Analysis on rapid cooling and epitaxial solidification of a plasmasprayed yttria stabilized zirconia splat on a high-temperature substrate

In many applications, it will be beneficial if the plasmasprayed Yttria stabilized zirconia (YSZ) coatings exhibit epitaxial growth. Early experiments in plasma spray have shown that a high initial substrate temperature may help develop epitaxial growth from the previous deposited splats. This paper has performed an experiment to demonstrate the possibility of epitaxial growth in plasma-sprayed YSZ coatings at high substrate temperatures. A quantitative analysis of splat cooling and rapid solidification of the YSZ splat is then presented. The analysis is based on a one-dimension heat transfer model of a thin liquid YSZ layer in contact with an YSZ substrate at fairly high initial temperature. The model calculations indicate that equilibrium solidification may take place on the YSZ substrate but with a solidification temperature that is much higher than the YSZ substrate temperature. Such equilibrium solidification requires nucleation of new crystalline YSZ and therefore only leads to non-epitaxial growth. Epitaxial growth, on the other hand, requires a large melt undercooling so the YSZ crystalline can grow directly from the substrate surface, which is at a much lower temperature than the equilibrium melting point. The effect of the substrate initial temperature on the development of melt undercooling within the splat is investigated in detail. Some interesting observations have been made which may explain the physics of epitaxial growth in YSZ coatings.