Micromechanics of friction: effects of nanometre-scale roughness

Abstract

Nanometre-scale roughness on a solid surface has significant effects on friction, since intersurface forces operate predominantly within a nanometre-scale gap distance in frictional contact. To study the effects of nanometre-scale roughness, two novel atomic force microscope friction experiments were conducted, each using a gold surface sliding against a flat mica surface as the representative friction system. In one of the experiments, a pillar-shaped single nano-asperity of gold was used to measure the molecular-level frictional behaviour. The adhesive friction stress was measured to be 264 MPa and the molecular friction factor 0.0108 for a direct gold-mica contact. The nano-asperity was flattened in contact, although its hardness at this length scale is estimated to be 3.68 GPa. It was found that such a high pressure could be reached with the help of condensed water capillary forces. In the second experiment, a micrometre-scale asperity with nanometrescale roughness exhibited a single-asperity-like response of friction. However, the apparent frictional stress, 40.5 MPa, fell well below the Hurtado-Kim model prediction of 208-245 MPa. In addition, the multiple nano-asperities were flattened during the frictional process, exhibiting load-and slip-history-dependent frictional behaviour.

Effects of surface structure deformation on static friction at fractal interfaces

“…At a molecular scale, asperity interactions are inadequate to explain frictional forces and mechanisms of adhesion are more apt to describe resistance to shear (Li & Kim, 2008). Atomic frictional phenomena differ from 2, 52-58, http://dx.doi.org/10.1680/geolett.13.016 classical Amonton-Coulomb friction.…”

“…At the finest scales, frictional stress t f is described by a load-dependent component, governed by the coefficient of molecular scale friction a and a materialinterface-dependent adhesive shear stress t 0 (Li & Kim, 2008) t f~t0 zaP…”

Integration of contact size dependence and thermal activation in atomic friction

“…[1][2][3][4][5] In the past several decades, the development of friction force microscopy (FFM) has enabled the quantitative measurement of friction behavior for nanoscale asperities with the contact sizes lying below ~100 nm. The first noteworthy observation is the serrated force-displacement curves with stick-slip periodicity matching the substrate lattice constants.…”

Scale dependence of interface dislocation storage governing the frictional sliding of single asperities

“…Friction is an inherently multiscale problem [1,2], because of the geometric irregularities (i.e., roughness, asperities) that prevail over multiple decades of length scales [3][4][5], the plastic responses that depend on intrinsic and extrinsic length scales such as geometrically necessary dislocations or material microstructures [6][7][8][9], and a variety of time-dependent processes that render peculiar rate-dependent, collective responses of multi-asperities at micro-and macroscales [10]. Much of the above complexities can be removed if a smooth single asperity can be examined at nanoscale.…”