Atomic-scale and microscale friction studies of graphite and diamond using friction force microscopy

Abstract

Friction between graphite and diamond surfaces against a sharp silicon nitride tip was measured using a friction force microscope (FFM). Atomic-scale friction images of a freshly cleaved highly oriented pyrolytic graphite exhibited the same periodicity as that of the graphite surface; however, the peaks in friction profiles and those in corresponding topography profiles were displaced relative to each other. Using the Fourier expansion of the interaction potential, the conservative interatomic forces between the FFM tip and the graphite surface have been calculated. It is shown that the variations in atomic-scale friction and the observed displacement between the peaks in the frictional (or lateral) force and those in the corresponding topography can be explained by the variations in interatomic forces in the normal and lateral directions. Thus, the observed variation in friction force may not necessarily occur as a result of the commonly believed atomic-scale stick-slip process, but can be due to variation in the intrinsic lateral force between the sample and the FFM tip. At large scan sizes (50×50 nm2 or larger), the variation of friction for graphite and a single-crystal (IIa) diamond was found to correlate with the variations in the local slope of the sample surface, suggesting that a ratchet mechanism is responsible for variations in microscale friction.