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.

Plasma-sprayed ceramic coatings were deposited on refractory substrates to improve their chemical resistance to molten glass. Mullite, coarse and fine alumina powders were employed. Different layers with step-wise varying compositions were deposited in order to avoid dilatometric incompatibility troubles. In all the samples, the top coating consisted of plasma-sprayed fine alumina powders, to achieve low porosity and confer adequate chemical resistance, while mullite was used to match the low thermal expansion coefficient of the porous refractory substrates. The coatings were characterized by SEM, XRD, fracture toughness, abrasion resistance, thermal shock resistance and resistance to chemical attack. Since the overall thermo-mechanical and chemical behaviour is greatly affected by microstructural features, such as porosity amount, stacking sequence of layers and their composition, a FEM simulation of the thermo-mechanical properties (with particular regard to thermal stresses-induced cracking) has been conducted and compared to the experimental results.