Structural characterisation of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

The microstructural features of TiO2 coatings, deposited by High Velocity Suspension Flame Spraying (HVSFS) from a suspension of titania nanoparticles, were investigated by Focused Ion Beam (FIB) + Scanning Electron Microscopy (SEM) techniques, by Transmission Electron Microscopy (TEM) and by micro-Raman spectroscopy, and were compared to those of conventional HVOF-sprayed TiO2. Proper selection of the HVSFS deposition parameters results in coatings consisting of a dense matrix, made up by the efficient superposition of well-flattened micrometric lamellae, with homogeneously distributed porosity containing sub-micrometric re-solidified spherical particles. Unlike conventional HVOF coatings, lamella boundaries are hardly discernible, no intralamellar cracking occurs and equiaxed crystals appear instead of columnar ones. A homogeneous distribution of anatase and rutile is also found. Modifications to the spray parameters can give rise to large, unmelted agglomerates, scattered throughout the coating and having poor cohesion to the surrounding material. These agglomerates retain the original phase composition of the nanopowder.