Transfer, mixing and associated chemical and mechanical processes during the sliding of ductile materials

This paper is based on a presentation designed to stimulate discussion among participants in an international symposium held at Hyderabad, India, 14-16 December 1998. The program focused on the genesis and role of transfer and mechanical mixing in the wear of materials. It is convenient to divide the changes arising from sliding contacts into two broad categories, the first in which the average chemical composition is unchanged and the second in which the chemical composition is modified by interactions with the counterface material and the environment. These are not completely independent. In fact, plastic deformation and changes in structure during early stages of sliding may be precursors to processes such as transfer and mechanical mixing in which the chemical composition of near-surface material is modified, together with further changes in structure, and further deformation of the tribe-chemically modified material. Also, one part of a wearing surface may have a given combination of chemical composition and structure, while a nearby one may have another. Advances in understanding sliding behavior have been greatly aided by the recent availability of improved techniques for structural, chemical and mechanical characterization of materials. There is now abundant evidence for large plastic strains, allowed by the imposed compressive and shear stresses, even in materials which are considered to be brittle in simple tensile tests. Thus, sliding encourages ductility adjacent to the sliding interface. The energy dissipated during continuing plastic deformation can account for the values of friction coefficients typically measured during the unlubricated sliding of ductile materials. To improve our understanding of sliding wear processes, it is helpful to monitor changes in structure and chemical composition of sliding test specimens and wear debris, using very short to very long sliding times and using both in situ and post-test techniques. Such work has suggested that sliding wear commonly involves development of a deformation substructure which is susceptible to shear instabilities, leading to transfer, chemical reactions, mechanical mixing and fracture. The processes are similar to those which occur in the early stages of the commercial process known as mechanical alloying. Evidence for such processes has accumulated over many years and is now more than sufficient to justify major efforts to incorporate transfer and mixing in quantitative wear models. Questions designed to stimulate such efforts are included in this paper. (C) 2000 Elsevier Science S.A. All rights reserved.