The fundamentals of friction
Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge December 2010
Leonardo da Vinci’s heirs explore the mechanisms behind friction and wear.
As tribologists, the challenges we work on have a worldwide impact, including developing lower-friction bearings to reduce energy losses, lower-wear mechanical systems to reduce maintenance costs and new lubricant additives to protect the environment.
Whether you are a scientist, engineer, bearing designer or molecular dynamicist, there is one thing to remember—fundamental research is the life-blood of our field.
Strangely enough, this is not a view that is universally shared by government agencies, and academia often has an extraordinarily hard time supporting fundamental research activities. As often as not, fundamental research is piggybacked onto industrially supported, problem-solving projects and, therefore, not always given the highest priority.
In Europe there is a glimmer of hope that this situation may not be inevitable. On the continent where Leonardo da Vinci first observed the friction-load proportionality (later rediscovered and codified by Amontons), the European Science Foundation (ESF) has formed a collaborative initiative known as Friction and Adhesion in Nanomechanical Systems (Fanas). In this context, the ESF sponsored a conference titled “Trends in Nanotribology” at the International Center for Theoretical Physics in Trieste, Italy. Selected papers from the meeting have recently appeared in a special issue of Tribology Letters.
New tribological phenomena are the focus of research in the fanas program. Topics include superlubricity, wearless sliding, control of frictional properties, bridging the gap between the nano-, micro- and macroscales, manipulation of nanoparticles on surfaces and aqueous lubrication. These are being addressed in collaborative projects taking place in many European countries and Israel, some with the involvement of researchers from the United States.
Highlights of the Trieste meeting included experimental and modeling studies addressing atomic-level understanding of both friction and wear. Italy Barel and Michael Urbakh from Tel Aviv University in Israel, in collaboration with Lars Jansen and André Schirmeisen at the University of Münster, Germany, contributed a paper investigating the temperature dependence of friction.
The focus of the study involved the thermally activated detachment and reattachment of multiple contacts during sliding, which leads to a complex energy landscape but one in which higher temperatures might be expected to lead to the surmounting of energy barriers, the breaking of contacts and, thus, more facile slip and lower friction.
Working against this trend is a thermally activated reattachment mechanism, which leads to higher friction at higher temperatures. The result is a peak in dry sliding friction at low temperatures (around 100 K), which the authors have demonstrated experimentally for a variety of different materials (silicon, silicon carbide, sodium chloride, graphite) and also modeled as a set of multiple contacts with different activation energies for formation and rupture.
The model was able to predict the experimental behavior with impressive accuracy using very few fitting parameters, thus initiating a new approach to understanding the dynamics of nanoscale dry sliding.
Thermal activation is also the central issue in a paper by Bernd Gotsmann and Mark Lantz from IBM Research in Zurich, Switzerland and Tevis Jacobs and Robert Carpick from the University of Pennsylvania in Philadelphia. In this case, atomic-scale wear is the focus of the investigation, and the authors ponder the extent to which transition-state theory—the chemist’s standard quantitative approach to understanding the temperature dependence of chemical reactions—could be applied to this phenomenon. Their answer was that the approach can, indeed, explain atomic-scale wear phenomena, although caution is required in applying this model.
Measurements of temperature-dependent wear in the atomic force microscope, which are quite scarce at present, are needed to identify relevant activated transitions and to disentangle thermally activated and velocity-dependent effects on wear.
The collection of papers in Tribology Letters is merely a reflection of some of the research taking place within the Fanas program, which involves further workshops, exchanges and training courses over the next year. Information can be found at www.esf.org/fanas.
FOR FURTHER READING:
Gulseren, O., Manini, N., Meyer, E., Tosatti, E., Urbakh, M. and Vanossi, A. (2010), “New Trends in Nanotribology,” Tribology Letters, 39 (3), p. 227.
Barel, I., Urbakh, M., Jansen L. and Schirmeisen, A. (2010), “Temperature Dependence of Friction at the Nanoscale: When the Unexpected Turns Normal,” Tribology Letters, 39 (3), pp. 311-319.
Jacobs, T.D.B., Gotsmann, B., Lantz, M.A. and Carpick, R.W. (2010), “On the Application of Transition State Theory to Atomic-Scale Wear,” Tribology Letters, 39 (3), pp. 257-271.
Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.
Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.