Graphene gives friction the slip

Super-smooth coatings made from single layers of carbon known as graphene could save energy by eliminating friction and reduce wear and tear on mechanical components. Now an international team of researchers thinks they have cracked the origins of graphene’s superlubricity, laying the groundwork for the realization of this type of frictionless coatings [Kawai et al., Science 351 (2016) 957, http://dx.doi.org/10.1126/science.aad3569].

The slipperiness of graphene has been put down to its high surface stiffness and weak interaction with other solid surfaces, which makes close contact very difficult. To test this hypothesis, the team of scientists from the Universities of Basel and Bern, Empa, PRESTO, Japan Science and Technology Agency, Technische Universitat Dresden, Instituto Madrileno de Estudios Avanzados en Nanoscience, Friedrich Schiller University Jena and the Max Planck Institute for Polymer Research used dynamic atomic force microscopy (AFM) under ultrahigh vacuum and low temperature conditions to investigate the mechanical properties of graphene at the nanoscale.

Graphene nanoribbons were attached to the tip of the AFM probe and dragged back and forth across a gold substrate, gathering information about frictional forces in the process.

‘‘Our approach has the advantage of providing full control — down to the atomistic level — of the structure of the ribbon (which is essentially defect-free) and very fine tuning of the sliding conditions, together with exceptional precision in the force measurement,’’ explains Daniele Passerone of Empa. ‘‘This allows us to disentangle and understand, at the fundamental level, the factors determining the observed superlubric behavior.’’

The friction force measurements enable atomically resolved images of the graphene nanoribbons and atomistic computer simulation of the sliding process to be generated. According to the researchers’ molecular dynamics simulations, the static friction force at the point of contact with the gold substrate is a tiny 100 pN, confirming the near-superlubricity of graphene on the nanoscale. Putting it another way, it requires a force of just 2—200 pN to drag a graphene nanoribbon across a gold surface.

The findings confirm superlubricity in graphene — because of its lateral stiffness, lack of contact with the substrate, and absence of defects — and open up the possibility of a new class of nanofunctionalized coatings for friction control.

‘‘Superlubric properties of graphene have been demonstrated before,’’ points out Anirudha V. Sumant of Argonne National Laboratory, ‘‘but these studies show that even on metal surfaces such as gold, graphene slides with almost no mechanical resistance.’’

The findings have important implications for nanoscale electrical contacts such as those found in nano- or microelectromechanical system (NEMS or MEMS) switches, he believes, and there is great potential for using graphene nanoribbons as nanoscale ‘freight trains’ to carry molecules from one place to another with far less energy as well.

Novel friction-control coatings based on graphene nanoribbons could increase the lifetime of larger components as well as NEMS or MEMS devices, suggests Passerone.

‘‘We gained control of a complex nanosystem,’’ he says, ‘‘paving the way to the fabrication of atomically defined complex nanostructures.’’

Mauricio Terrones of Pennsylvania State University agrees, commenting: ‘‘This work is important because it shows from an experimental/theoretical angle that chemically synthesized graphene nanoribbons could be used as superlubricants and reduce friction between two surfaces.’’

But although graphene nanoribbons could be used as coatings in the future, there are many challenges to overcome before we are likely to see them in use in our daily lives.

This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.04.010