A case of wear particle formation through shearing-off at contact spots interlocked through microroughness in adhesive wear

Abstract

The shape and size distribution of wear particles and the interface morphology enables different wear modes to be distinguished, and hence allows comparison with corresponding wear models. The particles must be collected as soon as possible after their formation, to minimize deformation and chemical attack between the generation and analysis of the particles, and the concurrent damage to the interface. To this end, a correlated study of surface morphology and of wear particles was performed on bundles of copper wires of various diameters subject to unlubricated sliding on a copper substrate in adhesive wear. Different sliding conditions were used. In some tests the contact spot temperature was artificially increased through the passage of current to generate Joule heat; other tests were performed in argon rather than in the unregulated atmosphere. The results strongly support the “shearing-off” model of wear, operating at contact spots mechanically interlocked through microroughness, as indicated by the following. 1. (1) There was no evidence of any clinging of wear particles or fragments thereof to the harder side, as expected if adhesive forces had played a significant role in the primary wear event. 2. (2) Numerous partially formed but not yet fully detached wear particles were observed whose shape conformed exactly to that predicted by the shearing-off model. 3. (3) Evidence of microroughness was discovered on the surfaces of partly formed particles, the corresponding surfaces of wear particles and those of the contact spots on the other side. 4. (4) By contrast, none of these surfaces showed evidence of tearing or puckering that would indicate breaking of adhesive bonds. 5. (5) For all wear conditions, the size of the wear particles matched the size of the contact spots on the opposing side. 6. (6) The wear particles exhibited “similitude”, i.e. their ratio of length to width to thickness was virtually independent of the wear conditions, as predicted by the shearing-off model. 7. (7) The ratio of length to width to thickness of the particles was about 1:0.5:0.3, which is in close conformity with expectations from the shearingoff model without assistance from adhesion, whereas strong adhesive forces are expected to generate equiaxed wear particles.