In a nutshell, Da Vinci proposed that if two contacting surfaces are geometrically similar, - commensurable - they will have a much higher coefficient of friction than two geometrically dissimilar surfaces, due to the fact that the similar surfaces have a tendency to interlock. To test this theory at the molecular level, the research team looked at a quasicrystalline material that exhibits both periodic and aperiodic configurations in its crystal structure. What they found, in results to be published in the August 26 issue of the journal Science, was that friction along the periodic surface was about eight times greater than the friction along the aperiodic axis.
Until now, one of the main problems in exploring this theory was finding a material that exhibits both periodic and aperiodic order in its crystal structure. In a periodic structure, the atoms align in a regular, repeating, three-dimensional pattern. The atoms in an aperiodic structure are ordered, but the pattern in which they form isn't regular. Preparing two different samples of the same material - one periodic and one aperiodic - won't work because differences in the chemical makeup of the samples' surfaces will affect surface frictional properties. The solution was to use a quasicrystalline material that simultaneously exhibits both periodic and aperiodic structure.
Ames Laboratory materials chemists Pat Thiel and Cynthia Jenks have studied the surface structure of quasicrystals for many years and were among of the first to demonstrate that the "clean" - unoxidized - surface of a quasicrystal showed distinct layers that were consistent in structure with the bulk material. They wondered if a quasicrystal's low coefficient of friction is related to its high hardness or its unique structure.
To find out, Thiel's group teamed with physicist Miquel Salmeron and his Berkeley Lab research group, that is world renowned for its expertise in the friction of crystalline materials. For this study, Ames Lab provided the quasicrystal expertise, preparing a single crystal quasicrystalline sample of aluminum-nickel-cobalt which typically exhibits a decagonal (10-sided) symmetry.
"By cutting it parallel to its 10-fold rotational axis, it produced a surface with one periodic and one aperiodic axis, separated by 90º," Thiel said. "To keep the surface oxide-free and eliminate the effects that an oxide layer produces, the studies were conducted under ultra high vacuum conditions."
To find out if Thiel's and Jenks' suspicions were correct, Salmeron and Jeong Young Park, used a combined atomic force-scanning tunneling microscope to conduct the friction studies. These two types of microscopes both use a probe that tapers to a single atom at its tip, but they perform different tasks.
The STM's probe hovers just over the surface, close enough that the electrons in the sample's atoms begin to "tunnel" or generate an electric current across the gap between the tip and the sample. Using the STM, Salmeron's group was able to produce a "topographical" map of the surface, allowing them to determine the orientation of the periodic and aperiodic axes.
In AFM mode, the probe's tip rests directly on the sample, allowing the frictional force to be measured as it moves over the sample's surface atoms. To reduce the possibility of damage to the sample, the titanium-nitride tip was coated with a layer of hexadecane thiol. After a series of scans, the results clearly showed the friction along the periodic axis was eight times greater than the friction along the aperiodic axis.
"We believe the source of this friction has both an electronic and a phononic contribution (phonons are vibrations in a crystal lattice, like an atomic sound wave)," Park said, adding that new theoretical models are needed to determine whether electrons or phonons are the dominant contributors to the frictional anisotropy.
Ames Laboratory is operated for the Department of Energy by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.