Atomic simulation of contact behavior during sliding inception of an asperity

Abstract

Advances in nanotribology prompt the understanding of lateral junction growth at the nanoscale an emerging issue. Our previous atomistic simulations have presented the atomic origins of the lateral junction growth mechanism at the nanoscale, which is mainly the slips of the atoms within the asperity. In the current study, we reveal that the lateral force increases with an increasing lateral displacement of the flat until the point at which junction growth ceases; corresponding to the maximum value of the tangential force coefficient. The transition of the lateral force profile from a smoothly increasing profile to a periodic sawtooth-like profile coincides with the point at which lateral junction growth ceases and stick-slip motion commences. However, the presence of an adsorbed layer on the nano-asperity surface suppresses the stick-slip motion and prompts a smooth sliding contact between the asperity and the flat.

Introduction

Amontons’ law holds over an extremely wide range of experimental conditions and is generally assumed to be applicable to all manner of contact surfaces. However, with the continuing miniaturization of many engineering and consumer products, the validity of the classical friction law at the nanoscale has emerged as a major concern. The breakdown of the continuum mechanics model in evaluating mechanical contacts at the nanoscale was evaluated by the molecular dynamics (MD) simulations results of Mo et al. [1], which is found to be supported by the experimental results of Schwarz et al. [2] and Carpick et al. [3]. These results [1], [2], [3] confirm the requirement for an accurate definition of the real contact area between two contacts under normal loading in order to extend the classical law of friction to the nanoscale regime.

Once two contact surfaces have relative motion, the sliding contact takes place. The real contact area is found to be changed with the change in a tangential load applied to the contact surfaces [4], [5]. McFarlane and Tabor [4] found that the contact area increases continuously with an increasing frictional force. Meanwhile, Parker and Hatch [5] showed that the application of a tangential load may prompt a growth of both the real and the apparent contact areas. Tabor [6] termed the phrase “junction growth” to describe the increase in contact area produced by an increasing tangential load and showed that for the case of a contaminated interface, the shear stresses induced by the applied tangential force are so small that little junction growth can occur before sliding takes place. In a later study, Johnson [7] showed that the tangential force causes a shift of the plastic zone in the direction of the force and causes a volume of material to be agglomerated at the leading shoulder of the wedge. Brizmer et al. [8] recently presented a theoretical description of the junction growth phenomenon and argued that the evolution of the contact area depends on the magnitude of the normal preload and is the result of points on the contact surface originally outside of the contact area coming into contact with the rigid flat during tangential loading. These theoretical predictions were later validated by the experimental results presented by Ovcharenko et al. [9]. These studies [4], [5], [6], [7], [8], [9] provide an excellent understanding of the junction growth phenomenon in macroscopic mechanical contacts. However, the evolution of the contact area at the nanoscale is still poorly understood.

Experimental investigations performed using a friction force microscope (FFM) have shown that nanoscale contacts exhibit an atomic stick-slip behavior [3], [10]. Thus, the lateral force acting on the contact has a sawtooth-like characteristic as the lateral displacement is increased. The MD simulation results presented by Li et al. [11] showed that the stick-slip phenomenon was the result of an elastic deformation of the surface layers of the contact pair. Socoliuc et al. [12] showed that when the stick-slip instabilities cease to exist, a new ultra-low friction regime is encountered. Mulliah et al. [13] presented MD simulation results which suggested that the stick-slip phenomenon is associated with the production of dislocations in the substrate below the indenter. Gerde and Marder [14] have found that the occurrence of the self-healing cracks at the atomic scale would result in solids that slip in accord with Coulomb's law of friction. Later, Marder [15] uncovered that the relationship between the movement of detachment front and speed of the surface wave, which was an important topic for advancing the nanomachines involving the sticking and rubbing phenomena.

In general, the specimens used in friction experiments [4], [5], [7] are exposed to air, and thus the effects of adsorbed layers should be taken into account when investigating the junction growth phenomenon. Previous studies have shown that the friction coefficient between two contact surfaces is largely governed by the shear strength of the adsorbed layers [16], [17]. According to Bowden and Young [16], the existence of an adsorbed film leads to a significant reduction in both the number of metallic junctions and the friction coefficient.

Based upon the findings presented in the literature for the nano-asperity contact area [1], the lateral junction growth [4], [5], [6], [7], [8], [9], and the stick-slip behavior [10], [11], [12], [13], and recognizing the significance of adsorbed layers in determining the behavior of interfacial contacts [18], Jeng and Peng [19] have performed an atomistic simulation to provide the underpinning of the lateral junction growth mechanism of a nano-asperity in contact with a rigid flat at the nanoscale with/without the adsorbed layer. In this study, we further investigated the effects of an adsorbed layer on the normal force fluctuation, the lateral force fluctuation, evolution of the real contact area and the onset of stick-slip motion.