Riassunto analitico
The project aims to investigate the influence of feedstock powder manufacturing route and nozzle configuration on deposition, microstructural features, hardness, sliding wear and erosion behaviour of high-velocity air fuel (HVAF) sprayed WC-CoCr coatings. The Master thesis is based on a project carried out through a joint partnership between the Engineering Department “Enzo Ferrari” (Modena, Italy) and the Production Technology Centre of the University West (Trollhättan, Sweden). This approach to the research required performing analysis at both the University of Modena and University West, due to the different facilities available at each location. Höganäs GmbH (Sweden) has provided three different WC-10Co4Cr fine powders, hereafter labeled as P1, P2 and P3, with the following characteristics: • P1: agglomerated and sintered, 5/20 μm • P2: sintered and crushed, 5/15 μm • P3: newly developed powder manufacturing route, 5/15 μm. The feedstock powders have been deposited employing two different HVAF guns (2nd and 3rd generation) with two distinct DeLaval nozzle configurations each, involving different lengths and/or exit diameters. Therefore, each run has been sprayed with two different targeted thicknesses in order to investigate how thinner coatings (50 μm) perform compared to conventional thick coatings (>100 μm, in this case, 150 μm). The deposition efficiency in terms of μm/pass for each run has been evaluated showing that, for P1 and P2 the trend between the different nozzle configurations and targeted thicknesses is similar with the configuration having the shortest length and smaller exit diameter that brings to highest value (≈ 25 μm/pass). However, regarding P3 the deposition efficiency doesn’t reveal any relevant differences between the different nozzle configurations for both 50 μm and 150 μm coatings due to colder conditions and lower particle velocity. The feedstock powder and deposited coatings were characterized in terms of microstructure using SEM, EDS and XRD. X-ray diffraction analysis shows negligible decarburization in all the HVAF-sprayed coatings. It has been noted that, regardless of the thickness, extremely dense coatings with few segregation binder areas and low porosity (<1 %) can be deposited employing HVAF and fine feedstock powder. The hardness of the thin and thick coatings has been measured using Vickers hardness method from the cross-section. It has been found that, in all the powders, the 3rd generation gun leads to a slight improvement in the hardness of the coating due to the higher velocity of the particles. Also, the peening effect derived from a different number of passes is clear only in P1 and P2 powders. Furthermore, the results suggest that the peening effect has a considerable influence on the mechanical performances, decreasing the hardness values from 150 μm to 50 μm coatings. Specific wear rate under sliding conditions and erosion rate under dry conditions have been determined and a comprehensive post-wear analysis was conducted. It has been shown that the powders haven’t been founded to be sensitive to the type of nozzle used while the wear resistance of the coatings slightly declines with decreasing thickness of the coating. Regarding erosion performances, no major differences are noticed in changing the type of powder or the nozzle configuration. On the other hand, the evolution of the influence of the impact angle between the three powders shows slight deviations. These tests are highly suited to outline the overall quality of the coatings, as they provide immediate and useful information on the defectiveness and the better correlation type of powder-nozzle configuration to obtain the greatest mechanical and tribological performances.
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