| Abstract: |
In hydroelectric power generation, cavitation erosion (CE) 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 the deposition time and is expensive. Recently, WC cermet coatings deposited by a high-velocity oxygen fuel (HVOF) thermal-sprayed process have attracted attention because such coatings provide good CE resistance and thicker coatings can be applied. These coatings have thus been applied to the surfaces of the abovementioned parts. Furthermore, WC cermet coatings improve the CE resistance by optimizing the microstructure. Few studies have been reported on the comprehensive CE characteristics of WC cermet coatings. In this study, the influence of the WC grain size, powder size, and spray condition on the CE resistance of WC-CoCr coatings was investigated via the change in mass loss with CE exposure time and SEM observation of the damaged surface and its cross section. It was found that the WC-CoCr coating deposited under a higher combustion pressure of 0.8 MPa and/or with smaller powder size and smaller WC grain size and had superior CE resistance. The CE damage mechanism derived on the basis of SEM results revealed that enhanced CE resistance could be achieved by increasing the cohesive strength between the WC grains and each splat (combustion pressure being higher) and by creating a dense microstructure (grain size being smaller).
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