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Philip Egberts1 Nicholas Chan1

1, University of Calgary, Calgary, Alberta, Canada

Comparison between experiments, analytical models, and atomistic simulations of thermally-activated single asperity friction have shown good overlap in their ability to describe the energetics of atomic stick-slip and minor differences in the force required to initiate slip. However, a poorer overlap is seen in the magnitude and frequency of thermal vibrations responsible for reducing friction with increasing temperature. This issue indicates that the number of atoms contributing to friction measured in experiments may not be accurately reproduced in simulations. Using conductive atomic force microscopy (AFM), we have performed friction measurements while simultaneously measuring the current through the sliding contact to observe the variation in contact area as the tip-sample junction is moved across the sample surface. More specifically, we have performed atomic stick-slip measurements of platinum and doped diamond AFM tips across highly oriented pyrolytic graphite in an ultra-high vacuum environment. Variations in the current measured through the contact show the same periodicity and structure as the underlying graphite lattice for each of the tips used. Through examination of current, a change in contact structure during the sticking and slipping segments, as characterized in the lateral force, were observed in the simultaneously recorded current signal. Furthermore, local anomalies in the lattice structure were observed in the measured current signal that were absent in the measured lateral force signal. These point towards small area defects that are typically hidden in friction experiments due to the averaging effect from a multi-atom contact.

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