Abstract
Cavitation erosion (CE) damage, which occurs in the main parts (made of high chromium cast steel) of hydroelectric power generation machine, is one of the serious problems. It is expected that life time of those parts would be prolonged if the suitable CE-resistant coating is applied on the surface of the cast steel. In this study, WC-cermet coatings (WC-CoCr and WC-Cr3C2-Ni), which were fabricated by high-velocity oxygen-fuel (HVOF) thermal spraying process, was interested in protecting CE attack to the cast steel. To clarify CE property of the WC-cermet coatings, the ultrasonic vibration tests were conducted, and the amount of volume loss characterized as CE damage was measured. The microstructure and the fracture toughness, which was evaluated by the indentation test method, of the coatings were related with the CE damage. As the results obtained in this study, the fragment which was spalled from the surface after CE test was almost flake-like shape, and its size was from 2µm to 50µm. SEM observation indicated that this fragment included both WC particle and metal binder, which means that WC particle and metal binder was still strongly bonded together. It was also confirmed that the amount of volume loss could relate directly with the fracture toughness KIC rather than Vickers hardness. It was considered that CE damage was progressed into the depth by throwing out the fragment originated from micro crack initiation. Thus, it was required that the CE resistance of the developed coatings could be labelled through the fracture toughness.
