Surface Science 500 (2002) 741–758 www.elsevier.com/locate/susc Surface science and the atomic-scale origins of friction: what once was old is new again Jacqueline Krim * Department of Physics, Box 8202, 104 Cox Hall––127 Stinson Drive, North Carolina State University, Raleigh, NC 27695-8202, USA Received 4 August 2000; accepted for publication 12 April 2001 Abstract Long neglected by physicists, the study of friction’s atomic-level origins, or nanotribology, indicates that sliding friction stems from various unexpected sources, including sound energy, and static friction may arise from physisorbed molecules. Progress in this field will be discussed, with an emphasis on how the field of surface science has influenced our understanding of friction. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Friction; Physical adsorption; Phonons; Energy dissipation 1. Introduction techniques should be able to detect. As I thought about Tabor’s research suggestions, I pondered The fax from David Tabor came promptly, in whether forty years hence, at an age exceeding response to my query about whether he and col- eighty, I too might still be investigating friction, league Ken Johnson might be able to contribute and how fine it was that Tabor lived to participate to a special issue on ‘‘Fundamentals of Friction’’, in the current day renaissance in the topic of his which I was guest editing for the Bulletin of the life’s work. Materials Research Society [1]. Tabor, whose re- By most recent estimates, improved attention to nowned work on friction spanned well over forty friction and wear would save developed countries years, unfortunately was not going to be able to upto 1.6% of their gross national product, or over complete a manuscript in the time frame allowed. $100 billion annually in the US alone [2]. The But his letter did not end with that. He went on to magnitude of the financial loss associated with describe how he had been thinking about my ex- friction and wear arises from the fact that entire periments and also about how lattice commensu- mechanical systems, be they coffee makers or au- rability effects should impact sliding friction levels. tomobiles, are frequently scrapped whenever only Solid–solid phase transitions, for example, should a few of their parts are badly worn. In the case of produce changes in friction that my experimental an automobile the energy consumed in its manu- facture is equivalent to that consumed in 100,000 miles of operation [3]. More extreme examples * Tel.: +1-9195132684; fax: +1-9195154496. include aircraft, which can be entirely destroyed E-mail address: [email protected] (J. Krim). for loss of one part. The consequences of friction 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S0039-6028(01)01529-1 742 J. Krim / Surface Science 500 (2002) 741–758 and wear also have great impact on national se- tems to be increasingly comparable to experiment curity and quality of life issues, so it can come as in a very direct fashion [7]. no surprise that tribomaterials, materials designed The increasing overlapof the previously distinct for use in moving contact (sliding, rolling, abra- areas of surface science and tribology has given sive, etc.), have for decades attracted the interest of rise to increased optimism that major break- materials scientists and mechanical and chemical throughs will be achieved in upcoming decades. engineers. What is surprising however, is how little Issues of particular importance include: (1) Un- is known even now about the fundamental origins derstanding the chemical and tribochemical reac- of friction and wear. tions which occur in a sliding contact, and the As important as tribomaterials are to techno- energy dissipation mechanisms associated with logy, their discovery has usually been serendipitous. such a contact. To fully characterize a system’s To be sure, materials scientists have frequently behavior, one must not only estimate friction been able to provide explanations for why tribo- levels but also account for the effects of the heat materials perform as well as they do [4,5], and that the friction generates. Are chemical reactions have also been able to substantially improve the triggered? Do the contact points melt, etc.? (2) performance of tribomaterials through the devel- Characterization of the microstructural and me- opment of new alloys, composites and/or im- chanical properties of the contact regions between proved surface engineering methods. They have, the sliding materials. (3) Merging and coordinat- however, been far less successful at a priori design ing information gained on the atomic-scale with of tribomaterials with improved performance, that observed at the macroscopic scale. Much of largely because friction and wear processes have the current information is fragmented, with link- not been understood at the molecular level. Why ages between individual experimental results yet to is so little known on the topic? The answer lies have been established. (4) Development of realistic primarily in the fact that friction and wear are interaction potentials for computer simulations of surface and interfacial phenomena which occur at materials of interest to tribological applications, a myriad of buried contacts which not only are and (5) Development of realistic laboratory test extremely difficult to characterize, but are contin- set-ups which are both well-controlled and rele- uously evolving as the microscopic irregularities vant to operating machinery. of the sliding surfaces touch and push into one another. The late 1980’s marked the advent of a renewed 2. Working outside of a vacuum: tribology before interest in fundamental areas of tribology, sparked 1970 by a number of new experimental and theoretical techniques capable of studying the force of friction The view of the Greek philosophers that vac- in geometries which were well defined even at na- uum was an impossibility hampered understanding nometer length scales [6]. These techniques bene- of its basic principles until the mid-17th century fited directly from advances in surface science (Fig. 1), greatly delaying progress in all vacuum- throughout the 1970’s, whereby improvements in related fields [8]. The field of tribology by com- ultra-high vacuum technology allowed scientists parison dates well back to the construction of the to prepare unprecedented, well-characterized crys- Egyptian pyramids, if not hundreds of thousands talline surfaces. Experimental techniques such as of years earlier to the discovery of the use of flint the quartz crystal microbalance (QCM), the sur- stone for the sparking of fires [9]. Indeed, such face forces apparatus and the lateral force micro- tribological advances as Leonardo da Vinci’s de- scope, could now record friction in geometries sign of intricate gears and bearings [9] (some of involving a single contacting interface, a vastly which were not built until the industrial revolution simpler situation than that of contact between provided sufficiently strong materials), and the macroscopic objects. Faster computers meanwhile landmark 18th century development of a timepiece allowed large-scale simulations of condensed sys- allowing accurate longitudinal positioning of ships J. Krim / Surface Science 500 (2002) 741–758 743 Fig. 1. A highly publicized demonstration by Otto Van Guericke, describing his 1672 invention of a vacuum pump (from Ref. [8]). The ability to produce vacuum in laboratory conditions would centuries later give rise to the discovery of the electron. This revolutionary advance in physics gave rise to the field of modern surface physics. Electrons travelling through ultra-high vacuum conditions and high potential differences were first demonstrated to be highly sensitive probes of surfaces in 1967 (Fig. 7), and provided the vast majority of information on surface structure and chemical composition up until the invention of the scanning probe microscope in the 1980’s. at sea (accomplished via a self-lubricating wooden is independent of velocity for ordinary sliding gear) [10] could easily be termed ‘‘modern’’, given speeds. Although the coefficient of friction is in- the overall longevity of the field. Modern study of dependent of the apparent contact area, loading friction began perhaps 500 years ago, when Leo- force, and the sliding speed, it does in fact depend nardo da Vinci deduced the laws governing the on whether the force is applied to an object at rest motion of a rectangular block sliding over a planar (‘‘static friction’’) or already moving (‘‘sliding fric- surface (Fig. 2). Da Vinci’s work had no historical tion’’). Considering its simplicity, Amontons’ law influence, however, since his notebooks remained is amazingly well obeyed for a wide range of ma- unpublished for hundreds of years. The French terials such as wood, ceramics, and metals. physicist Guillaume Amontons is credited with the Amontons’ and Coulomb’s classical friction first published account of the classic friction laws, laws have far outlived a variety of attempts to which in 1699 described his observations of con- explain them on a fundamental basis. Surface tacting solid surfaces [11]. roughness, which Coulomb unsuccessfully at- Amontons observed that (1) the friction force tempted to attribute friction to (Fig. 3), was ruled that resists sliding at an interface is proportional out definitively as a possible mechanism for most to the ‘‘normal load’’, or force which presses the friction in the mid 1950’s. Molecular adhesion, surfaces together, where the ‘‘coefficient of fric- though, remained a strong possibility, a conclusion tion’’ is defined as the ratio of the frictional force due in large part to Bowden, Tabor and coworkers to the load, and (2) the friction force is indepen- at Cambridge University, England. Their group dent of the apparent area of contact: A small block found that friction, although independent of ap- experiences as much friction as does a large block parent macroscopic contact area, is in fact pro- of the same material so long as their weights are portional to the true contact area.