Quite a lot of unexpected detail when dealing with friction here. Not sure where it can take us.
The field of graphene still awaits application solutions as might be expected. That can take decades unfortunately.
All good.
.
Uncovering the secrets of friction on graphene
23-Nov-2016
Sliding on flexible graphene surfaces has been uncharted territory until now
Now, using powerful computer simulations, researchers at MIT and
elsewhere have made significant strides in understanding that process,
including why the friction varies as the object sliding on it moves
forward, instead of remaining constant as it does with most other known
materials.
The findings are presented this week in the journal Nature,
in a paper by Ju Li, professor of nuclear science and engineering and of
materials science and engineering at MIT, and seven others at MIT, the
University of Pennsylvania, and universities in China and Germany.
Graphite, a bulk material composed of many layers of graphene, is a
well-known solid lubricant. (In other words, like oil, it can be added
in between contacting materials to reduce friction.) Recent research
suggests that even one or a few layers of graphene can also provide
effective lubrication. This may be used in small-scale thermal and
electrical contacts and other nanoscale devices. In such cases, an
understanding of the friction between two pieces of graphene, or between
graphene and another material, is important for maintaining a good
electrical, thermal, and mechanical connection. Researchers had
previously found that while one layer of graphene on a surface reduces
friction, having a few more was even better. However, the reason for
this was not well-explained before, Li says.
"There is this broad notion in tribology that friction depends on
the true contact area," Li says -- that is, the area where two materials
are really in contact, down to the atomic level. The "true" contact
area is often substantially smaller than it would otherwise appear to be
if observed at larger size scales. Determining the true contact area is
important for understanding not only the degree of friction between the
pieces, but also other characteristics such as the electrical
conduction or heat transfer.
For example, explains co-author Robert Carpick of the University of
Pennsylvania, "When two parts in a machine make contact, like two teeth
of steel gears, the actual amount of steel in contact is much smaller
than it appears, because the gear teeth are rough, and contact only
occurs at the topmost protruding points on the surfaces. If the surfaces
were polished to be flatter so that twice as much area was in contact,
the friction would then be twice as high. In other words, the friction
force doubles if the true area of direct contact doubles."
But it turns out that the situation is even more complex than
scientists had thought. Li and his colleagues found that there are also
other aspects of the contact that influence how friction force gets
transferred across it. "We call this the quality of contact, as opposed
to the quantity of contact measured by the 'true contact' area," Li
explains.
Experimental observations had shown that when a nanoscale object
slides along a single layer of graphene, the friction force actually
increases at first, before eventually leveling off. This effect lessens
and the leveled-off friction force decreases when sliding on more and
more graphene sheets. This phenomenon was also seen in other layered
materials including molybdenum disulfide. Previous attempts to explain
this variation in friction, not seen in anything other than these
two-dimensional materials, had fallen short.
To determine the quality of contact, it is necessary to know the
exact position of each atom on each of the two surfaces. The quality of
contact depends on how well-aligned the atomic configurations are in the
two surfaces in contact, and on the synchrony of these alignments.
According to the computer simulations, these factors turned out to be
more important than the traditional measure in explaining the materials'
frictional behavior, according to Li.
"You cannot explain the increase in friction" as the material begins
to slide "by just the contact area," Li says. "Most of the change in
friction is actually due to change in the quality of contact, not the
true contact area." The researchers found that the act of sliding causes
graphene atoms to make better contact with the object sliding along it;
this increase in the quality of contact leads to the increase in
friction as sliding proceeds and eventually levels off. The effect is
strong for a single layer of graphene because the graphene is so
flexible that the atoms can move to locations of better contact with the
tip.
A number of factors can affect the quality of contact, including
rigidity of the surfaces, slight curvatures, and gas molecules that get
in between the two solid layers, Li says. But by understanding the way
the process works, engineers can now take specific steps to alter that
frictional behavior to match a particular intended use of the material.
For example, "prewrinkling" of the graphene material can give it more
flexibility and improve the quality of contact. "We can use that to vary
the friction by a factor of three, while the true contact area barely
changes," he says.
"In other words, it's not just the material itself" that determines
how it slides, but also its boundary condition -- including whether it
is loose and wrinkled or flat and stretched tight, he says. And these
principles apply not just to graphene but also to other two-dimensional
materials, such as molybdenum disulfide, boron nitride, or other
single-atom or single-molecule-thick materials.
"Potentially, a moving mechanical contact could be used as a way to
make very good power switches in small electronic devices," Li says. But
that is still some ways off; while graphene is a promising material
being widely studied, "we're still waiting to see graphene electronics
and 2-D electronics take off. It's an emerging field."
###
Besides Li and Carpick, the research team
included former MIT and University of Pennsylvania visiting student
Suzhi Li, now a Humboldt Research Fellow in Germany; Qunyang Li at
Tsinghua University in China; Xin Liu at the University of Pennsylvania
and now at Intel; Peter Gumbsch at Karlsruhe Institute of Technology in
Germany; and Xiangdong Ding and Jun Sun at Xi'an Jiaotong University in
China.
The work was supported by the National Science Foundation.
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