In an effort to obtain a series of plasma sprayed coatings of controlled microstructure, three mono-sized sapphire powders were deposited using an axial injection torch in which the plasma gas composition and nozzle diameter were the only processing parameters varied. The effects of changes in these parameters on the resultant coating splat morphology, porosity, angular crack distribution and hardness are reported. The uniform, dense microstructure and high hardness of 14 GPa - a level usually only associated with chromia thermal spray coatings - of the best alumina coatings results from use of the combination of tightly controlled processing conditions and mono-disperse precursor powders. The microstructural quality of plasma sprayed coatings and, hence, their properties can be improved significantly by minimizing variations in processing and raw material parameters.

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