Abstract
A model for the fomation of thermally sprayed alumina coatings is proposed. The spreading and crystallization of liquid droplets on impact with the substrate are analysed and the thermal history of individual particles related to the kinetics of nucleation of γ-Al2O3 to other forms. The results suggest that under the usual spraying conditions undercooling of the liquid droplets is such that γ-Al2O3 nucleates in preference to α-Al2O3 and the cooling rate after solidification is sufficiently rapid to prevent transformation to δ-Al2O3 or α-Al2O3. Transformation of initially formed γ-Al2O3 to α-Al2O3 appears to be possible only if the lamellae formed on impact are thicker than about 10 μm if the substrate is heated to about 1000° C, or if the thickness is greater than about 20 μm on an unheated substrate. The α-Al2O3 generally observed in thermally sprayed coatings is the result of crystallization from pre-existing nuclei arising from incomplete melting of the feed material.
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McPherson, R. On the formation of thermally sprayed alumina coatings. J Mater Sci 15, 3141–3149 (1980). https://doi.org/10.1007/BF00550387
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DOI: https://doi.org/10.1007/BF00550387