'Quasicrystal' Metal Computer Model Could Aid Ultra-low-friction Machine Parts

Quasicrystals, like normal crystals, consist of atoms that combine toform structures -- triangles, rectangles, pentagons, etc. -- thatrepeat in a pattern. However, unlike normal periodic crystals, inquasicrystals the pattern does not repeat at regular intervals. So,while the atomic patterns of two crystalline materials rubbing togethercan line up and grind against one another, causing friction,quasicrystalline materials do not, and thus produce little friction.

Quasicrystalline metalic alloys are already used in a handful ofcommercial products, including as a coating for some non-stick fryingpans because they combine the scratch- and temperature- resistantproperties of a polymer such as Teflon with the heat conductionproperty of metals.

However, a major technical obstacle remains to using quasicrystalmaterials to minimize friction between surfaces sliding against oneanother, the scientists said. Microscopic surface contaminants, such asatmospheric gases, can come between the surfaces and interfere with thematerials' high lubricity. The gases form a thin layer of moleculesover the alloy surface-- typically in a crystalline pattern -- whichmasks the desirable surface properties of the underlying quasicrystal,they said.

The researchers' computer model of the effect of adsorbed gas on thequasicrystal alloy of aluminum, nickel and cobalt will be published inan upcoming issue of the journal Physical Review Letters. Theirresearch was funded by the National Science Foundation.

"We are interested in quasicrystals because they are scratch-resistantand they have very little friction," said Stefano Curtarolo, leadauthor of the paper and a professor of materials science in Duke'sPratt School of Engineering. "So they are promising for slidinginterfaces in machines and applications where the potential forscratching might be involved."

Metals were believed to have only periodic crystalline structures until1984, when materials scientists reported discovery of the firstmetallic alloy with a quasicrystalline structure. Since then,scientists, including Curtarolo, have sought to explore the propertiesand applications of quasicrystals.

The challenge Curtarolo, Duke graduate student Wahyu Setyawan and theircolleagues at Penn State University address in their paper is how topreserve the low-surface-friction property of a quasicrystal in thepresence of a gas.

In previous experiments, Curtarolo's Penn State colleagues NicolaFerralis, Renee D. Diehl, Raluca Trasca and Milton W. Cole had foundthat when xenon gas is exposed to their quasicrystal alloy, a singlelayer of xenon first forms in a quasicrystal pattern on top of thealloy, but by the time two or more layers formed, the xenon atomsdevelop a crystalline structure.

They chose to experiment with xenon, which does not react chemicallywith most metals, so they could consider the physical interaction ofthe gas and the metallic alloy, without complicating chemicalinteractions. In the experiments, the number of layers formed by thexenon atoms varies with the experimental temperature and pressure.

"If you have very little xenon gas, it's going to follow the aperiodicsymmetry of the quasicrystal; if you have a lot, it's going to followthe periodic structure of xenon," Curtarolo said. "This change fromquasicrystal to periodic crystal -- that's what we want to know about."

Cutarolo and his colleagues modeled in their computer simulation thistransition from a single layer of xenon with quasicrystallineproperties to multiple layers with crystalline properties. Thesimulation is consistent with experimental data.

The simulation is available online at http://nietzsche.mems.duke.edu/SCIENCE/movies/XeQC/isotherm_T77K_big.mpg.In the simulation, the image on the left is of the average position ofthe xenon atom, the image on the right is of the electron diffractionpattern used to determine the position of the atoms and the graph onthe bottom gives the density of the xenon gas.

"This model tells us how we might be able to control the transition andpreserve the low-friction property of quasicrystals," Curtarolo said."It's a step towards understanding how quasicrystals interact withgases in the atmosphere and how we could eventually use them in realmachines."