Graphene, a two-dimensional carbon sheet, has received much attention in the past few years due to its unique mechanical, electronic properties. Earlier work has been found that the frictional behaviour of graphene exhibits traits unlike those of conventional bulk materials. Although a wide range of suggestions have been proposed, the mechanism behind this remains subject to debate.
Recently, Dr. Suzhi Li, a member of Prof. Jun Sun’s group, supervised by Prof. Ju Li at Massachusetts Institute of Technology (USA), and collaborators have investigated the friction on graphene and revealed the mystery using computer simulations. The research team also included: Dr. Qunyang Li at Tsinghua University; Prof. Robert W. Carpick and Dr. Xin-Zhou Liu at the University of Pennsylvania (USA); Prof. Peter Gumbsch at Karlsruhe Institute of Technology (Germany). Besides, Prof. Xiangdong Ding and Prof. Jun Sun at Xi’an Jiaotong University were also involved in the research. They found that the unique frictional behaviour of graphene is strongly related to the re-adjustment of its configurations as a direct consequence of its greater flexibility. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. The findings are presented in the journal Nature.
Figure. Frictional behaviour for a Si tip sliding over a graphene/a-Si substrate system.
Friction occurs when surfaces of solid bodies touch and move against each other. Energy is thereby converted into heat, which is lost. In order to reduce the friction in the metallic elements during sliding, for example in automobiles or industrial machines, materials with a lamellar structure are frequently used as the solid lubricants.
One of the solid lubricants is graphite, which is commonly used as dry lubricant agents for macroscale metallic sliding components and high pressure contacts. Graphite has a natural appearance of carbon with a three-dimensional, laminated structure. It is theoretically composed of several layers of graphene, which are slightly stacking over one another. Graphene is a well-known two-dimensional material, which consists of only one layer of carbon atoms arranged in hexagonal form such as honeycombs. It does not exist in nature as an isolated single-layer material, but can be produced by various methods.
It has been shown experimentally that monolayer graphene shows higher friction than multilayer graphene and graphite. When an atomic force microscope (AFM) tip slides on few-layer graphene loosely adhering to a substrate, the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases. By using atomistic simulations, the researchers revealed that such effect arises primarily from increased pinning between tip and graphene atoms and the simultaneity of these pinning sites, which happens more easily for fewer layers as they are more flexible, and not just from increased contact area as previously proposed.
The current findings provide new insights into the origins of friction on graphene. They are critical for explaining the time-dependent friction of configurationally floppy interfaces. The research also suggests a means of controlling friction of two-dimensional materials via strain engineering.
The research was funded by the 973 Programs of China, the NSFC (grant numbers 11422218 and 51021003, the International Joint Laboratory for Micro/Nano Manufacturing and Measurement Technologies and 111 project.
The paper is available at：
http://www.nature.com/nature/journal/v539/n7630/full/nature20135.html
Nature | News & Views by Dr. Astrid S. de Wijn
http://www.nature.com/nature/journal/v539/n7630/full/539502a.html
