| Université du Québec
Institut national de la recherche scientifique
Énergie, Matériaux et Télécommunications
1650, boulevard Lionel-Boulet
Varennes, Québec, Canada
J3X 1S2
Telephone: +1 (514) 228-6946
Fax: +1 (450) 929-8102
charnagea@sympatico.ca
www.emt.inrs.ca
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Friction is a ubiquitous phenomenon represented by a force opposing the
relative lateral (tangential) displacement of two bodies contacting each
other. Due to its omnipresent nature, much is known from an experimental/empirical
viewpoint; however, the origin of friction is still poorly understood.
Macroscopic tribology usually focuses on determining the friction coefficient
and wear rate for materials of interest. An important subfield of engineering
where friction becomes crucial is the fabrication of small scale devices
such as MEMS/NEMS (micro/nano-electromechanical systems). As the dimension
of the individual components decreases, the surface/volume ratio increases,
thus the processes taking place at the surface become dominant. It is
well known that silicon-based MEMS exhibit undesirable stiction and high
wear, which typically leads to device failure.
To elucidate the origin of friction, experiments at well-defined interfaces
are required. Further simplification is achieved by reducing one of the
surface in contact with the other down to a single asperity. Such single-asperity
contact measurements are performed using an atomic force microscope (AFM),
which is a unique tool to detect atomic-scale forces. In this technique,
a sharp probe (“tip”) is raster-scanned across a surface while
its position above the substrate is regulated such that the interaction
force between the tip and sample is maintained constant. The AFM tip becomes
the asperity needed for reliable measurements in nanotribology that will
lead to understanding, and thus enabling friction control.
The role of charges or charge flow on the frictional forces is much less
studied and this is what this project is focused on. The application of
a voltage bias during nanotribologic measurements are limited so far to
the simple determination of the contact area from the magnitude of the
current flowing between tip and surface. However, charges and currents
should have a significant effect on the frictional forces, because at
the nanoscale electrostatic forces become significant; consider, e.g.
that the force between two electrons one nanometer apart is ~0.2 nN, thus
comparable with the friction forces involved in AFM experiments. I also
expect that the high current density flowing through nanoscale contacts
(~109A/m2) will generate significant thermal effects (local heating/dissipation)
thus again influencing the interaction force.
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