Using an atomic force microscope, we have observed atomic-scale features on the frictional force acting on a tungsten wire tip sliding on the basal plane of a graphite surface at low loads, ${<10\mathrm{}}^{\mathrm{-}4}$ N. The atomic features have the periodicity of the graphite surface and are discussed in terms of a phenomenological model for the tip motion described by the sum of a periodic tip-surface force and the spring force exerted by the wire.

• Received 31 August 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.1942