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 μm grain-size powder of commercially available MoSi2. The oxidation behaviour of MoSi2 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 MoSi2 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 MoO3. The formation of Mo5Si3 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 Mo5Si3 phase did not appear anymore at 1400 °C, indicating that another oxidation mechanism has to be proposed.

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► Pesting does not occur in samples that are free of cracks and residual stresses.
► The volatilisation of MoO3 from 550 °C allows the development of protective scales.
► Selective oxidation of MoSi2 is possible only from 800 °C.
► High temperature oxidation of MoSi2 is governed by solid state diffusion in silica.
► Thermodynamic data are contradicted at 1400 °C; Mo5Si3 does not form anymore.