Ultra-low friction and stiffness dependence of interlayer friction in graphite flakes under various rotation angles

Abstract

The 60-degree rotational symmetry of graphene layers leads to remarkable physical and mechanical phenomena. One corresponds to ultralow sliding friction and friction coefficient between incommensurate graphene layers. A pick-up method was proposed to pick graphite flake to microsphere probe and achieved graphite and graphite tribo-pair. Ultralow friction was attained for various rotation angles, and the friction coefficient was kept at the range of 10−3. Density functional theory (DFT) calculation was employed to reveal the angular dependence of friction, and the calculations are partially consistent with the experimental results. The difference of angle-dependence ultralow friction between experimental results and DFT calculations was observed, which was greatly influenced by the stiffness of the system. Friction hysteresis, abnormal shape of pull-off curves and long jumps in stick-slip were further demonstrated the stiffness of the system had changed during the friction measurements. The contact geometry in our experiment was a finite, symmetric, nanometer-sized (less than 1 nm), flexible graphite flake and a rigid graphite surface. The tribological system offers an experimental platform to study interlayer friction and contact geometry.