SLIPPING, SLIDING, AND SUPERLUBRICITY: Friction on the Atomic Scale
Joost Frenken
Kamerlingh Onnes Laboratory, Leiden University, The Netherlands
http://www.physics.leidenuniv.nl/sections/cm/ip

We have constructed a frictional force microscope (FFM) that is able to quantitatively track the forces between a sharp tip and a flat sample in three dimensions, with a friction force resolution as low as 15 pN, even under normal loads up to several tens of nN. Measurements performed with this new instrument for tungsten tips sliding on graphite surfaces, show a big surprise. When we rotate the graphite, the average friction force varies between a high and a near-zero value. We interpret this vanishing of the friction force as the first direct observation of "superlubricity": a special force cancellation phenomenon predicted for nano-scale contacts between stiff, incommensurate surfaces. This peculiar effect may also be the origin of the excellent lubricating properties of graphite in practical applications...