Dr. Martin Dienwiebel†,
Namboodiri Pradeep,
Gertjan S. Verhoeven & Joost W.M. Frenken
Kamerlingh Onnes
Laboratory, Leiden University
†present
address: Institut für angewandte Verschleißforschung
(IAVF) AG, Karlsruhe, Germany
Many technical applications demand the use of dry lubricants rather than liquids (oils). Most dry lubricants consist of lamellar materials such as graphite or molybdenum disulphide (MoS2). For decades, the low-friction behavior of these lamellar solids has been ascribed to the low resistance to shear between neighboring atomic layers. However, this rather simple concept might not be the complete explanation of dry lubrication. With a newly developed Friction Force Microscope that allows the measurement of frictional forces with a very high resolution, we have found that the origin of the ultra-low friction of graphite lies in an effect proposed under the name of ‘superlubricity’ [1,2]. This phenomenon has been proposed to occur when two parallel surfaces slide over each other in incommensurate contact. In such geometry, the lattice mismatch can prevent collective, atomic-scale stick-slip motion of the two surfaces, and thus the kinetic friction force can be vanishingly small. Our experimental results clearly show the effect of commensurability between graphite surfaces (see figure) and are confirmed by model calculations. We speculate that this effect plays a significant role in the lubricating properties of solid lubricants.
Friction force versus rotation angle of the graphite surface around an axis normal to the sample. Two narrow peaks of high friction were observed at 0 and 61 degrees, respectively, consistent with the phenomenon of superlubricity
[1] M. Hirano, K. Shinjo, Phys. Rev. B 41, 11837 (1990).
[2] M. R. Sørensen, K.W. Jacobsen , P. Stoltze, Phys. Rev. B 53, 2101 (1996).
Interface
Physics Group
Joost Frenken:
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