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We describe the results of atomic-level stick-slip friction measurementsperformed on chemically-modified graphite, using atomic force microscopy (AFM).Through detailed molecular dynamics simulations, coarse-grained simulations,and theoretical arguments, we report on complex indentation profiles during AFMscans involving local reversible exfoliation of the top layer of graphene fromthe underlying graphite sample and its effect on the measured friction forceduring retraction of the scanning tip. In particular, we report nearly constantlateral stick-slip magnitudes at decreasing loads, which cannot be explainedwithin the standard framework based on continuum mechanics models for thecontact area. We explain this anomalous behavior by introducing the effect oflocal compliance of the topmost graphene layer, which varies when interactionwith the AFM tip is enhanced. Such behavior is not observed for non-lamellarmaterials. We extend our discussion toward the more general understanding ofthe effects of the top layer relaxation on the friction force under pushing andpulling loads. Our results may provide a more comprehensive understanding ofthe effectively negative coefficient of friction recently observed onchemically-modified graphite.

Effects of surface compliance and relaxation on the frictional properties of lamellar materials