In this study, a TiO 2 (anatase) nanopowder suspension was processed by high velocity suspension flame spraying (HVSFS). The resulting coatings were characterized and compared to conventional HVOF and atmospheric plasma sprayed layers. It is shown that the HVSFS operating parameters can be adjusted to achieve dense titania with a near nanostructure and homogeneous distribution of anatase and rutile phases. These coatings have lower pore interconnectivity and higher wear resistance than the APS and HVOF layers. Alternatively, large unmelted agglomerates of anatase nanoparticles can be embedded in the coating, increasing the porosity and anatase content for enhanced photocatalytic efficiency.

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 Al 2 O 3 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 oxides are effective coatings against wear and corrosion. Low particle velocity in the plasma jet causes a limited interlamellar cohesion. HVOF-sprayed ceramic coatings emerged as an improved alternative. In this paper, microstructural characteristics and tribological performances of HVOF sprayed Al 2 O 3 , nanostructured Al 2 O 3 and Cr 2 O 3 coatings are compared to reference plasma-sprayed Al 2 O 3 and Cr 2 O 3 . The microstructure is analysed by SEM, EDS and XRD. Hardness and fracture toughness are investigated by instrumented indentation and elastic modulus by 3-point bending. Steel wheel and rubber wheel tests have been used to assess dry particle abrasion resistance. Sliding wear resistance is tested by pin-on-disk at room temperature and at 400°C, against SiC and 100Cr6 steel balls. HVOF-sprayed coatings are denser and have better interlamellar cohesion thanks to increased particle velocity. They are harder, tougher, possess a higher elastic modulus and lower porosity. Dry particles abrasion resistance is definitely superior to plasma-sprayed ceramics due to higher toughness; sliding wear resistance is higher, particularly at 400°C.

HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) have been compared with various kinds of industrially manufactured hard chrome coatings (HCC), whose substrate preparation, deposition process, post deposition treatments greatly affect their characteristics. Microstructure, micromechanical properties, tribological behaviour and corrosion resistance (electrochemical polarization tests and Corrodkote test) have been studied. HVOF-sprayed cermets are harder but less tough than HCC, Co-28Mo-17Cr-3Si are less hard than HCC. Splats detachment causes a comparable or higher mass loss in three-body abrasion than HCC coatings. Forming a uniform surface film, cermet coatings definitely overcome HCC in two-body sliding, while Co-28Mo-17Cr-3Si has insufficient hardness to display good sliding wear resistance. HVOF coatings show no passivation in corrosive media but cermets posses more noble corrosion potentials than HCC, and undergo generalized corrosion in HNO 3 and HCl, with similar corrosion current densities (I C ). HCC passivate and resist well in HNO 3 0.1N, but undergo pitting corrosion in 0.1N HCl. Definitely different E C and I C are recorded for various HCC in HCl. HVOF-sprayed cermet coatings show lower I C in 0.1N HCl solution than several kinds of HCC. No visible damage occurs on HVOF-sprayed coatings after the Corrodkote test, while non de-hydrogenated HCC suffered pitting corrosion.

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.

Plasma-sprayed alumina-waste glass composite coatings on ceramic substrates were produced. Two kinds of alumina powders, different alumina volume fractions, and two glass powders particle size distributions were tested. Post-process thermal treatments were performed. The coatings were characterized by SEM, XRD, Vickers microhardness, fracture toughness, abrasion resistance tests. Coatings superior to traditional tile glazes were obtained with as high as 50 vol.% of waste glass. Fine glass powders (<45µm) must be employed to achieve adequate toughness. A low-cost spray-dried alumina can be used instead of the expensive commercial powders. The thermal treatment enhances the coating properties. A FEM thermo-mechanical simulation was performed. Elastic modulus calculations show a definite coating anisotropy (higher mechanical properties in the longitudinal direction). Compressive residual stresses in the alumina and tensile ones in the glass are developed after the thermal treatment. Crack propagation studies based on Griffith model show cracks initiating from larger pores and propagating easily through the glass, thus explaining the coating toughening achieved through the employment of finer glass powders. Cracks are stopped by alumina; this effect is enhanced in the thermally treated coatings. The numerical and experimental (from indentation fracture toughness test) crack propagation patterns are in good agreement.