Structural lubricity: Role of dimension and symmetry
نویسنده
چکیده
– When two chemically passivated solids are brought into contact, interfacial interactions between the solids compete with intrabulk elastic forces. The relative importance of these interactions, which are length-scale dependent, will be estimated using scaling arguments. If elastic interactions dominate on all length scales, solids will move as essentially rigid objects. This would imply superlow kinetic friction in UHV, provided wear was absent. The results of the scaling study depend on the symmetry of the surfaces and the dimensionalities of interface and solids. Some examples are discussed explicitly such as contacts between disordered threedimensional solids and linear bearings realized from multiwall carbon nanotubes. Introduction. – Many small-scale devices cannot be miniaturized further, because friction and wear appear to be exceedingly large in nanoscale machines. This limitation seems to arise, because the surface to volume ratio is large in small systems and thus surface forces such as friction become relatively large. However, many theoretical studies [1, 2, 3, 4, 5] and most recently an increasing number of experiments [6, 7, 8, 9, 10, 11] indicate that shear forces can be tremendously small between two atomically flat surfaces. These findings spur the hope for new avenues to reduce friction in nano-scale applications. Superlow friction was first suggested by Hirano and Shinjo [1, 2]. Their calculations indicated that no instabilities occur when two atomically smooth copper solids slide past each other provided that they are sufficiently misaligned. The absence of instabilities in such contacts implies that kinetic friction approaches zero at infinitely small velocities, even when thermal fluctuations are absent. Hirano and Shinjo called such superlow friction superlubricity. This term is often considered unfortunate, because one may expect zero friction in the sliding state in analogy to superconductivity and superfluidity. However, due to the emission of sound waves, which also occur when instabilities are absent, a drag force linear in velocity will persist. It might therefore be more appropriate to call the effect structural lubricity, as the low friction arises from the structural incompatibility of the two contacting solids rather than from a Bose Einstein condensate of bosons. The main reason for the low friction is that lateral forces between two non-matching, rigid solids cancel systematically for incommensurate interfaces and stochastically between disordered surfaces, so that the average force per unit area decreases quickly with increasing contact area.
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