Graphene and diamonds prove a slippery combination
12 June 2015
Scientists have used tiny diamonds and graphene to create a new material combination that demonstrates the rare phenomenon of 'superlubricity'.
Led by nanoscientist Ani Sumant of the US Department of Energy’s Argonne National Laboratory Center for Nanoscale Materials (CNM) and Argonne Distinguished Fellow Ali Erdemir of Argonne’s Energy Systems Division, the five-person Argonne team combined diamond nanoparticles, small patches of graphene and a diamond-like carbon material to create superlubricity, a highly-desirable property in which friction drops to near zero.
According to Erdemir, as the graphene patches and diamond particles rub up against a large diamond-like carbon surface, the graphene rolls itself around the diamond particle, creating something that looks like a ball bearing on the nanoscopic level. “The interaction between the graphene and the diamond-like carbon is essential for creating the ‘superlubricity’ effect,” he says. “The two materials depend on each other.”
At the atomic level, friction occurs when atoms in materials that slide against each other become 'locked in state', which requires additional energy to overcome. Diana Berman, a post-doctoral researcher at the CNM and co-author of an article describing the work in the online issue of Science Express, likens it trying to slide two egg cartons against each other bottom-to-bottom. “There are times at which the positioning of the gaps between the eggs – or in our case, the atoms – causes an entanglement between the materials that prevents easy sliding,” she says.
By creating the graphene-encapsulated diamond ball bearings, or 'scrolls', the team found a way to translate the nanoscale superlubricity into a macroscale phenomenon. Because the scrolls change their orientation during the sliding process, enough diamond particles and graphene patches prevent the two surfaces from becoming locked in state.
The team used large-scale atomistic computations on the Mira supercomputer at the Argonne Leadership Computing Facility to prove that the effect could be seen not merely at the nanoscale but also at the macroscale.
“A scroll can be manipulated and rotated much more easily than a simple sheet of graphene or graphite,” Berman says.
However, the team was puzzled that while superlubricity was maintained in dry conditions, in a humid environment this was not the case. Because this behaviour was counter-intuitive, the team again turned to atomistic calculations. “We observed that the scroll formation was inhibited in the presence of a water layer, therefore causing higher friction,” says co-author, Argonne computational nanoscientist, Subramanian Sankaranarayanan.
While the field of tribology has long been concerned with ways to reduce friction – and thus the energy demands of different mechanical systems – superlubricity has been treated as a tough proposition. “Everyone would dream of being able to achieve superlubricity in a wide range of mechanical systems, but it’s a very difficult goal to achieve,” says Sanket Deshmukh, another CNM post-doctoral researcher on the study.
“The knowledge gained from this study,” Sumant added, "will be crucial in finding ways to reduce friction in everything from engines or turbines to computer hard disks and micro-electromechanical systems [MEMS]."