Evaluation of metastable phase and microhardness on plasma sprayed titania coatings

TiO2 coatings were plasma sprayed using two spray-dried powders as feedstock materials with either a dense and solid shape or a hollow morphology. To achieve interesting photocatalytic properties, coatings must be in the anatase phase. The effects of the powder characteristics, especially shape, density and size distribution, and of the plasma conditions on the anatase ratio and porosity rate of the coatings were investigated. To better qualify the characteristics of these coatings produced from spray-dried powders, microhardness was also studied. A bimodal microstructure of the coatings was identified related to the incorporation of non-molten particles that allow to maintain a high ratio of anatase phase. As a consequence a bimodal distribution of Knoop microhardness was also reported for coatings composed of 40 to 60 vol.% anatase.