Sep 26, 2013
Atomic roughness increases friction on hydrogenated graphene
Graphene, a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick, is well known for its low-friction properties at the nanoscale. A team of researchers in the US has now found that when its surface is modified with hydrogen, adhesion to graphene decreases by a factor of two, but friction increases by an order of magnitude because of the atomic roughness induced by hydrogenation. The discovery may help in the design of more efficient mechanical components for micro- and nanoelectromechanical systems.
Friction is an old, yet still very relevant topic thanks to its importance in our everyday lives. The development of nanotechnology has enabled researchers to look into the origins of friction at the atomic level for the first time and much work has also gone into controlling friction by engineering materials at the nanoscale.
The present work is inspired by research conducted by Dr Kim’s and Dr Park’s teams at KAIST in the Republic of Korea. These scientists found that fluorinating graphene increases friction on the carbon material at the nanoscale. However, it was extremely difficult to identify the atomic level origin of this friction in experiments.
Atomic roughness increases friction
Researchers from the University of Akron, Purdue University and the University of California, Merced, have now employed molecular dynamics simulations to model these previous experimental measurements. The simulations, which capture the dynamics of each atom, allowed them to correlate the friction behaviour with atomic level information. The results reveal that it is atomic roughness – rather than variations in rigidity or adhesion – that produces the increased friction observed.
After chemically modifying the graphene surface with hydrogen atoms, its two-dimensional structure evolves into a three-dimensional one with much more surface roughness. Even though roughness decreases the real contact area, and thus adhesion properties, it causes so-called atomic interlock at the interface and so ultimately leads to increased friction on the material.
The discovery of this new mechanism also challenges a widely accepted belief that friction is proportional to contact area at the nanoscale. The results show that, while contact quantity and real contact area play an important role in atomic friction, interlock at interfaces is another hitherto unsuspected factor.
More information can be found in the journal Nanotechnology 24 375701.
About the author
Dr. Yalin Dong is an assistant professor at the Department of Mechanical Engineering at the University of Akron. His research group, the Micro/Nano Engineering Lab works on applying theoretical and numerical methods to investigate mechanical, thermal, and electrical properties of materials at the micro- and nanoscales. Xiawa Wu is a PhD student at Purdue University supervised by Prof. Ashlie Martini at the University of California, Merced.