Hydraulic turbines, valves, and pumps operate in environments where they are exposed to cavitation phenomena and corrosion, which can result in mass loss, leading to reduced performance and failure. HVOF spraying has been used to repair eroded surfaces on such components and new alloys are being developed to reduce repair costs. This investigation assesses the cavitation resistance of FeMnCrSiNiB alloy coatings deposited by HVOF spraying. Corrosion rates and oxidation potentials are measured under different conditions and compared to stainless steel coatings normally used on water turbine runners.

In this study, two coatings, one produced by HVOF spraying, the other by physical vapor deposition (PVD), were applied on a nickel superalloy substrate in order to compare their hot corrosion behavior. The coating samples were initially characterized by OM and SEM-EDS analysis, then a mixture of V 2 O 5 , Na 2 SO 4 , and NaCl was deposited on the surface and the samples were to 900 °C for 35 h. The results show that the PVD CrN coating provided better corrosion protection than HVOF-sprayed CrC-NiCrAlY.

Thermally sprayed coatings rely on mechanical anchoring as the main bond mechanism, which depends on previous surface preparation, i.e. cleaning, roughening and preheating. In this work, aluminum oxide (Al 2 O 3 ) grit was used to blast surfaces in order to show the influence of the abrasive reuse on the decrease in granulometry, surface roughness, cleanliness and consequently on the coating adhesion strength. SEM images illustrate the difference in grain sizes and EDS was used to study the contamination of the abrasive with the substrate elements. To understand the granulometry reduction, the samples were blasted and after each cycle the abrasive was classified with different sieves, and then the mass was measured. The abrasive was mixed and the process was repeated for five cycles. Sa, Sz, Ssk and Sku were the parameters chosen for the roughness characterization and were compared to the grain size decrease, showing a significant correlation between them. Lastly, the adhesion strength of the coatings was measured according to ASTM-C633 and compared to both the roughness and blast cycle. The results show that a small fraction of abrasive with finer grit reduces the roughness significantly. However, the adhesion strength was not affected.

The HVOF process is characterized by the deposition of a low porosity and oxide content coating and the particles projected by this process have high kinetic energy, resulting in a high density coating. However, the presence of non-melted particles can affect the porosity, tensile adhesion, hardness and surface finishing. In this article, the influence of the shot peening treatment on morphology, porosity, tensile adhesion and roughness of some FeMnCrSi(Ni/B) HVOF coatings have been studied. High carbon steel particles with spherical geometry were used for shot peening. The results show that the shot peening reduces the porosity, mainly in the coatings with higher porosity level, and generates an increase of hardness with no phase transformations. The increase in adherence is also a benefit generated by the treatment. The surface quality analyses show a significant roughness reduction. With shot peening a compressive residual stress state was achieved in the coating, improving mechanical properties.

Cavitation erosion is a mass loss process that occurs in a component subject to a liquid pressure variation. The mass loss phenomena occurred by extensive microstructure deformation with combination of shock loading and fatigue caused by the impact and collapse of the bubbles at surface. Many studies have been done to evaluate the cavitation resistance of the thermally sprayed coatings. Oxide formation, microstructure and tensile adhesion are important characteristics for coatings against cavitation. In this work some Fe-Mn-Cr-Si alloys, that is a class of steel with strain induced phase transformation, were used to evaluate the chemical composition influence on the microstructure, oxide formation, chemical composition and tensile adhesion of HVOF coatings. It was observed a significant reduction on the porosity, from 3% to 0,1%, and area fraction oxide from 15% to 7%, with Boron and Nickel addition. A combination of Nickel and Boron addition improve the wettability of the splats with increase on the tensile adhesion of the coatings, up to 60MPa, otherwise, higher levels of Boron content reduce the adhesion of the coatings.