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≈10) of isolated, typically microscopically 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 wear are generated. The intermittent local shear strains at the contact spots in the course of sliding, and eventually through their statistical movements 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 stacked foils of pure copper and silver sheared under high superimposed pressure in a Bridgman-anvil apparatus. Strain hardening curves were obtained and the samples, now equivalent to material at wear tracks and specifically ‘mechanically mixed layers’ (MMLs), were examined microscopically by means of a variety of techniques. From the workhardening curves the coefficient of friction 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.