the silica scale (amorphous and β cristobalite) governed the oxidation kinetics. The Mo 5Si 3 phase did not appear anymore at 1400 °C, indicating that another oxidation mechanism has to be proposed.