Atomic-scale friction of a tungsten tip on a graphite surface

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^-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.