Comparison between APS, HVOF and High-Velocity Suspension Flame Spraying (HVSFS)-Deposited Al₂O₃ Coatings

The High-Velocity Suspension Flame Spraying (HVSFS) technique, a recently-developed modification to the standard HVOF process enabling the use of suspension feedstock, was employed in order to deposit Al₂O₃ coatings from a nanopowder suspension. These coatings were compared to conventional APS and HVOF-sprayed ones. HVSFS coatings possess lower overall porosity and lower pore interconnectivity degree. Indeed, most of the nanoparticles were fully melted by the gas jet, thus forming very thin, well-flattened lamellae, having smaller columnar crystals than conventional coatings. Accordingly, HVSFS coatings possess higher hardness and elastic modulus, as determined from nanoindentation tests. Ball-on-disk tribological tests also indicate that HVSFS coatings possess much better sliding wear resistance than conventional ones, because they are capable of forming denser and more protective surface tribofilms during dry sliding.