Nanotribology: friction at the atomic level

Friction between two sliding surfaces is probably one of the oldest problems in physics. For example, there is evidence that already the ancient Egyptians used some sort of lubricants to move the large blocks used in the construction of the pyramids [1]. This has certainly contributed to the common view of friction as a phenomenon very important from a practical point of view but, nowadays, scientifically dull. It is usually briefly mentioned at the beginning of any introductory physics classes where it is described in terms of simple empirical laws which are said to be the results of many effects caused by the complexity of the two real surfaces in contact. The implicit message is that the sliding friction of an ideal system comprised by a very tiny particle lying on top of a flat, uncontaminated surface kept in vacuum conditions will be trivial. Yet, investigations about the origins of friction are still scarce and controversial [2]. For instance, if one asks whether friction depends on the conducting properties of the surface, there is no unanimous answer. In principle, if the substrate is metallic there are two different processes through which part of the kinetic energy of an object moving on a metal can be transferred to the internal degrees of freedom of the substrate and therefore transformed into heat: excitation of lattice vibrations and induction of electronic currents at the interface [3]. Despite recent advances [4], in experimental probes, the relative importance of these two contributions is still far from being settled even for the idealized system of a nanoscale object sliding over a well characterized surface. To quantify this ratio experimentally has proven difficult because the phononic and electronic dissipation channels are both generally active.

Arguably, the most direct way to determine the importance of electronic friction is to work across the superconducting phase transition. When the substrate becomes a superconductor, the electronic mechanism is frozen out while that phononic is essentially unaffected. Krim and coworkers have studied with a quartz microbalance technique (QCM) the friction between a lead substrate and an adsorbed film of solid nitrogen a few layers thick and found a sharp drop in friction by a factor ∼2 when lead became superconducting [5]. This work triggered significant debate because the observed behavior did not show the predicted temperature dependence around the critical temperature Tc [6].Furthermore, the same system was studied in a different QCM experiment [7], with improved cryogenics, but complete pinning of the nitrogen film to the lead substrate was reported in the temperature range 4-14 K.

In order to provide some firm data on this topic, we decided to repeat these experiments in much more controlled conditions. Starting with the Advanced Research Project Nanorub funded by INFM in 2000, followed by the funding of other local and national projects, we have assembled a completely new set-up which allows QCM measurements in ultra-high vacuum and at temperatures as low as 5 K [8].It also fixes some of the drawbacks of precedent experiments like i) the film coverage was not measured but only estimated; ii) the superconducting state of lead during the thermal scans was not directly probed; iii) the measurements were only taken during thermal warm-up cycles after cooling down the sample cell to 4.2 K.

So far, our attention has been mainly devoted to the study of the nanofriction of Neon monolayers deposited on a bare Lead surface across its critical temperature because Ne, unlike heavier adsorbates like Nitrogen, Argon, Krypton, Xenon…, is found to reproducibly slide at such low temperatures. References[9-13] contain the principal results obtained till now. Currently, we are studying other fundamental issues particularly relevant for the atomic superlubricity.

1. D.Dowson, History of Tribology, (Longman, NY, 1979).
2. M. Kisiel, E. Gnecco, U. Gysin, L. Marot, S. Rast and E. Meyer, Nature Mater. 10, 119 (2011).
3. See e.g., “Physics of Sliding Friction”, ed. by B. N. J. Persson and E. Tosatti, eds., (Kluwer, Dordrecht, 1996); B.N.J. Persson, Sliding Friction (Springer, Berlin, 1998).
4. "Nanotribology: Friction and Wear on the atomic scale", ed. by E. Meyer and E. Gnecco, (Springer, New York, 2007).
5. A. Dayo, W. Alnasrallah and J. Krim, Phys. Rev. Lett. 80,1690 (1998).
6. B.N.J. Persson and E. Tosatti, Surf. Sci. 411, L855 (1998); B.N.J. Persson, Solid State Comm. 115,145 (2000).
7. Renner, J. Rutledge and P. Taborek, Phys. Rev. Lett. 83,1261 (1999).
8. L. Bruschi, A. Carlin, F. Buatier de Mongeot, F. dalla Longa, L. Stringher and G. Mistura, “UHV apparatus for QCM measurements in the temperature range 4-400K”, Rev. Sci. Instrum. 76, 023904 (2005).
9. L. Bruschi, G. Fois, A. Pontarollo, G. Mistura, B. Torre, F. Buatier de Mongeot, C. Boragno, R. Buzio and U. Valbusa, “Structural Depinning of Ne Monolayers on Pb at T < 6.5 K”, Phys. Rev. Lett. 96, 216101 (2006).
10. G. Fois, L. Bruschi, L. d'Apolito, G. Mistura, B. Torre, F. B. de Mongeot, C. Boragno, R. Buzio and U. Valbusa, “Low-temperature static friction of N-2 monolayers on Pb(111)”, J. Phys.: Condens. Matter 19, 305013 (2007).
11. L. Bruschi, M. Pierno, G. Fois, G. Mistura, C. Boragno, F. Buatier de Mongeot, and U. Valbusa, “Friction reduction of Ne monolayers on preplated metal surfaces”, Phys. Rev. B. 81, 115419 (2010).
12. M. Pierno, L. Bruschi, G. Fois, G. Mistura, C. Boragno, F. Buatier de Mongeot, and U. Valbusa, “Nanofriction of Neon Films on Superconducting Lead”, Phys. Rev. Lett. 105, 016102 (2010).
13. M. Pierno, L. Bruschi, G. Mistura, C. Boragno, F. Buatier de Mongeot, U. Valbusa, and C. Martella, “Nanofriction of adsorbed monolayers on superconducting lead”, Phys. Rev. B. 84, 035448 (2011).
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