Dominance of Phonon Friction for a Xenon Film on a Silver (111) Surface

Abstract

Molecular dynamic simulations for a Xe film sliding on an Ag(111) substrate are performed from the submonolayer through the bilayer regime, which, when compared to both friction and surface resistivity measurements, demonstrate that the friction in this system is dominated by phonon excitations. Slip times are found both by direct calculation of the decay of the center-of-mass velocity, as well as from the decay of the velocity correlation function. Agreement of the slip times from the two methods supports the occurrence of a friction force linear in velocity over a wide velocity range.[S0031-9007 (97)04756-X] PACS numbers: 46.30.PaAlthough tribology, the study of friction and wear, has been of technological interest since ancient times [1,2], the topic continues to rouse interest today [3][4][5][6][7][8][9][10][11][12]. Rapid progress in experimental, theoretical, and computational methods provides new insights into the atomic origins of frictional energy dissipation. When a thin film slides on a metal substrate there exists dissipation of energy via two mechanisms: (i) electronic excitations in the metallic substrate [4,9], and (ii) phonon excitations in the film or in the substrate [10]. The dissipation of energy can be characterized by the slip time t (i.e., the time it takes for the film's speed to fall to 1͞e of its original value, assuming it is stopped by friction) or equivalently by a damping coefficient h ϳ 1͞t.In this Letter we study the phonon contribution to friction for Xe films sliding along an Ag(111) substrate using molecular dynamics simulations. It is of great interest to determine the relative contributions of the phonons and electrons to friction, since to date it is not clear which is dominant. To this end, we compare our results with the slip time versus coverage data reported by Daly and Krim [6], and with the electrical resistivity versus coverage data of Dayo and Krim [7]. The results of this comparison provide convincing evidence that phonon excitations make a dominant contribution to the friction for this system.We determine the slip time as a function of coverage, defined as the number of atoms in the film per unit area. We treat a range of film coverages, from submonolayer to bilayer. The slip time is determined by two methods. In the first method, an initial center-of-mass velocity V 0 is produced by an external force exerted on the film for t , 0. The external force is turned off for t . 0, and t is determined by the resulting velocity decay V 0 e 2t͞t . In the second method, no external force is applied. The slip time is determined by the behavior of the thermal equilibrium autocorrelation for the center-of-mass velocity as a function of time. The autocorrelation function is related to the linear response of the system to a small perturbation by the fluctuation-dissipation theorem [12,13], which holds in the limit of zero applied force and, hence, zero velocity. This method is the only one which is valid at the 1 cm͞s velocities appropriate for experiments using the quartz crystal...

Nanoscale sliding friction versus commensuration ratio: Molecular dynamics simulations

“…8 Despite the fact that they used similar models and similar simulation techniques to calculate the phononic contribution to friction, they got quite different results. Persson and Nitzan 3 found that the phononic friction was not significant in comparison with the electronic contribution, while Tomassone et al 4 found the opposite. On the other hand Liebsch et al 5 concluded that both contributions are important, but that the phononic friction strongly depends on the substrate corrugation amplitude.…”

“…[2][3][4][5] Subsequently, a huge body of new and fascinating experiments on nanoscale friction has seen the light during the last 15 years. However, the theoretical interpretation of nanoscopic sliding experiments has evidenced some degree of disagreement, mainly related to incompatible results between different simulations, [3][4][5] in spite of the fact that they were performed for the same system with similar models and techniques. With the aim of finding some plausible explanation for these discrepancies we set out to study the relation between the topology and sliding friction coefficient .…”

“…6,7 Issues like how the energy dissipates on the substrate, which is the main dissipation channel ͑electronic or phononic͒, and how the phononic sliding friction coefficient depends on the corrugation amplitude were addressed, and partially solved, by several groups. [2][3][4][5] Yet in the specific case of Xe over Ag, three different groups tried either to explain or to estimate theoretically the experimental observations of Daily and Krim. 8 Despite the fact that they used similar models and similar simulation techniques to calculate the phononic contribution to friction, they got quite different results.…”

Nanoscale Friction Behavior of the Ni-Film/Substrate System Under Scratching Using MD Simulation

“…In past decades, considerable research has been focused on investigating the friction behaviors at the microor nano-scales based on theory and simulations. The concept of friction has been extended in a number of associated areas, such as using surface roughness [3,4], phonon and electron excitation [5][6][7], and dislocation interaction theories [8][9][10].…”

Nonlinear friction of a damped dimer sliding on a periodic substrate

“…2 Microscopic experiments on friction, 3 and subsequent theory and simulation were started only in the last 15 years. [4][5][6][7] Of the work that remains to be done to achieve a coherent picture of this complex phenomenon, an important aspect concerns nonlinear sliding friction. In his seminal paper, 8 Persson presented numerical simulations in the nonlinear regime of an adsorbate sliding over a periodic potential.…”