"Nanomechanics at Interfaces:
New Insights into Friction and Adhesion for Carbon-Based
Systems"
Prof. Robert W. Carpick
Associate Professor, College of Engineering
University of Wisconsin-Madison
Abstract
Designing materials and devices at the atomic scale presents
tremendous opportunities and challenges. A key barrier to
the success of nanotechnology lies in developing a fundamental
understanding of the mechanical behavior at interfaces.
I will highlight recent atomic force microscopy experiments
which demonstrate how nano-scale mechanical behavior is
manifested in unique and surprising ways, and discuss the
impact for the design of nanomechanical systems.
For example, friction at the nanoscale can exhibit clear
transitions from smooth sliding to single slips and then
multiple slips. The slips are directly correlated with the
atomic lattice of the sample, in this case pure graphite.
The observation of the transition to multiple slips is new,
and is understood by considering the competition between
the “stiffness” of the interatomic potential
across the interface and the elastic stiffnesses of the
contacting materials and the force sensor itself. The transition
to smooth sliding with ultralow dissipation without the
need for ultrahigh vacuum conditions is observed for the
first time, and atomic-scale stick-slip is observed for
interfaces orders of magnitude larger than any previously
tested. Atomic-scale stick-slip may therefore be a far more
prevalent phenomenon than initially appreciated.
We have also extensively studied the nanotribological behavior
of other carbon-based systems, including single crystal
and nanocrystalline diamond. The atomic structure of the
surface, verified by detailed surface spectroscopy, critically
affects friction and adhesion. Hydrogen termination is particularly
effective in reducing friction and adhesion to the limit
of van der Waals’ interactions. Friction is also affected
by the crystal orientation. Surprisingly, continuum mechanics
models of contact area can be applied to understand the
interfacial mechanics of these nano-scale contacts, as evidenced
by the observation of direct proportionality between friction
and contact area, a phenomenon known as “interfacial
friction”.
Finally, we have found that the molecular architecture
of interfaces plays a key role in controlling friction in
the case of self-assembled monolayers. These hydrocarbon
layers are extremely effective in reducing friction. Sliding
appears not only to be controlled by interfacial friction,
but also by an additional contribution that we propose arises
from energy dissipation due to molecular plowing.
Thursday, April 20, 2006
2 PM, 337 Towne Bldg.
