14 April 2010—The moving parts of micromechanical machines tend to seize up under the forces of sticking and friction that engineers call stiction. The problem yields to solid lubricants, notably graphite (sheets of carbon atoms called graphene stacked in layers), although for a long time no one understood exactly why this happens.
Now nanotechnology researchers, led by Professor Robert Carpick at the University of Pennsylvania and Professor James Hone at Columbia University, in New York City, have shown that how effective the lubrication is depends on the number of layers of graphene in the graphite. In particular, more layers means better lubrication. Because the same relationship between layers and lubrication occurs in thin sheets of molybdenum disulfide, niobium diselenide, and boron nitride—materials of widely differing properties—the workers conclude that this behavior is a fundamental aspect of friction. They expect that the discovery will lead to better lubrication of tiny moving parts. The researchers published details of their experiments in a recent issue of Science.
The work is ”top notch,” says Martin Mueser, a theoretical physicist at University of Western Ontario, who was not involved in the research. He says the researchers have ”finally succeeded in categorizing friction at the atomic level, in thin sheets of material.”
As the semiconductor industry continues down the path of miniaturization, mechanical components are getting smaller as well. Today, tiny machines called microelectromechanical systems (MEMS) are routinely used to make sensors such as those used in accelerometers, gyroscopes, and air bags. The next stage of research has already begun into even smaller machines—nanoelectromechanical systems (NEMS). Stiction is arguably the single greatest impediment to progress.
Stiction is caused by a number of different forces, including the electrostatic force and the van der Waals force. The electrostatic force, which is due to the buildup of charges on a surface, is what makes a sheet of plastic film stick to a person’s hand, to name one effect. The van der Waals attraction is what binds uncharged molecules in nature; it is the force responsible for geckos’ ability to defy gravity and stick to the ceiling. It dominates at intramolecular distances, which are typically less than 10 nanometers.
Water and other liquids cannot be used in microscopic machines, because they ”tend to be squeezed out,” says Mueser. And ”if you have a small device and you rub off one to two layers, the device will fail,” he adds. Therefore, engineers get around this problem by using solid lubricants, such as graphite.
To understand how solid lubricants work, the Penn and Columbia researchers began in 2008 by studying the microscopic behavior of graphene. They noticed that ”some graphene flakes had higher friction than others,” says Carpick. ”The thinner flakes had higher friction.”
Using atomic-force microscopy, they found that friction increased as the number of graphene layers decreased. A single layer of graphene showed the highest friction of all. Thin sheets of molybdenum disulfide, niobium diselenide, and boron nitride also exhibited the same basic frictional behavior despite having varying electronic properties, Carpick says.
”We call this mechanism, which leads to higher friction on thinner sheets, the puckering effect,” he says. ”Interatomic forces, like the van der Waals force, cause attraction between the atomic sheet and the nanoscale tip of the atomic-force microscope.”
The thinness of the sheet causes it to deflect toward the tip of the microscope, leading to a wrinkled shape, he explains. This change of shape increases the area of interaction between the tip and the sheet, increasing the friction. Thicker sheets are stiffer and thus do not bend as easily, so in these the increase in friction is less pronounced.
Carpick says the team thinks it has found a way of controlling the friction in future generations of MEMS machines—and ultimately that in NEMS machines—by changing the number of sheets of lubricants.
Carpick says the challenge is in laying down the right number of sheets for the purpose—a manufacturing challenge that he says a number of teams are already investigating. Meanwhile, the Penn-Columbia researchers are continuing their fundamental investigations. ”We want to look at how temperature and environment affect friction,” Carpick says. ”We would like to see if this effect can be put to use in an actual device.”