Epitaxial grain growth during the rapid solidification of molten TiO_2 in plasma spraying was studied. The crystallographic structure of the TiO_2 splats deposited on rutile and α-Al_2O_3 substrates at 150, 300 and 500°C was characterized by high resolution transmission electron microscopy and electron back scattering diffraction. The results reveal that homo-epitaxial and hetero-epitaxial TiO_2 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 TiO_2 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.
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