|
Our study found that the larger mass of the terminating atoms at the
surface, in this case deuterium, led to less energy lost to heat in
the system, Robert Carpick, associate professor of mechanical
engineering and applied mechanics at Penn, said. The larger atomic
mass of deuterium results in a lower natural vibration frequency of
the atoms. These atoms collide less frequently with the tip sliding
over it, and thus energy is more slowly dissipated away from the
contact.
The single layer of atoms at the surface of each crystal acts as an
energy transfer medium, absorbing kinetic energy from the tip of the
atomic force microscope. The tips were less than 50nm in radius at
their ends. How much energy is absorbed is dependent, researchers
found, on the adsorbates natural atomic vibration frequencies. The
heavier an atom, the lower its vibrational frequency. The lighter an
atom, the faster the vibrations and thus the faster the dissipation of
energy from the contact in the sample. Keeping the atoms chemically
similar avoided any changes arising from chemical bonding.
The Penn findings provide a better understanding of the nature of
friction, which lacks a comprehensive model at the fundamental level.
We know how some properties - adhesion, roughness and material
stiffness for example - contribute to friction over several length
scales, but this work reveals how truly atomic-scale phenomena can and
do play a meaningful role, Matthew Brukman, a contributor to the
research, said.
Industry has long been concerned with ways to reduce friction between
objects, both to maintain the energy of the system as well as to
reduce heat-generation and wear, which can weaken machinery and
materials to the breaking point. The authors note that improved
friction models can be used for the opposite effect; makers of some
mechanical components such as automobile clutches may be interested in
techniques to increase friction without changing the wear or adhesion
of materials.
Even in the absence of rough edges or wear between sliding bodies,
friction between the atoms at the surface causes vibrations which
dissipate energy, but the exact mechanisms of this process remain
unresolved. Scientists continue to explore the details of friction,
and other open questions include the effects of environmental
variables such as temperature and atmosphere.
The research was performed by Carpick and Brukman of the Department of
Materials Science and Engineering in Penn's School of Engineering and
Applied Science; Rachel J. Cannara, now of the IBM Zurich Research
Laboratory; Anirudha V. Sumant, now at Argonne National Laboratory;
and Steven Baldelli and Katherine Cimatu of the University of Houston.
The research was supported by the National Science Foundation, an NSF
Graduate Research Fellowship and the Air Force Office of Scientific
Research.
|
