An international team of scientists investigating the friction properties of two-dimensional carbon graphene has explained the reasons for friction being dependent on the number of layers of the material. Using simulations, they found that, when in contact with monolayer graphene, friction was found to be greater than for multi-layered graphene, or graphite. In addition, the friction force increased for continued sliding, which was put down to the real contact area and the evolving nature of frictional contact.

Friction takes place when the interfaces of solids come into contact and move in opposite directions, with energy being converted into heat before being lost. Of course, friction also results in wear, and to reduce this for metallic sliding elements and also high-contact pressures such as in machines or automobiles, substances with a lamellar structure are often used as dry lubricants since their particles can easily slide on each other.

“Our concept of evolving contact quality can be used to explain why friction of interfaces of a less constrained structure changes with time”Suzhi Li

One such common lubricant is graphite, the 3D layered structure of carbon. Monolayer graphene has been found to exhibit higher friction than multilayer graphene or graphite, which increases with continued sliding. Although the causes of this are unknown, this new study produced insights into layer-dependent friction and the increase in friction force for graphene.

In their paper, “The evolving quality of frictional contact with graphene”, published in Nature [Li et al. Nature (2016) DOI: 10.1038/nature20135], researchers from Karlsruhe Institute of Technology in cooperation with colleagues at the Fraunhofer Institute for Mechanics of Materials and others in China and the US simulated a silicon tip sliding over graphene applied to a non-crystalline silicon substrate. Although it had been assumed that friction between interfaces depends on the true contact area, and the increased friction of monolayer graphene was thought to be due to the larger true contact area, this work showed that the evolving contact quality is also significant.

It was previously thought that the true contact area between two rough bodies controls interfacial friction, which is can be defined by the number of atoms within the range of interatomic forces, and is difficult to visualize directly but characterizes the quantity of contact. However, its greater flexibility means that the thinner and less constrained monolayer graphene manages to readjust its configuration. Carbon atoms also stick more strongly to the atoms of the silicon tip, and also show greater synchronicity in their behavior. As researcher Suzhi Li said, “Our concept of evolving contact quality can be used to explain why friction of interfaces of a less constrained structure changes with time”.