Despite being one of the oldest phenomena known to mankind and its vast use, there still are open questions about the frictional process between two surfaces. At the nanometer scale for instance, the energy dissipations mechanisms, the influence of the crystalline orientation and the correlation between macroscopic and microscopic scales are still under debate.
Recently, development of 2D materials such as graphene has been widely studied due to its prominent properties and potential applications. Therefore, it is necessary to understand how such materials behave when in contact. Their tribological behavior at the nanoscale may present novel and unexpected features as compared to their bulk counterparts.
In this work we study the interaction between silicon tips and graphene by friction force microscopy. A giant anisotropy according to the crystallographic direction is observed, which is counterinuitive due to the isotropic behavior of its elastic properties. This behavior is associated with a nonlinear mechanism for energy disspation during scanning.
The velocity dependence of friction was also measured. The effective interaction potential between the tip and the surface was observed to vary linearly with the normal force, ranging from ~ 0.5 to ~0.6 eV. A critical velocity related to thermally activated dynamics of the contact atoms have also been determined to vary linearly with the applied load in the range of 1.5 to 2.0 µm/s.