HVOF hard coatings for the enhancement of erosion resistance of components to be applied in water treatement implants

The aim of this work was to evaluate the hydro-erosion resistance of HVOF thermal sprayed coatings suitable to coat parts to be applied in the water treatment equipments and implants. Several coating materials such as Inconel 625, Inconel 625+10%vol. Al2O3, Stellite 6, AISI 316 stainless steel and WC-Co were applied on steel or cast iron substrates with different geometries. The derived specimens were tested in a special slurry abrasion/hydro-erosion test rig, that simulates the operating conditions of wet erosive wear. The behaviour of different coatings was evaluated in terms of weight losses vs test duration. Through this test, the influence of coatings surface roughness on the performances in the slurry hydro-erosion test was also evaluated. For this part of the research, reference AISI 304 uncoated samples, roughened with different Ra values, were used. Finally, on the transverse section of the samples morphological analyses were performed through optical and SEM microscopies, so as to study the layers characteristics. Furthermore, the adhesion strength of the coatings-substrate interface was assessed through the tensile method. The Stellite 6 and WC-Co systems had shown the best performances in the hydroerosion test. The composite coating improved the simple Inconel system, but didn't overcome the best performing systems. In this composite coatings a reduction of oxide formation and an homogeneous structure were achieved. However, large alumina agglomerates formed in the layer structures and might be detrimental in terms of the coating toughness.