Usually, people would spray either corundum or transitional phase alumina would get multi or single phase coatings.
No or very little of α-Al2O3 phase can form during extremely rapid cooling of Al2O3-melt, because there is not enough time for this process. During the crystallization of the melt, γ-Al2O3 is formed first, but the nanosecond time, the material remains hot above 900°C is not enough to form α-Al2O3.
Thank you Vadim, Starting with corundum and ending up with a multi-phase coating is suggesting feedstock is heated above crystallization temperature in-flight and would thus start crystallizing as it leaves the heating core of the spray flame until impact with the substrate. It would reach the initial phase if the process is kinetically favoured else amorphous or metastable phase would result.
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