RIPPLE FORMATION IN SOLID LIQUID EROSION

The formation of ripples on the surface of materials subjected to small-angle impingement erosion has been widely observed. In order to determine the origin and development of surface ripples, ductile (stainless steels and copper-based alloys) and brittle (thermal spray ceramics and cermets) materials were eroded in laboratory and fields tests. Scanning electron microscopy investigation revealed that the predominant wear mechanisms were scratching in metallic alloys, spray particle break-out in ceramics and removal of the binding matrix in cermets. Despite substantial differences in erosion mechanism, all the materials tested formed ripple patterns on their surfaces. It was determined that the ripple size increases with time and can attain a steady state that reflects local fluid flow conditions. Cermets and ceramics display the onset of rippling under more severe erosive conditions than metals. Ripple formation can induce cavitation, particularly on the lee side of ridges. Tests using water free of solid particles also gave rise to ripples, thereby confirming the importance of flow in their formation. Several hypotheses and models are discussed with respect to the results and a model based on the interaction between eddies and surface profile is presented.