Logo: University of Southern California

USC Supercomputer Simulations Probe the Basis of Brittleness

Understandings May Lead to Materials that are Both Hard and Flexible

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.