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

The scan-directional dependence of the superlubricity of a C-60 molecular bearing system (graphite/C-60/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 [10 (1) over bar0] direction. For other regions, (F-L) has a nearly constant value of less than 1 pN. In particular, in the [12 (3) over bar0] direction, (F-L) reaches a minimum of nearly zero. It is clarified that (F-L) reflects the following types of C-60 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. (C) 2009 The Japan Society of Applied Physics