August 08, 2005 —
 |
POINT
OF IMPACT: Indenter drives down into silicon carbide ceramic in an 18.7
million atom simulation created by supercomputers at the USC
Collaboratory for Advanced Computing and Simulations
|
Ceramics are both hard and brittle. Now supercomputer modeling
of the activity of millions of individual atoms in a ceramic hints that
it may be possible to get rid of some of the brittleness.
The August 5 issue of
Science reports simulation experiments by researchers using large cluster computers at the University of
Southern California to visualize what happens to an 18.7-million-atom
piece of silicon carbide when a hard square “indenter” hits its surface.
The simulation revealed a surprise. Even though the material was
uniformly structured throughout with granules of a uniform size, the
behavior of the atoms varied drastically, as the indenter drove farther
down into the material. There was a “crossover” effect.
“If we can understand this crossover better, we can potentially tailor
nanostructured ceramics with dramatically new properties,” said
materials science computer simulation specialist Priya Vashishta, who
is the director of the Collaboratory for Advanced Computing and
Simulations (CACS), and holds joint appointments in the USC Viterbi
School of Engineering and the USC College of Letters Arts and
Sciences. Vashishta and his longtime CACS collaborator Aichiro
Nakano (also of the Viterbi School and the College) worked with Izabela
Szlufarska, a former post-doctoral researcher in CACS at USC who is now
an assistant professor in engineering at the University of Wisconsin.
Silicon carbide, a ceramic material nearly as hard as diamond, is
attracting attention as a potential material for many applications now
filled by metal — if the brittleness factor can be controlled.
"Nanostructuring" new, ultra-uniform forms of carbide has already
produced less brittle forms of the substance. But the CACS research
points a direction for much more improvement.
According to Vashishta, a surface layer of carbide atoms yields
smoothly and elastically to an incoming square point, in a pattern
called “intergranular continuous deformation.”
But after the indenter drives a certain critical distance down into the
nanostructured material, the pattern changes. Instead of flowing out of
the way of the indentation point, the material clumps into discrete
chunks (“grains) which pop apart in fits and starts, in “intragrain
discrete deformation.”
Detailed analysis of the impact disclosed four distinct sub-phases, two
in the elastic, non-brittle response region and two more in the clumped,
embrittled region.
In phase one, the initial impact, material simply flowed as individual
atoms. A little farther down, in phase two, the material crystallized
into planes, which flow smoothly past each other.
But this stopped in phase three, when the material stopped crystallizing
and instead begins to clump. At first, the grains are simply
compressed, and the intergranular spaces between them disappear. But
when this space is exhausted, phase four begins. The small grains jam
together into bigger ones which, as the impact continues, begin to
shatter, catching on each other and then slipping past each other in
sharp mini-earthquakes.
The behavior observed was created by modeling an initial material with
grains of a specific size – approximately eight nanometers
(0.000008 millimeters) in diameter. Large-scale molecular dynamics
simulations can provide a level of detail that is not yet available in
experiments. The multimillion atom simulations were carried out on
parallel supercomputers of the CACS and USC High Performance Computing Center.
“We expect to see different patterns starting with different sized
grains,” said Vashishta, saying that adding non-carbide material to
the mix also had possibilities for improving performance.
Funders of the research included the Air Force Office of Scientific
Research’s Nanotechnology Initiative, the Army Research Office, the
Defense Advanced Research Projects Agency (DARPA), the US Department of
Energy, and the National Science Foundation.
