A model of friction with plastic contact nudging: Amontons-Coulomb laws, aging of static friction, and non-monotonic Stribeck curves with finite quasistatic limit

Abstract

We introduce a model of friction between two contacting (stationary or co-sliding) rough surfaces, each comprising a random ensemble of polydisperse hemispherical bumps. In the simplest version of the model, the bumps experience on contact with each other only pairwise elastic repulsion and dissipative drag. These minimal ingredients are sufficient to capture a static state of jammed, interlocking contacting bumps, below a critical frictional force that is proportional to the normal load and independent of the apparent contact area, consistent with the Amontons-Coulomb laws of friction. However, they fail to capture two widespread observations: (i) that the dynamic friction coefficient (ratio of frictional to normal force in steady sliding) is a roughly constant or slightly weakening function of the sliding velocity $U$, at low $U$, with a non-zero quasistatic limit as $U\to 0$, and (ii) that the static friction coefficient (ratio of frictional to normal force needed to initiate sliding) increases ("ages") as a function of the time that surfaces are pressed together in stationary contact, before sliding commences. To remedy these shortcomings, we incorporate a single additional model ingredient: that contacting bumps plastically nudge one another slightly sideways, above a critical contact-contact load. With this additional insight, the model also captures observations (i) and (ii).

5 pages, 5 figures

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• Interfacial charge density and its connection to adhesion and frictional forces
• How collective asperity detachments nucleate slip at frictional interfaces
• Transition from Static to Dynamic Friction in an Array of Frictional Disks
• Sliding friction in the hydrodynamic lubrication regime for a power-law fluid