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)+ScanningElectron Microscopy (SEM) techniques, by Transmission Electron Microscopy (TEM) and by micro-Ramanspectroscopy, and were compared to those of conventional HVOF-sprayed TiO2. Proper selection of theHVSFS deposition parameters results in coatings consisting of a dense matrix, made up by the efficientsuperposition of well-flattened micrometric lamellae, with homogeneously distributed porosity containingsub-micrometric re-solidified spherical particles. Unlike conventional HVOF coatings, lamella boundaries arehardly discernible, no intralamellar cracking occurs and equiaxed crystals appear instead of columnar ones. Ahomogeneous distribution of anatase and rutile is also found. Modifications to the spray parameters can giverise to large, unmelted agglomerates, scattered throughout the coating and having poor cohesion to thesurrounding material. These agglomerates retain the original phase composition of the nanopowder.