A direct experimental link between atomic-scale and macroscale friction
Date
2019
Authors
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Publisher
University of Delaware
Abstract
This dissertation describes the development and application of new experimental tools designed to close the gap between industrially-relevant friction and wear and their molecular origins. ☐ Industrially relevant friction and wear account for a significant fraction of global energy consumption and, as a result, have massive economic consequences. The complexities, uncertainties, and inaccessibility of these tribological contacts makes fundamental studies of friction and wear inherently difficult. One approach is to isolate and a study a single-asperity interaction under well-controlled nanotribological conditions. Although nanotribological interfaces have given important insights into the physics of friction, it remains unclear how their responses relate to those of industrially-relevant multi-asperity ensembles. ☐ This dissertation describes the development and application of new experimental methods to address the most important experimental barriers, which include: (1) controlling load, probe material and geometry, and sliding speed; (2) quantifying interfacial forces at conditions that vary by many orders of magnitude within a single interface; (3) controlling the transition between interfacial slip and wear. ☐ These new methods were used to construct tribological maps that bridge the gaps in load, size, speed, pressure, and number of asperities in contact between the nanoscale and the macroscale. The first challenge was identifying a model interface capable of surviving a wide spectrum of frictional conditions without wear – we identified single-crystal MoS2. Using this model material system, friction was mapped for varying loads (10 nN to 10 mN), contact sizes (10 nm to 100 µm diameters), and sliding speeds (25 µm/s to 100 mm/s) to bridge the gap between the nanoscale and the macroscale. Without the confounding effects of wear, the results showed that atomic scale friction is likely connected to macroscale friction. Analysis of the contact areas suggest that friction is sensitive to both the contact area and the way in which contact forces are distributed between asperities. Future work will be needed to elaborate this connection and the apparently dominant effect of multi-asperity versus single asperity interactions. ☐ The major contribution of this thesis is twofold: (1) the discoveries that provide an elucidation of macroscale friction and wear as a function of atomic-scale phenomena and (2) the various technologies that were developed along the way that now enrich the family of available tribology, and not only, research methods.