Frictional anisotropy of crystalline surfaces and its relationship with surface structure studied through atomic force microscopy

Friction forces play a key role in mechanical phenomena occurring on all scales, from the operation of microelectromechanical systems to interplate earthquakes. The study of friction forces is also useful for understanding a large spectrum of physical properties of surfaces. Thanks to the development of nanoscale investigation tools such as scanning probe microscopy, the investigation of friction and wear phenomena down to the atomic scale is becoming a leading topic within the field of surface physics, and has given rise to the science called nanotribology. We focus here on the anisotropic aspects of nanotribology, related in particular to the surface of crystalline materials. Frictional anisotropy is characterized by a dependence of the friction force intensity on the sliding direction and friction force components orthogonal to the sliding direction. We show how an atomic force microscope can be used to map in two dimensions the frictional anisotropy at the nanoscale and how to carry out data interpretation to unravel the friction– surface structure relationship. As model systems, we analyzed the basal plane of organic molecular single crystals, often characterized by marked corrugations, and interpreted the data in terms of constitutive models of anisotropic friction.