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Materials Science Division |
HighlightsFundamental Investigations on Nanotribology using In Situ TEMFundamental Investigations on
Nanotribology using In Situ TEM Scientific Achievement The
ongoing research in
understanding the fundamental origins of nanotribological phenomena
using in situ TEM is enabled by
using a STM or
AFM probe mounted on a TEM sample holder.
The obvious merits of in situ
tribological studies using a TEM include dynamic monitoring of
structural and
chemical changes that occur due to sliding interfaces at a higher
spatial
resolution. These
studies are performed
using the Tecnai F20 TEM available at the Argonne National Lab. Several
prototypical
materials have been analyzed in this study including pyrolytic graphite
(HOPG),
gold and amorphous carbon films. Sharp
tungsten STM probes (r < 100 nm) were used as a single asperity
contact to
investigate the tribological behaviors of the chosen materials. A series of indents and
sliding passes were
performed to induce tribological phenomena on the nanoscale. Some of the prominent
results are highlighted
below. HOPG
samples showed evidence
of wear by flaking. Layers
of graphite
between two and fifteen nanometers in thickness were removed by passing
the
tungsten tip across the sample surface.
Transfer of graphite to the tungsten tip was directly
observed,
identifying a mechanism responsible for a large reduction in friction
forces. The
thickness of the wear layers is
correlated to an analysis of interfacial dislocation standoff
distances. The
deformation of HOPG
samples under a variety of contact modes using electron diffraction was
also
examined. The
lattice straining,
rotational disordering of the grains, and cracking and roll-up of the
film edges
were systematically investigated.
In
order to correlate the effects with the magnitude of the corresponding
forces,
a contact mechanics based model was used.
A more rigorous approach in the determination of the
forces and thereby
enabling quantitatively analyses will be performed in the future using
the
AFM-TEM holder. High
resolution imaging of
the sliding interface and chemical analyses through EELS will allow the
investigation of friction-induced phase transformation.
An early result from such studies
demonstrated an increase in the sp2 bonding
character in the
energy-loss spectra after several hundred sliding passes between DLC
and
tungsten probe proved confirming chemical changes due to friction. Currently, a systematic
approach in the
investigation of surface phase transformation due to friction is being
pursued. Significance Tribological
experiments on
the nanoscale have been traditionally hampered by the inability to
directly
characterize the sliding interfaces.
The
in situ TEM studies currently being performed at the
Argonne EM facility
significantly enables direct structural and chemical analysis at a
higher
spatial resolution. Therefore,
specific
materials science aspects of sliding interfaces can be investigated
systematically. Performers 
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