Contact ageing in atomic friction

The stick - slip motion of an atomic force microscope cantilever on highly oriented pyrolytic graphite is investigated experimentally and theoretically. It is shown that the interstitial slips of the cantilever tip at high pulling velocities are thermally activated events and can be described with a single- step rate equation. On the other hand, at slow pulling, the statistics of the slip events deviates from the predictions of the single- step rate approach, signaling the onset of contact ageing, that is, gradual changes of the contact properties in each stick phase. A model is introduced which takes the contact ageing effect into account, and whose predictions are in good agreement with the experimental results for all pulling velocities.