Researchers in China and Australia have observed superlubricity – the dropping of friction to near zero – on length scales much larger than before. They say that the phenomenon, which they measured in sheared pieces of graphite, could find applications in sensitive microscopic resonators or nanoscale gyroscopes.

Superlubricity is sometimes used to mean simply very low friction, but the original meaning is that the friction between two surfaces disappears almost completely. Proposed in the early 1990s by Motohisa Hirano, then at the Nippon Telegraph and Telephone Corporation in Tokyo, Japan, and others, it relies on a special arrangement of atoms on a material's surface. In graphite, for instance, the surface atoms have a bumpy hexagonal arrangement like egg-boxes. In certain orientations, two surfaces of graphite can mesh in such a way that the "bumps" can slide past one other effortlessly – and friction drops towards zero.

Since it was first proposed, superlubricity has been observed on the nanoscale, mostly under high-vacuum conditions. Now, however, Quanshui Zheng at Nanchang University in China and others have observed the phenomenon on the microscale, in ambient conditions.

"Big advance"

"This is a big advance beyond the nanometre-scale superlubricity experiments," says Hirano, who was not involved with the latest study. "It could lead to implementing superlubricity [as a] lubricant for future practical use in mechanical engineering, including [devices for] saving energy."

In its experiments, Zheng's group used pyrolitic graphite, a type of graphite manufactured under high temperature that has particularly well-aligned crystal planes. Using lithography, the researchers made square columns – or mesas – of graphite up to 20 µm wide and up to 400 nm in height. They transferred these mesas to a scanning electron microscope or an optical microscope and, with a tungsten probe, sheared them into flakes, which they could rotate into different orientations.

Zheng's group found that the flakes orientated symmetrically with respect to the underlying mesa stayed still, even when poked with the probe. However, when the researchers misorientated the flakes and poked them, the flakes retracted to their original, lowest energy position. This could only happen because of the extremely low friction, the researchers say – that is, because the surface "bumps" could mesh together and allow superlubricity.

Easy and practical

"The ultimate significance of these results is they imply that the conditions for superlubricity are more easily created, and more reproducible, than previously supposed," says Zheng. "This implies a much wider practical significance for the phenomenon of superlubricity, for example in nano- and micromachines."

Other specialists in superlubricity seem to agree. "I think this is very interesting and promising work, which could lead to a breakthrough in the field of superlubricity and more generally in the control of friction properties," says Michael Urbakh of Tel Aviv University in Israel, who had previously published a theory suggesting microscale superlubricity is possible. "This work may open a new way for preparation of graphite lubricants with improved lubrication properties."

Zheng points out that his group's results may also be applicable to graphene – a single layer of graphite with superlative properties that was the subject of the 2010 Nobel Prize for Physics. However, for applications with regular graphite, he points to high-frequency microscopic resonators and nanoscale gyroscopes, to which superlubricity could offer reduced wear and lower actuation energies.

"The conventional wisdom so far has been that friction is a major hurdle to shrinking mechanical systems to the micro- and nanoscale," he says. "This is because of the increasing surface-to-volume ratio of smaller components, which favours friction – a surface-dependent force. [Our work] provides new avenues to produce practical micro- and nano-scale mechanical devices that rely on the ultra-low friction of superlubricity."

Source: Physical Review Letters