Despite the fact that many machine components are designed to operate with a comparatively thick film of lubricant between the loaded surfaces these often still deteriorate with time as hard particulate contaminants are swept through the bearing gap. Such particles may arise from the external environment or represent wear debris from other pairs of surfaces lubricated by, or in contact with, the same fluid. In order to investigate this phenomenon experimentally it is necessary to develop a predictable hydrodynamic film between the test surfaces which can be contaminated by small volumes of carefully graded abrasive particles. A foil bearing has been used to generate such films between 10 and 50 microns thick and to which contaminants such as powdered quartz or finely divided diamond can be added. As the ratio of the characteristic particle size to film thickness is varied not only do the wear rates of the solid surfaces change but examination of the wear tracks suggests that very different mechanisms of material loss come into operation. When the size ratio is low the worn surface consists of a large number of small pits and indentations; these display virtually no alignment in the direction of relative sliding and it appears that the particles tumble and roll freely through the gap. Above some critical value of the particler:film size ratio the appearance of the surface changes dramatically to a grooved or micro-machined surface with all the grooves aligned in the sliding direction. A relatively simple theoretical model is developed, based on what happens to a typical particle, which goes some way to explaining these observations. As well as being consistent with the observed transition from “tumbling” to “grooving”, the model can also explain why increasing the hardness differential between the hard and the soft surfaces does not always lead to a reduction in damage to the harder member of the pair.