Simulation of Scan-Directional Dependence of Superlubricity of C₆₀ Molecular Bearings and Graphite

The scan-directional dependence of the superlubricity of a C₆₀ molecular bearing system (graphite/C₆₀/graphite interface) is studied and compared with that of a graphite system (graphite/graphite/graphite interface) by molecular mechanics simulation. The mean lateral force <F_L > reaches a maximum within a narrow region approximately in the [1010] direction. For other regions, <F_L > has a nearly constant value of less than 1 pN. In particular, in the [1230] direction, <F_L > reaches a minimum of nearly zero. It is clarified that <F_L > reflects the following types of C₆₀ motion: sliding above the carbon bond and a discrete slip to the neighboring AB-stacking position. The load dependence of <F_L > also exhibits marked anisotropy. The orders of magnitude of the simulated friction coefficients are comparable to those obtained in our previous experiments.