Particle motion and modes of wear in the dry sand–rubber wheel abrasion test

Abstract

In the dry sand–rubber wheel test, the particles are free to move between the wheel and testpiece. The particles may either roll as they pass through the contact, or they may temporarily embed in the rubber wheel and groove the sample; the motion of the particles will govern the modes of deformation of the sample and thus the dominant mechanisms of wear. In experiments concerning the abrasion of a range of steels with widely varying hardnesses, it has been shown that the motion of the particles through the contact depends not only upon the details of the testing conditions (for example particle feedrate, particle size and shape, applied load) but also upon the testpiece material properties themselves, such as hardness. Such a dependence upon testpiece material properties is a cause for concern for those who use the test, and indicates that observations of the mechanisms of wear are an essential part of this test methodology. Particle rolling through the contact is favoured by low applied loads and low testpiece hardness whereas particle sliding through the contact is favoured by high applied loads and high testpiece hardness. The wear coefficients in situations where particle sliding (grooving) occurs are not significantly higher than those for situations where particle rolling occurs and it is argued that this is associated with the way in which grooving particles orient themselves with respect to the testpiece as they pass through the contact. This is in contrast to fixed-particle grooving abrasion (such as might be observed in tests using abrasive papers), and as such, it is argued that what is commonly termed two-body abrasion should be subcategorised into “fixed-particle grooving abrasion” and “free-particle grooving abrasion”. The paper then proceeds to provide an analysis of the motion of particles in the dry sand-rubber wheel abrasion test, and seeks to understand the mechanics controlling their motion, and thus the dependence of particle motion upon external factors, focussing on the effects of testpiece hardness and applied load. The effect of hardness on particle rotation is well predicted by the model, but the effect of the applied load on particle motion observed experimentally is opposite to that which is predicted by the model. The shortcomings of the model are discussed, and the model has been qualitatively modified to account for this discrepancy. The modifications centre around the significant changes in rubber wheel–particle contact geometry as the applied load is changed; such changes are difficult to model analytically due to the large strains associated with such a contact, and it is suggested that finite element modelling may be required to fully understand and model the complex contacts occurring in this simple and widely employed test method.