Surface finishing can alter the cavitation erosion resistance of metallic materials by affecting both the bubble dynamics and the mechanical properties. Smooth surfaces are assumed to provide less nucleation sites from which cavitation bubbles may form, which can reduce cavitation activity and damage. On the other hand, grinding is known to result in strain hardening of the subsurface material, which may also contribute to lower cavitation erosion rates. Laboratory experiments using a sonotrode to induce cavitation in a liquid are commonly employed to study the cavitation erosion behaviour of various materials. Typically, samples with polished or ground surfaces are used, but surface finish varies significantly between studies. The present work aims at identifying the effects of ground and polished surface finish on cavitation erosion test results.
Two already thoroughly investigated alloys, austenitic steel 316L and NiAl-bronze, are exposed to ultrasonic cavitation. Two different surface finishes are compared: metallographic polishing using Silica, and grinding by 80 grit sandpaper. The microstructure below the surface after grinding is analysed by scanning- and transmission-electron microscopy (SEM & TEM). Cavitation erosion rates are recorded and surface features after different testing times are investigated by SEM.
The grinding marks act as preferential sites of early material removal, leading to an increase in early erosion rate for the NiAl-bronze, but not for the steel. Grain refinement below the steel surface hinders the formation of slip steps, which lowers the erosion rate of the ground steel. After 8.5 h cavitation erosion testing time, a lower erosion mass loss was obtained for both alloys in the ground state as compared to the polished state. The effect of grinding on cavitation erosion test results is obviously not straight forward, and depends on the respective alloys as well as the chosen testing time.