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Nanotribology: Friction and Wear on the Atomic Scale
Friction and wear processes on the nanometer scale can be investigated by atomic force
microscopy. When the probing tip of the microscope slides over a flat surface, the
cantilever sustaining the tip undergoes a torsion, which is directly related to the
frictional force between tip and surface. Figure 1A shows a ‘friction loop’ recorded on a
sodium chloride crystal. The tip was first moved to the right and then back to its
initial position. The sawtooth shape corresponds to the atomic surface lattice. The peak
height is not uniformly distributed, which is due to thermally activated jumps across
the lattice. This effect is also responsible for the logarithmic velocity dependence of
friction, observed at low scan speed [1].
Increasing the normal force, the surface can be permanently damaged. In some cases the
same tip can be used for both scratching and imaging the surface [2]. Figure 2A shows how
the debris rearranges at the end of a groove formed by repeatedly scanning a potassium
bromide surface. From the friction loops acquired while scratching, the energy dissipated
in the wear process can be quantified. These results can be compared with computer
simulations, in which tip and surface are reproduced at atomic levels (Figure 2B).
On the opposite side, when the normal force is kept below a certain threshold, the
stick-slip motion is “smoothed” out, and the energy dissipation becomes negligible [3].
This regime was recently achieved also by applying an ac voltage between the tip and a
counterelectrode on the other side of the crystal sample. When the actuation frequency
corresponds to a normal resonance of the system, a dramatic decrease of friction is
observed (Figure 4). Interesting applications to micro-electromechanical devices, which
up to now exploit only stator/rotor geometries, are expected.
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Figure 1:
A typical “friction loop” acquired on a
crystal surface. |
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a)
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Figure 2: (a) The atoms dragged by a sharp tip repeatedly moved
back and forth on a straight line reorganize in atomically flat terraces at the end of the
line; (b) Snapshot of a computer simulation reproducing the sliding motion of a tip on a
surface on the atomic scale.
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Figure 3:
Friction can be “switched” on and off while
scanning by mechanically exciting the cantilever at its contact resonance
frequency. |
[1] |
Velocity Dependence of Atomic Friction
E. Gnecco, R. Bennewitz, T. Gyalog, Ch. Loppacher, M. Bammerlin, E. Meyer, and H.-J. Güntherodt
Phys. Rev. Lett. 84, 1172–1175
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[2] |
Abrasive Wear on the Atomic Scale
E. Gnecco, R. Bennewitz, and E. Meyer
Phys. Rev. Lett. 88, 215501 (2002)
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[3] |
Transition from Stick-Slip to Continuous Sliding in Atomic Friction: Entering a New Regime of Ultralow Friction
A. Socoliuc, R. Bennewitz, E. Gnecco, and E. Meyer
Phys. Rev. Lett. 92, 134301 (2004)
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Contact:
Enrico Gnecco |
Ernst Meyer |
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Institute of Physics
University of Basel
Switzerland
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