It turns out
that diamond polishing is stranger than we imagined. An amorphous layer is produced that can then
be removed. This suggests a new strategy
for preparing fine surfaces on nano engineered material.
It is a
little bit like using pressure to induce failure and electrostatic stickiness
to remove debris from the finished surface.
It lends itself to fine work.
This implies
that we will see this used in technology rather quickly on even quite ordinary
materials. It is a new useful method.
Secret of diamond polishing revealed
Dec 2, 2010
It is the hardest everyday material on Earth, so
why does diamond glisten when rubbed against another diamond? Now, the ancient
but mysterious process of diamond grinding may have been explained by
physicists in Germany
For centuries precious-stone merchants have
polished diamonds by grinding them with cast-iron wheels embedded with coarse
diamond fragments. It is not clear why this procedure is so effective at
cleaning diamonds, but experience suggests that it works far better when the
diamond is fixed at certain angles to the wheel than others.
This directional dependence of diamond grinding
has now been investigated by Lars Pastewka at the Fraunhofer Institute for
Mechanics of Materials who set out to investigate the phenomenon. Working with
colleagues at several other institutes across Germany
Diamond in the rough
But when the researchers applied their model to
diamond itself, they were surprised to find that it accurately predicted the
experimental wear rates for this material – even though the exact wear
mechanism has so far remained poorly understood. "At this point we became
very excited about this work and analysed our simulations in much more detail
to uncover the details of the process," Pastewka toldphysicsworld.com.
Pastewka's team set about simulating diamond
grinding using 70 computer processors running for a year, and discovered that
during the grinding the diamond surfaces were being transformed into soft,
amorphous layers. These thin films can then be easily removed by either
chipping them away, or through carbon molecules bonding with oxygen in the
atmosphere, leaving behind clean diamond surfaces.
This creation of the amorphous film occurs because
of existing imperfections at the diamond surface, including the build-up of
dirt over time. As a diamond atom slides over the surface it repeatedly pulls
at the diamond crystal's atoms, and sometimes removes an atom from the crystal
surface, which becomes part of the amorphous layer.
Like a stack of paper clips
"Imagine you have a stack of paper clips
neatly arranged on your desk," explains Pastewka. "Now you take a
magnet and move that over these clips at a certain height. You cannot keep the
height ideally constant, so if the height is right you will pull some paper
clips to your magnet and others will remain on the desk."
Changfeng Chen, a materials scientist at the University of Nevada
in the US
To develop the work, however, Pastewka's team
intends to further investigate diamond's surface chemistry, and is currently
writing a paper on the oxidation of the amorphous layer.
This research is described in a research paper in Nature Materials.
About
the author
James Dacey is
a reporter for physicsworld.com
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