Abstract
The anisotropy of nanoscale friction is analyzed for different samples using friction force microscopy under ultrahigh-vacuum conditions with varying scan directions. For both $\mathrm{Mo}{\mathrm{S}}_{2}$ and highly oriented pyrolytic graphite surfaces, our experiments reveal a sixfold symmetry of the friction vector plots in accordance with solutions of the Langevin equation, where the atomic force microscope tip is treated as a point mass elastically driven on the hexagonal surface lattices. In addition, the effect of temperature is analyzed both by experiment and by simulations. In both cases we find that increased temperature not only lowers the absolute friction values but also decreases the friction anisotropy. In contrast, experiments on NaCl cannot be explained on a similar basis. While the square symmetry is clearly revealed in the first measurements, it progressively deteriorates with repeated scanning, which is possibly due to abrasive wear occurring at the interface.
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