PerspectiveMaterials Science

Understanding friction in layered materials

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Science  08 May 2015:
Vol. 348, Issue 6235, pp. 632-633
DOI: 10.1126/science.aab0930

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As we move to the digital age, we may forget that a lot of correspondence was accomplished by pencil and paper. Nonetheless, when we do still write with “lead” pencils, we are making use of the weak interlayer forces in graphite to strip off tiny flakes of graphite and deposit them on paper in a complex interplay between adhesion and friction. Understanding the nature of such weak interlayer forces and the attendant opportunities afforded by such understanding are becoming more important with the advent of nano- and microelectromechanical systems (NEMS/MEMS); machines whose moving components are at the nanometer and micrometer scales embodied in their names. Because the surface area of such components is much larger than their volume, surface forces (for example, van der Waals, electrostatic, and capillary forces) become the dominant loading to be considered in the design and reliability of NEMS and MEMS devices (1). On page 679 of this issue, Koren et al. (2) have devised an innovative experiment that quantifies our day-to-day experience with lead pencils, but with much higher fidelity than we might expect. They show that two adjacent planes of carbon atoms can be sheared in highly ordered pyrolytic graphite (HOPG) and observe the interplay between adhesion and friction forces in this layered material. This experiment with HOPG can be considered as a model for two planes of graphene or other so-called two-dimensional materials sliding over one another. The findings are exciting because they point to new ideas for data storage at small scales, as well as novel actuation concepts in electromechanical devices.