Atomic Scale Friction

In this proposal we intend to understand friction and wear properties in the extreme case of atomic
scale friction [1], where only a few atoms constitute the tip-sample contact. Despite recent successes
in the understanding atomic friction processes, where the velocity dependence [2], load dependence
[3] and new effects like superlubricity [4,5] (structural and externally induced) have been targeted,
where many phenomena are still under dispute. At this point, a multitude of experimental and
theoretical work exists, however, only a few papers report on the direct overlap of experiments and
theory. Atomic scale friction is particularly well suited for direct comparison, since the contact size is
as small as possible, and thus is much better defined than in conventional tribology experiments. This
invites direct comparison of atomic friction experiments with first principles and molecular dynamics
simulations (MD) based on discrete atom geometries. A central question is the role of defects and
interfaces. We will investigate atomic friction in the vicinity of defects under ultrahigh vacuum
conditions and compare with theoretical studies. The important role of thermal actuation was
indirectly derived from the velocity dependence of atomic friction [2]. Temperature studies were
performed on glassy polymers, where hindered rotation was found to be the relevant mechanism [6].
However, temperature studies of atomic friction under ultrahigh vacuum studies are still missing.
Therefore, an important goal of this project will be the study of atomic friction at temperature from
25K up to 1000K. Recently, high temperature friction was studied theoretically, where a strong
reduction is predicted by the skating effect [7]. At very low velocities, a drop of friction due to
thermal excitation is also predicted theoretically, the thermolubricity effect [8]. We will try to verify
these effects experimentally. Nc-AFM measurements have shown that dissipation of the order of 1 eV
per cycle is found in near contact. First principles and MD-simulations with realistic tip geometries
were compared with nc-AFM experiments, which demonstrated that adhesion hysteresis due to tip
configuration changes is the origin for dissipation [9]. Our aim is to perform nc-AFM experiments
with small amplitudes [10] and to directly compare with simulations. The other extreme regime of
sliding nanometer-sized contacts is at high normal forces, where the onset of wear occurs [11]. Here,
we plan to perform experiments as a function of load and speed and to extend contact areas with the
use of a UHV-microtribometer to explore the beviour of multi-asperity contacts.

[1] M. Mate, G. Mc Clelland, R. Erlandsson, S. Chiang, Atomic-Scale Friction of a Tungsten Tip on a
Graphite Surface, Phys. Rev. Lett., 59, 1942 (1987).
[2] E. Gnecco, R. Bennewitz, T. Gyalog, Ch. Loppacher, M. Bammerlin, E. Meyer, and H.-J.
Güntherodt. ,Velocity Dependence of Atomic Friction, Phys. Rev. Lett. 84, 1172 (2000) .
[3] A. Socoliuc R.Bennewitz, E.Gnecco, and E.Meyer, Transition from stick-slip to continuous
sliding in atomic friction: Entering a new regime of ultra-low friction, Physical Review Letters 92,
134301 (2004).
[4] M. Dienwiebel, G.S. Verhoeven, N. Pradeep, J.W.M. Frenken, J. Heimberg and H. W.
Zandbergen, Superlubricity of Graphite, Phys. Rev. Lett. 92, 126101-1 (2004).
[5] A. Socoliuc E. Gnecco, S. Maier, O. Pfeiffer, A. Baratoff1, R. Bennewitz and E. Meyer A.
Atomic-scale control of friction by actuation of nanometer-sized contacts, Science, 313, 207 (2006).
[6] S. Sills and R. Overney, Creeping Friction Dynamics and Molecular Dissipation Mechanisms in
Glassy Polymers, Phys. Rev. Lett. 91, 095501-1 (2003).
[7] T. Zykova-Timan, D. Ceresoli, E. Tosatti, Peak effect versus skating in high-temperature
nanofriction, Nature Materials 6 (3): 230 (2007).
[8] S. Y. Krylov, K. B. Jinesh, H. Valk, M. Dienwiebel, and J. W. M. Frenken, Phys. Rev. E 71,
065101R (2005).
[9] N. Oyabu, P. Pou, Y. Sugimoto, P. Jelinek, M. Abe, S. Morita, R. Perez and O. Custance.
"Single Atomic Contact Adhesion and Dissipation in Dynamic Force Microscopy".
Physical Review Letters, 96(10) : Art. No. 106101 MAR 17 2006.
R. Perez, M.C. Payne, I. Stich et al. Role of covalent tip-surface interactions in noncontact atomic
force microscopy on reactive surfaces. Physical Review Letters, 78 (4): 678-681 (1997).
[10] P.M. Hoffmann, S.M. Jeffery, J.B. Pethica, H.O. Özer, and A. Oral, Phys. Rev. Lett. 87, 265502
(2001).
[11] E. Gnecco, R. Bennewitz, E. Meyer, Abrasive wear on the atomic scale. Phys. Rev. Lett. 88
215501(2002).