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

• Published in: Wear Vol. 246; no. 1; pp. 74 - 90
• Main Authors: Young, John L., Kuhlmann-Wilsdorf, Doris, Hull, R.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 01.11.2000; Amsterdam Elsevier Science; New York, NY
• Subjects: Applied sciences; Contact spots; Exact sciences and technology; Focused ion beam microscopy; Friction, wear, lubrication; Machine components; Mechanical engineering. Machine design; Mechanically mixed layers; Secondary ion mass spectrometry; Secondary ion mass spectrometry; Focused ion beam microscopy; Contact spots; Mechanically mixed layers; Statistical analysis; Shear; Point contact; Probabilistic approach; Solid solid interface; Mechanical contact; Hardness; Friction material; Dry friction; High pressure; Lubricant; Crystal growth from melt; Silver; Sliding contact; Wear; Boundary lubrication; Copper; Strain hardening; Sliding friction
• ISSN: 0043-1648; 1873-2577
• DOI: 10.1016/S0043-1648(00)00456-7

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. Microscopic analysis by means of focused ion beam microscopy (FIBM) and secondary 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 discovery was the bodily migration of volume elements of one of the metals through the other, e.g. of lumps of silver through copper and vice versa, without leaving a trace. This phenomenon was enhanced through oil lubrication.