Epitaxial growth during the rapid solidification of plasma-sprayed molten TiO₂ splat

Epitaxial grain growth during the rapid solidification of molten TiO₂ in plasma spraying was studied. The crystallographic structure of the TiO₂ splats deposited on rutile and α-Al₂O₃ substrates at 150, 300 and 500 °C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homoepitaxial and hetero-epitaxial TiO₂ splats can be formed at the deposition temperature of 500 °C. Epitaxial growth is significantly influenced by the crystal orientation. It is easier to form an epitaxial TiO₂ splat with a <001> orientation in the direction perpendicular to the substrate surface. In order to explain the formation of epitaxial splat during plasma spraying, a competition mechanism between heterogeneous nucleation and epitaxial growth was proposed. It was indicated that the face (001) of rutile crystal exhibits the largest growth velocity, which is conducive to form an epitaxial splat for the melt with a largest undercooling degree. In addition, the effect of deposition temperature and crystalline orientation on the epitaxy was simulated. The simulation results are in agreement with the experimental observations.