Friction across length scales: connecting practical friction with its fundamental origins
Date
2022
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Friction dictates machine performance and is a primary source of global energy consumption. Indeed, most major civilizational and technological advancements have required some corresponding advancement in our understanding of and ability to manage friction. While it is easy to measure and study friction, frictional phenomena occur at the intersection of disparate fields, including physics, chemistry, mechanics, and materials science. They occur between rough surfaces at inaccessible interfaces where contacts occur at multiple unknown locations under ever changing contact conditions. This challenge has motivated single asperity friction studies using the Atomic Force Microscope (AFM). In these measurements, sliding of the asperity occurs through a series of stick-slips, and basic science studies have convincingly demonstrated that friction is fundamentally attributable to the energy dissipated by these slip events. This frictional energy dissipation process is defined as Mode 1 friction in this thesis. At the practical scale, however, frictional energy can be dissipated by many other irreversible processes, including plastic deformation, wear, transfer film formation, and tribochemistry to name the most common. Frictional dissipation from irreversible surface changes is defined as Mode 2 friction. The current consensus is that atomic-scale friction and practical friction come from fundamentally distinct modes of energy dissipation (mode 1 and mode 2, respectively) and are, therefore, unrelated. This thesis challenges this consensus. Specifically, it aims to: 1) demonstrate that atomic stick-slip contributes to friction in practical multi-asperity contacts and; 2) quantify that contribution over a wide range of length scales. ☐ To achieve these aims, I had to address the long-standing challenge of multi-scale lateral force calibration. Existing calibration methods lack traceability to a force standard, reliability, or robustness across length scales. I successfully developed and validated a robust and traceable lateral force calibration method that is used throughout this thesis. ☐ Next, I devised a vibration-based approach to isolate and study the effect of atomic-scale stick-slip on multi-asperity friction. Specifically, I subjected the contact to nanoscale oscillations that would, by design, eliminate atomic-scale stick-slip without affecting other known contributors, including mode 2 contributors (e.g. plowing) and stick-slip at scales of the roughness and larger. The application of these nanoscale oscillations nearly eliminated multi-asperity friction just as it eliminates single-asperity friction. Thus, by the same logic used in the nanotribology literature, the majority of friction in these multi-asperity contacts can be attributed to atomic scale stick-slip, not plowing, wear, transfer, or any of the other mechanisms multi-asperity friction is commonly attributed to. More importantly, this study shows that multi-asperity and single asperity friction have the same physical origins. ☐ In the following chapter, I use this insight to quantify how this fundamental mode of frictional dissipation varies across length scales. Friction between Alumina and gold, silicon, molybdenum disulfide, or steel was quantified for 200 nN-75 mN of normal force under otherwise identical conditions. Prior studies varying loads by a similar or greater extent were forced to change other important variables simultaneously. This is the first study to isolate the effect of load on friction over this range. Additionally, I used varying probe radii to discriminate between the effects of load, area, and contact pressure. ☐ Interestingly, friction did not completely vanish with the isolation of mode 1 friction. An ultra-low but quantifiable friction component was observed. The next chapter assesses the relative contributions of viscous dissipation, large-scale stick-slip, and wear) to this residual friction. ☐ Finally, I show a transition from mode 1 to mode 2 dominant friction with the onset of severe wear. This transition occurred when friction coefficients transitioned from relatively low values characteristic of run-in (~0.2) to relatively high values characteristic of severe wear during unlubricated sliding (~0.5). An in-situ study shows that the transition typically corresponds to the onset of material transfer. ☐ In summary, under low to moderate friction coefficient conditions (0.1-0.25), this thesis presents the first experimental evidence that fundamental single asperity friction and practical multi-asperity friction share common physical origins – frictional dissipation is due to atomic stick-slip. Dissipation due to irreversible surface changes dominate friction following the transition to severe wear, which appears to be highly correlated to the onset of material transfer. Atomic scale and practical friction are only fundamentally distinct phenomena in cases involving severe wear.
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Keywords
Friction, Calibration methods, Fundamental modes, Atomic-scale stick slip