Microstructure of plasma-sprayed titania coatings deposited from spray-dried powder

TiO coatings were prepared by plasma spraying using a spray-dried powder as feedstock material. A systematic study has 2 been performed to determine how the titania slurry formulation (e.g. dispersant level, pH, binder addition) affects the granule characteristics. Aqueous slurries consisting of 50 wt.% of TiO particles, 0–1.2 wt.% ammonium polyacrylate as a dispersant and 2 up to 15 wt.% styrene–ester acrylic copolymer as a binder were investigated using settling experiments. It was shown that when the relative sedimentation height (RSH) value is below 0.45 the fully dispersed slurry leads to the fabrication of a hollow powder whereas when the RSH is over 0.50 the flocculated slurry leads to a dense powder. TiO coatings were prepared from a batch of 2 hollow granules of anatase. The microstructure and crystallographic phases of these deposits were characterised using scanning electron microscopy, three porosimetry methods (image analysis, Archimedean and calculation from thickness and X-ray diffraction, XRD) and quantitative XRD. The results showed that the TiO coatings consist of a mixture of anatase and rutile. 2 The content of anatase was varied from 35 to 50 vol.% depending on the operating conditions (mainly plasma power and cooling rate). These great amounts of anatase phase were obtained along with rather high percentage of porosity (between 13 and 25%)