Structural and morphological studies of the growth of MoO3 scales during high-temperature oxidation of molybdenum

The oxidation of polycrystalline Mo plates and of Mo(100), Mo(110), and Mo(111) single-crystal plates in pure oxygen at 8 × 10⁴ and 2.7 × 10³ Pa, at 743–1023 K leads to the growth of orthorhombic MoO₃ only, as shown by X-ray diffraction and SEM observations. The stable oxides MoO₂ and Mo₄O₁₁ were not identified. At each side of the molybdenum plate, the oxide scale is a stacking of MoO₃ crystals with their [100] axes oriented normal to the surface of the initial Mo plate. The MoO₃ crystals are very thick in the [010] direction, compared with the well-known shape of the MoO₃ crystals grown from the vapor phase. Two main factors determine the oriented growth of MoO₃ crystals from Mo oxidation. A growth mechanism involving a structural rearrangement of the Mo atoms at the reactional interface and oxygen diffusion through the oxide is proposed.