Thick and thin

Science 328, 76–80 (2010)

Understanding friction at the atomic scale is important for the design of devices that involve the movement of nanoscale components across surfaces, and for the development of nanolubricants. Solid materials are often used for lubrication at the nanoscale because liquid lubricants can be forced out of the gaps between the moving parts. Robert Carpick of the University of Pennsylvania and co-workers in the US, the Netherlands and Switzerland have now explored the frictional properties of four solid lubricants — graphene, molybdenum disulphide (MoS₂), hexagonal boron nitride (h-BN) and niobium diselenide (NbSe₂) — at the nanoscale.

All four lubricants showed similar behaviour, even though NbSe₂ is metallic, MoS₂ is a semiconductor, h-BN is an insulator and graphene is a semimetal. Using friction force microscopy, Carpick and co-workers found that the friction between thin films of the lubricant and a silicon oxide surface increased as the film became thinner. They argue that this is because it is easier for the thinnest layer to deform. The results of measurements of graphene on a mica substrate — to which graphene binds strongly — are consistent with this interpretation.