G. Bolelli, V. Cannillo, L. Lusvarghi, University of Modena and Reggio Emilia, Modena, Italy; R. Gadow, J. Rauch, Universität Stuttgart, Stuttgart, Germany; A. Killinger, University of Stuttgart, Germany, Stuttgart, Germany
The innovative High-Velocity Suspension Flame Spraying (HVSFS) process, whereby a conventional gas-fuelled HVOF torch is modified in order to allow the processing of liquid feedstock, was employed in order to spray a TiO2 nanopowder suspension. Three different sets of parameters were employed. The structure, microstructure, nanohardness, tribological properties and photocatalytic activity of the resulting coatings were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO2 coatings, manufactured using commercially available feedstock.Compared to the APS and HVOF techniques, HVSFS enabled the deposition of thinner (20 μm – 60 μm thick), yet high-quality layers.
Moreover, it was found that the HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced, in order to achieve higher photocatalytic efficiency than conventional APS and HVOF TiO2.
Alternatively, it is possible to deposit dense layers, with lower porosity and pore interconnectivity and better hardness and wear resistance than as-deposited APS and HVOF coatings.
Summary: This study deals with the characterisation of the microstructure, the micromechanical properties, the dry sliding tribological behaviour and the photocatalytic activity of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings. In the HVSFS process, a conventional gas-fuelled HVOF torch is modified in order to enable the use of a liquid feedstock. In the present case, the starting feedstock was an isopropanol-based nanoparticle suspension (average particle size 25 nm). These coatings were produced using three different parameter sets, and they were also compared to conventional APS and HVOF-sprayed coatings, deposited using commercially-available dry powder feedstock.
SEM observations of polished cross-sections and of fractured sections, as well as XRD analyses, suggest that the HVSFS coatings are formed by the impact of nanoparticle agglomerates. The liquid suspension drops injected into the combustion chamber of the HVOF torch are fragmented into micrometric droplets because of aerodynamic effects; subsequent evaporation of the solvent produces fine agglomerates. The agglomerates have various sizes, also because they could be partially disrupted during spraying, and, depending on their size and on the torch parameters, they attain different melting degrees. Small, fully molten agglomerates produce dense regions consisting of fine lamellae, which have excellent cohesion. Other agglomerates remain partly unmelted, thus resulting in the retention of anatase nanoparticles. Consequently, HVSFS-deposited coatings have lower defectiveness than conventional ones, yet they retain significantly larger amounts of anatase phase. Electrochemical impedance spectroscopy analyses confirmed that, in all cases, the HVSFS coatings have lower defectiveness and lower amount of interconnected porosity than conventional APS and HVOF ones.
However, significant differences exist between the three HVSFS coatings, deposited using different sets of parameters. Depending on the choice of deposition parameters, indeed, coatings may either contain mostly fully-molten agglomerates, or retain a significant amount of partially unmelted material. In the former case, the layer is dense, harder than conventional APS and HVOF coatings, and therefore shows better dry sliding wear resistance than as-deposited APS and HVOF coatings. In the latter case, the coating displays lower mechanical and tribological properties but better photocatalytic activity.
The HVSFS process seems therefore to possess excellent flexibility: depending on the selection of parameters, opposed requirements (high density and wear resistance / high retention of unmelted nanoparticle agglomerates for the photocatalytic performance) can be satisfied, achieving results which are, in both cases, superior to conventional plasma-spraying or HVOF-spraying techniques.
