AVS 56th International Symposium & Exhibition
    Tribology Focus Topic Wednesday Sessions
       Session TR+NS-WeM

Paper TR+NS-WeM1
Nonlinear Contact Area Dependence of Sliding Friction for Metallic Nanoparticles

Wednesday, November 11, 2009, 8:00 am, Room C4

Session: Nanomechanics and Nanotribology
Presenter: D. Dietzel, University of Münster, Germany
Authors: D. Dietzel, University of Münster, Germany
T. Moenninghoff, University of Münster, Germany
A. Schirmeisen, University of Münster, Germany
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The existence of superlow friction under appropriate interface conditions is one of the most intriguing concepts in nanotribology. If an interface between two incommensurate surfaces is atomically clean, a state of virtually frictionless sliding is anticipated, often referred to as ‘superlubricity’ or ‘structural lubricity’. But although superlubricity is a widely accepted theoretical concept, an unambiguous and direct verification has been difficult in the past. Theory predicts that the lattice mismatch at the interface causes a decrease of the potential barrier between stable states with increasing contact size that ultimately leads to vanishing friction. Therefore, analyzing the contact area dependence of superlubric friction might be a straightforward approach to confirm the frictional conditions.

Unfortunately, conventional friction force microscopy (FFM) has limitations inherent to the experimental configuration when it comes to contact area dependent measurements: Apart from the rather limited variety of material combinations, the fixed tip radius makes it especially difficult to analyze effects as a function of the contact area. In order to analyze the contact area dependence of interfacial friction, the friction between two objects in relative motion with a well-defined contact area should be measured instead of the friction between tip and surface. Therefore we have manipulated nanometer scale metallic particles on atomically flat surfaces by contact mode atomic force microscopy techniques and quantitative information on interfacial friction has been extracted from the lateral manipulation of these nanoparticles1. In previous experiments2 we found two distinct frictional states during particle sliding of Sb-particles on HOPG substrate: Some particles show finite friction increasing linearly with interface area, thus reinforcing Amonton’s law at the nanoscale, other particles assume a state of frictionless or ‘superlubric’ sliding.

In this contribution we show new measurements which were concentrated on the particles exhibiting vanishing friction. By optimizing our experimental sensitivity we succeeded for the first time to analyze the contact area dependence of friction force of these low friction particles. In contrast to the ‘Amonton’-like particles, interfacial friction of the low friction particles showed strongly nonlinear contact area dependence. The experimental results are compared to theoretical considerations, which predict that the shear stress of sliding superlubric particles should decrease with increasing particle size.

1Dietzel et al., J. Appl. Phys.102, 084306 (2007).

2Dietzel et al., Phys. Rev. Lett. 101, 125505 (2008).