Superlubricity (also known as structural superlubricity) is a phenomenon in which the friction between two solid surfaces moving across each other almost vanishes. Superlubricity has mainly been observed on the micro- and nanoscales, and mostly under high-vacuum conditions. Indeed, many researchers believed that it could not exist on the macroscale at all because large objects are intrinsically rough. A team led by Fei Wei of Tsinghua University has now turned this idea on its head.

Double-walled carbon nanotubes (DWCNTs), which are composed of two coaxial rolled-up sheets of carbon (graphene) with a high-aspect ratio, appear to have perfect crystalline structures on macro length scales – something that makes them ideal for studying superlubricity. For most DWCNTs, the interfaces between the inner and outer shells are "incommensurate", explains Wei, which means that the two surfaces have no energetically preferred position with respect to each other. They can thus slide almost effortlessly across other. Such lack of friction is a very good property to have in a nanomachine. What is more, for an ideal DWCNT with a partly extruded inner shell, the shear stress in the overlapped section vanishes thanks to repeated breaking and reforming of van de Waals interactions between the adjacent shells.

"All this means that only the edge section of a CNT is responsible for intershell interactions during the pulling-out process," says Wei, "and since friction between DWCNT shells does not depend on the length of a tube, they will show superlubricity on centimetre length scales."

Pulling out inner shell

The researchers obtained their results by pulling out a DWCNT inner shell using a probe in a scanning electron microscope (SEM) equipped with nanomanipulators, and then analysing the friction force between the pulled out shell and the outer "host" shell. "We found that the inner shell could be continuously pulled out of the DWCNT – in theory, even if the tube is infinitely long," Wei told nanotechweb.org.

The team found that the measured frictional forces between the inner and outer DWCNT shells were almost independent of the pull out length and could be as low as I nN. These forces only fluctuate by about 2 nN or so – even though the contact length changes by six orders of magnitude (from a few tens of nm to a few mm).

"We believe that this result is important for fundamental studies on friction and for technological applications of superlubricity," added Wei. "For example it might help us to make superlubricous micro- and nanodevices based on DWCNTs in the future."

The team now plans to study superlubricity in other few-walled carbon nanotubes, such as triple- or four-walled tubes, rather than just double-walled ones. "We also plan to design and fabricate micro- and nano-electromechanical devices (M/NEMS) based on DWCNTs taking into account our results," said Wei.

The present work is detailed in Nature Nanotechnology doi:10.1038/nnano.2013.217.

Further reading

• Atomic roughness increases friction on hydrogenated graphene (Sep 2013)
• Nanomachines could benefit from superlubricity (April 2012)
• Double graphene coat is slippery stuff (Mar 2009)