12 - Superlubricity between graphite surfaces

Abstract

This chapter presents measurements that demonstrate the effect of (in)commensurability on friction between two nanoscopic graphite sheets. In the framework of the Prandtl–Tomlinson model, friction between the tip of a friction force microscope (FFM) and the surface is the result of many rapid atomic slips from one minimum of the potential energy landscape to the next. However, when the amplitude of the potential energy surface is below a certain threshold or if a very stiff FFM cantilever is used, the tip can slide continuously over the surface, without the typical slip events that are accompanied by energy dissipation. The transition from a stick-slip motion with energy dissipation to a smooth, frictionless sliding occurs when the so-called Tomlinson parameter becomes lower than unity. The measurements performed with a unique FFM allows quantitative tracking of the forces on the scanning tip in three directions, with a high resolution in the lateral forces, down to 15 pN. The friction force images are recorded as a function of the normal force on highly oriented pyrolytic graphite using a chemically etched tungsten tip, which is glued in the force sensor.