A mechanical model for particle motion in the micro-scale abrasion wear test

Abstract

The micro-scale abrasion test has attracted much attention recently as a test ideally suited to the examination of both thin wear resistant coatings and monolithic materials with fine abrasives. However, care is needed in the interpretation of experimental data since changes in the conditions of the test, or even the material under test, may radically alter the behaviour of the abrasive particles in the test. At high loads, ridge formation may be observed which effectively invalidates the test as a measure of abrasion resistance. However, at lower loads, particles are observed to roll through the contact zone in some cases and groove (slide) through in others; such changes in particle motion mean that comparisons of wear rates become fraught with difficulty. This work presents a mechanical model which predicts the mode of motion of a particle between two surfaces loaded against each other, where the particle supports the load; this is in contrast to the early work of Williams and Hyncica [J. Phys. D: Appl. Phys. 25 (1992) A81; Wear 152 (1992) 57] which addressed the problem of particles passing through a lubricated contact where the fluid film itself supported the load. The shape of the particle has been assumed to be a prism with a parallelogram cross-section. Even with this simplistic assumption, it has been shown that the shape of the particle exerts a strong influence on the particle motion. Further work is required to allow the model to operate with particle shapes which have been derived from experimental observations. The behaviour of the particles has been mapped as a function of a number of parameters (applied load per particle, particle shape, hardnesses of the two surfaces, coefficient of sliding friction between the particle and the surface) which describe the wear system and thus the influence of these parameters on particle motion has been identified. Comparisons have been made between the loads predicted to cause particle sliding (grooving) by the model and experimental data from the literature; whilst assumptions concerning amongst others, particle shape, particle concentration in the contact zone and coefficient of friction have had to be made, the model and experimental results are in good agreement.