event
Friday 24 Aug 2012: Negative nanoscale friction coefficients from reversible local exfoliation of graphite
Dr Rachel Cannara - Project Leader - Nanofabrication Research Group, Center for Nanoscale Science and Technology,National Institute of Standards and Technology (NIST), Gaithersburg, MD USA
Harrison 170 (3D Visualisation Suite) 11:00-11:30
While frictional forces always oppose motion, we observe that friction increases linearly with decreasing load on aged graphite using atomic force microscopy (AFM). This results in an effectively negative nanoscale coefficient of friction. The absolute magnitude of the friction coefficient increases with tip-sample adhesion, which we control by varying oxygen content on the graphite surface. Through molecular dynamics and finite element simulations, we demonstrate that the unusual negative coefficient of friction is a mechanical effect caused by a change in deformability of the top layer of graphite with applied normal load. Viscoelastic waves which dissipate energy during sliding intensify in the top layer of graphite when it is partially (and reversibly) exfoliated by the AFM tip. As the magnitude of this negative coefficient depends on the ratio of the work of adhesion to the exfoliation energy, we will explore the potential use of friction force microscopy for experimentally determining exfoliation energies of layered materials.
http://www.nist.gov/cnst/cannara.cfm
