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Friction at the nanoscale For large objects sliding over one another, the friction force is proportional to the true contact area between the two bodies — which is smaller than the apparent contact area because the surfaces are rough, consisting of a large number of smaller features (asperities) that actually make the contact. The situation for nanomaterials, however, has been unclear, since the continuum contact theory that can account for macroscale effects has been predicted to break down at the nanoscale. Using large-scale molecular dynamics simulations of scanning force microscopy experiments, Yifei Mo et al . show that, despite this, simple friction laws do apply at the nanoscale: the friction force depends linearly on the number of atoms, rather than the number of asperities, that are chemically interacting across the sliding interfaces. For large objects sliding over one another, the friction force is proportional to the true contact area between the two bodies — this is smaller than the apparent contact area as the surfaces are rough, consisting of a large number of smaller features (asperities) that actually make contact. Here a related idea holds for contacts at the nanoscale: the friction force depends linearly on the number of atoms (rather than asperities) chemically interacting across the sliding interfaces. Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale 1 , they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance 2 , 3 . Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments 4 , 5 , 6 , 7 . We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories 8 , 9 , 10 of friction can be applied at the nanoscale.

Forces acting within the area of atomic contact between surfaces play a central role in friction and adhesion. Such forces are traditionally calculated using continuum contact mechanics 1 , which is known to break down as the contact radius approaches atomic dimensions. Yet contact mechanics is being applied at ever smaller lengths, driven by interest in shrinking devices to nanometre scales 2 , 3 , creating nanostructured materials with optimized mechanical properties 3 , 4 , and understanding the molecular origins of macroscopic friction and adhesion 5 , 6 . Here we use molecular simulations to test the limits of contact mechanics under ideal conditions. Our findings indicate that atomic discreteness within the bulk of the solids does not have a significant effect, but that the atomic-scale surface roughness that is always produced by discrete atoms leads to dramatic deviations from continuum theory. Contact areas and stresses may be changed by a factor of two, whereas friction and lateral contact stiffness change by an order of magnitude. These variations are likely to affect continuum predictions for many macroscopic rough surfaces, where studies 7 , 8 show that the total contact area is broken up into many separate regions with very small mean radius.

The Prandtl–Tomlinson (PT) model has been widely applied to interpret the atomic friction mechanism of a single asperity. In this study, we present an approximate explicit expression for the friction force in the one-dimensional PT model under quasi-static conditions. The ‘stick–slip’ friction curves are first approximated properly by sawtooth-like lines, where the critical points before and after the ‘slip’ motion are described analytically in terms of a dimensionless parameter η . Following this, the average friction force is expressed in a closed form that remains continuous and valid for η  > 1. Finally, an analytical expression for the load dependence of atomic friction of a single asperity is derived by connecting the parameter η with the normal load. With the parameters reported in experiments, our prediction shows good agreement with relevant experimental results.

Atomic force microscopes are used, besides their principal function as surface imaging tools, in the surface manipulation and measurement of interfacial properties. In particular, they can be modified to measure lateral friction forces that occur during the sliding of the tip against the underlying substrate. However, the shape, size, and deformation of the tips profoundly affect the measurements in a manner that is difficult to predict. In this work, we investigate the contribution of these effect to the magnitude of the lateral forces during sliding. The surface substrate is chosen to be a few-layer AB-stacked graphene surface, whereas the tip is initially constructed from face-centered cubic gold. In order to separate the effect of deformation from the shape, the rigid tips of three different shapes were considered first, namely, a cone, a pyramid and a hemisphere. The shape was seen to dictate all aspects of the interface during sliding, from temperature dependence to stick–slip behavior. Deformation was investigated next by comparing a rigid hemispherical tip to one of an identical shape and size but with all but the top three layers of atoms being free to move. The deformation, as also verified by an indentation analysis, occurs by means of the lower layers collapsing on the upper ones, thereby increasing the contact area. This collapse mitigates the friction force and decreases it with respect to the rigid tip for the same vertical distance. Finally, the size effect is studied by means of calculating the friction forces for a much larger hemispherical tip whose atoms are free to move. In this case, the deformation is found to be much smaller, but the stick–slip behavior is much more clearly seen.

This report describes an observation of alternating transitions between linear (Amontons) and non-linear friction-load behavior during Lateral Force Microscope experiments using a silicon tip sliding on a quartz surface. Initially, a transition from linear to non-linear behavior was attributed to nanoscale ‘running-in’ of the tip to form a single contact junction at the interface. Once this had occurred, a non-linear relationship between friction and applied load was observed during a number of loading and unloading cycles. For higher compressive loads, a further transition to a more linear friction-load behavior was attributed to nanoscale wear in the contact zone. Notably, when applied load was reduced below this ‘high-load’ transition point, the same non-linear friction-load behavior was again observed, but with a larger (friction per load) magnitude than seen previously. This cycle was repeated five times in these experiments, and each time, switching between non-linear and linear friction-load behavior occurred, along with a progressive increase in friction (per load) each time load was reduced below the transition point. The progressive increase in friction is attributed to an increased area of contact, caused by nanoscale wear at higher applied loads. An increase in tip size was confirmed by tip profiling before and after experiment. By progressively wearing the asperity at higher loads, the (interfacial or true) contact area, A, between the surfaces could be progressively increased, and as a result, a progressive increase in interfacial sliding friction, Ff, was obtained at lower loads (according to Ff = τA).

Friction force measurements were performed on 2-hydroxy stearic acid (2-HSA) and 12-hydroxy stearic acid (12-HSA) coated silica surfaces in air using an atomic force microscope. The 2-HSA displayed viscoelastic behaviour with a yield point as the static–dynamic friction transition. Steady sliding motion was replaced by microscopic stick–slip at lower velocities and higher loads. Stick–slip motion was successfully described and fitted to a phenomenological model ascribed to interfacial material melting and freezing in periodic cycles. The stick–slip periodicity is of the same order as the contact diameter. The 12-HSA did not experience a yield point and exhibited steady sliding over the entire load and velocity regime. We attribute these observations to the difference in molecular configuration, shear strength and adsorption density of the stearic acid layers.

This chapter presents a review of significant experimental and theoretical aspects of nanotribology. Innovative experimental techniques including surface force apparatus (SFA) and atomic force and friction force microscopes (AFM/FFM), and molecular dynamics (MD) computer simulations are used to study the interaction of materials ranging from atomic scales to microscales. First, the chapter talks about SFA experiments used to study adhesion and friction of a molecularly thick liquid film confined between two smooth surfaces. Next, the chapter describes two commercial AFM/FFMs commonly used for measurements of nanotribological and nanomechanical properties ranging from micro‐ to atomic scales. Finally, the chapter presents an overview of MD simulation modeling and selected results dealing with various friction, wear, and indentation studies.

This chapter presents a review of significant experimental and theoretical aspects of nanotribology. Innovative experimental techniques including surface force apparatus (SFA) and atomic force and friction force microscopes (AFM/FFM), and molecular dynamics (MD) computer simulations are used to study the interaction of materials ranging from atomic scales to microscales. First, the chapter talks about SFA experiments used to study adhesion and friction of a molecularly thick liquid film confined between two smooth surfaces. Next, the chapter describes two commercial AFM/FFMs commonly used for measurements of nanotribological and nanomechanical properties ranging from micro‐ to atomic scales. Finally, the chapter presents an overview of MD simulation modeling and selected results dealing with various friction, wear, and indentation studies.

There is a huge interest in developing superrepellent surfaces for antifouling and heat-transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be inferred indirectly using Furmidge’s relation. While easy to implement, contact angle measurements are semiquantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an atomic force microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micrometer-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning–depinning dynamics as the droplet detaches from or moves across the surface.

Room-temperature ionic liquids and their mixtures with organic solvents as lubricants open a route to control lubricity at the nanoscale via electrical polarization of the sliding surfaces. Electronanotribology is an emerging field that has a potential to realize in situ control of friction—that is, turning the friction on and off on demand. However, fulfilling its promise needs more research. Here we provide an overview of this emerging research area, from its birth to the current state, reviewing the main achievements in non-equilibrium molecular dynamics simulations and experiments using atomic force microscopes and surface force apparatus. We also present a discussion of the challenges that need to be solved for future applications of electrotunable friction. This Review discusses the development of electronanotribology, its intersection with room-temperature ionic liquids and how such collaboration can be used to electrically control friction at the nanoscale.