Conventional lateral force experiments give insufficient insight into the fundamental reasons for graphite’s outstanding qualities as a solid lubricant due to an averaging effect caused by the finite contact area of the tip with the sample. To overcome this limitation, we used a noncontact atomic force microscopy-based approach that enables use of atomically sharp tips. The new technique [1], performed using a home-built low temperature, ultrahigh vacuum atomic force microscope [2], allows the measurement of normal and lateral surface forces in a dense three-dimensional raster with picometer and piconewton resolution.

In this presentation, we analyze the height and lattice site dependence of lateral forces, their dependence on normal load, and the effect of tip shape in detail. The lateral forces are found to be heavily concentrated in the hollow sites of the graphite lattice, surrounded by a matrix of vanishingly small lateral forces. It will be argued that this astonishing localization may be a reason for graphite’s excellent lubrication properties. In addition, the distance and load dependence of the lateral forces experienced along possible “escape routes” from the hollow sites, which would be followed by a slider that is dragged out of them, are studied. Surprisingly, the maximum lateral forces along these escape routes, which ultimately determine the static friction, are found to depend linearly on normal load, suggesting the validity of Amontons' law in the noncontact regime.

[1] B. J. Albers et al., Nature Nanotechnology 4, 307 (2009).

[2] B. J. Albers et al., Rev. Sci Instrum. 79, 033704 (2008).