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

TiO2 coatings were prepared by plasma spraying using a spray-dried powder as feedstock material. A systematic study has 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 TiO2 particles, 0–1.2 wt.% ammonium polyacrylate as a dispersant and 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. TiO2 coatings were prepared from a batch of 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 TiO2 coatings consist of a mixture of anatase and rutile. 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%).