Influence of chemical composition, grain size, and spray condition on cavitation erosion resistance of high-velocity oxygen fuel thermal-sprayed WC cermet coatings

Abstract

In hydroelectric power station, cavitation erosion (CE) and slurry erosion (SE) damage often occurs in components such as turbine runners, guide vanes, and stay vanes. Such surface damage causes serious problems to the machine because it decreases the power generation efficiency. To protect against such damage, the parts are generally subjected to chromium plating. However, a thick coating is required to ensure long-term CE resistance, which needs more deposition time and is expensive. Recently, WC cermet coatings deposited by high-velocity oxygen fuel (HVOF) thermal-sprayed process have attracted attentions because such coatings provide good CE resistance and thicker coatings can be applied. These coatings have been applied to surfaces of abovementioned parts. Furthermore, WC cermet coatings improve CE resistance by optimizing microstructures. Few studies have been reported on the comprehensive CE characteristics of WC cermet coatings. In this study, the influence of chemical composition (WC-CrC-Ni and WC-CoCr), WC grain sizes, and spray conditions on CE resistance of WC cermet coatings was investigated via the change in volume loss with CE exposure time and SEM observation of damaged surfaces and cross sections. It was found that WC-CoCr coating deposited under higher combustion pressure of 0.8 MPa had superior CE resistance. CE damage mechanism derived on the basis of SEM results revealed that enhanced CE resistance could be achieved by increasing cohesive strength between WC grains (combustion pressure being higher) and by creating a dense microstructure.