▎ 摘　　要

The friction properties of graphene at the microscale are important for its application as a solid lubricant in micro-/nano-electromechanical system components. In this work, the friction properties of graphene under the action of a 5-mu m-diameter microsphere probe are investigated using atomic force microscopy. A special phenomenon was found that the friction coefficient changed from positive to negative during the unloading process on different thicknesses of graphene. The applied normal load corresponding to the turning point of friction coefficient was not subject to the variation of the unloading range but increased with the increment of graphene thickness. Different stick-slip behaviors were observed, demonstrating the main reason for the above frictional characteristics is out-of-plane deformation. The anomalous friction between the contact interface of the microsphere probe and the graphene discussed broadens new horizons for the frictional properties of graphene.