QCM STUDIES OF PHONONIC AND ELECTRONIC CONTRIBUTIONS TO SLIDING FRICTION IN ADSORBED MONOLAYERS Jacqueline Krim Department of Physics, Box 8202 North Carolina State University Raleigh, NC 27695-8202 ABSTRACT Studies of the fundamental origins of friction have undergone rapid progress in recent years with the development of new experimental and computational techniques for measuring and simulating friction at atomic length and time scales. The increased interest has sparked a variety of discussions and debates concerning the nature of the atomic-scale mechanisms, which dominate the dissipative process by which mechanical energy is transformed into heat. Measurements of the sliding friction of physisorbed monolayers and bilayers provides information on the relative contributions of electronic to phononic dissipative mechanisms, since phonon dissipation is present at all film coverages, while electronic dissipation primarily impacts the first monolayer. These systems exhibit virtually no static friction and in fact the sliding friction can be up to a million times less than for macroscopic lubricants such as graphite. Exceptionally low phononic friction, as is typical for incommensurate interfaces, has been referred to as superlubricity by some. This is unfortunate, in this author's opinion, since the resistance to sliding does not drop strictly to zero. 1. INTRODUCTION One of the simplest geometries in which friction can occur, and thus be studied, is that of a molecularly thin adsorbed layer sliding along an ideal, atomically flat surface. A first principles knowledge of sliding friction in this system is key to a vast range of fundamental and applied areas in physics, spanning the molecular origins of static friction[1] to the design of atomic-scale automobiles [2]. Sliding adsorbed monolayers are moreover accessible to experimental methods by means of the Quartz Crystal Microbalance (QCM) technique[3] and to theoretical treatments including both analytic calculations and molecular- dynamics simulations.[ 4] 2. QUARTZ CRYSTAL MICROBALANCE EXPERIMENTS & RESULTS The Quartz Crystal Microbalance QCM is an instrument that operates on a time scale short enough to detect phonons, whose lifetimes in the best of cases are no longer than a few tens of nanoseconds. The basic component of a QCM is a single crystal of quartz that has very little internal dissipation and oscillates at an extremely stable resonance frequency. The oscillations are excited by applying a voltage to thin metal electrodes on the surfaces of the QCM that are prepared so as to exhibit a crystalline texture, generally (111) in nature. Atomically thin films of a different material are then adsorbed onto the electrodes. The extra mass of the adsorbed layer lowers the resonance frequency of the microbalance, and the resonance is broadened by any frictional dissipation due to sliding of the adsorbed layer and the microbalance. The friction can be measured only if it is sufficiently low so as to result in significant sliding, which is accompanied by a measurable broadening of the resonance. For this reason, QCM measurements of sliding friction tend to be carried out on systems exhibiting very low friction, such as incommensurate rare-gas solids adsorbed on noble metals. For the vast majority of other systems which exhibit higher friction (chemically bonded layers, etc.) the slippage of an adsorbed monolayer on the surface of the QCM is too small to produce a measurable broadening. In this case interfacial slippage and/or bond breaking can be detected by performing measurements on substantially thicker films, where the larger inertial masses can more readily overcome the stronger frictional forces. [5] The first measurements of sliding friction in adsorbed monolayers were reported in 1991 and 1996, for the systems Kr/Au(111) [6] and Xe/Ag(111) [7] respectively. The experiments generated much theoretical interest, and a variety of theoretical approaches were employed in order to understand the fundamental energy dissipation processes that gave rise to the reported friction levels. One of the surprising results revealed by the experiments was the fact that both solid and liquid monolayers of the adsorbed films were well-described by the (static- friction free) “viscous friction” law F = -(m/τ) v. In this relation F is the friction force, m is the mass of the adsorbed film and v is the average film sliding speed. The slip time τ , which is inversely proportional to the shear stress (the amount of force per unit area of true contact needed to maintain sliding), is a characteristic time for friction to decrease the sliding speed v. Proceedings of WTC2005 World Tribology Congress III September 12-16, 2005, Washington, D.C., USA WTC2005-64097 1 Copyright © 2005 by ASME