The generation of mechanically mixed layers (MMLs) during sliding contact and the effects of lubricant thereon

In unlubricated and boundary lubricated sliding, materials touch only at a restricted number (typically n approximate to 10) of isolated, typically mi croscopically small 'contact spots' that occupy but a small fraction of the macroscopic interfacial area. It is here that the load is supported by the local hardness of the softer of the two materials, and that friction and w ear are generated. The intermittent local shear strains at the contact spot s in the course of sliding, and eventually through their statistical moveme nts of the entire top layers of wear tracks, are very large. This behavior has been simulated, both for dry sliding and lubrication, by means of stack ed foils of pure copper and silver sheared under high superimposed pressure in a Bridgman-anvil apparatus. Strain hardening curves were obtained and t he samples, now equivalent to material at wear tracks and specifically 'mec hanically mixed layers' (MMLs), were examined microscopically by means of a variety of techniques. From the workhardening curves the coefficient of fr iction as well as the hardness of the MMLs was inferred. The experiment is complicated by a strong shear strain anisotropy, in fact comparable to that found at actual contact spots, namely rising from near zero strain at the anvil-sample interfaces and at the axial center of the samples to a maximum at the mid-plane and the circumference. Microscopic analysis by means of focused ion beam microscopy (FIBM) and sec ondary ion mass spectrometry (SIMS) revealed that in the course of sliding, MMLs are formed through the proliferation of 'tongues' where local folding of the material occurs. An unexpected and potentially highly important dis covery was the bodily migration of volume elements of one of the metals thr ough the ether, e.g. of lumps of silver through copper and vice versa, with out leaving a trace. This phenomenon was enhanced through oil lubrication. (C) 2000 Elsevier Science S.A. All rights reserved.