Oxidation mechanism of Mo5Si3 particle in Si3N4 matrix composite at 750 °C

The oxidation behavior of a submicrometer-sized Mo5Si3-particle-reinforced silicon nitride (Si3N4) composite was studied. After oxidation at 750 °C for 10 h in air, the Mo5Si3 particles, which existed at the grain-boundary junctions on the surface of the composite, disintegrated catastrophically. During the oxidation procedure, a complex oxide containing molybdenum and silicon, with a dual-phase microstructure, first formed from the Mo5Si3 single crystals, after which molybdenum oxide agglomerated to form nanometer-sized, spherical MoO2 and MoO3 particles, which were surrounded by an amorphous silicon oxide. Vaporization of the MoO3 at 750 °C induced the formation of spherical voids in the oxidation products and, thus, a porous, amorphous silicon oxide. The poor oxidation resistance of the Mo5Si3 is thought to have resulted from the formation and vaporization of the MoO3 particles. Because of MoO3 vaporization, an oxidation temperature <750 °C was needed to obtain nanometer-sized, spherical MoO3 particles dispersed on the surface layer of the Mo5Si3–Si3N4 composite, for decreasing the friction resistance.