From the early tribological studies of Leonardo da Vinci to Amontons’ law, friction has been shown to increase with increasing normal load. This trend continues to hold at the nanoscale, where friction can vary nonlinearly with normal load1,2. Here we present nanoscale friction force microscopy (FFM) experiments for a nanoscale probe tip sliding on a chemically modified graphite surface in an atomic force microscope (AFM). Our results demonstrate that, when adhesion between the AFM tip and surface is enhanced relative to the exfoliation energy of graphite, friction can increase as the load decreases under tip retraction. This leads to the emergence of an effectively negative coefficient of friction in the low-load regime. We show that the magnitude of this coefficient depends on the ratio of tip–sample adhesion to the exfoliation energy of graphite. Through both atomistic- and continuum-based simulations, we attribute this unusual phenomenon to a reversible partial delamination of the topmost atomic layers, which then mimic few- to single-layer graphene. Lifting of these layers with the AFM tip leads to greater deformability of the surface with decreasing applied load. This discovery suggests that the lamellar nature of graphite yields nanoscale tribological properties outside the predictive capacity of existing continuum mechanical models.
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The authors thank F. Sharifi for providing invaluable insights regarding this work, and K. Steffens for XPS measurements. We further thank R. Carpick, C. Righi and J. Killgore for stimulating discussions. N. Faralli created the rendering software used to visualize the molecular dynamics simulations. Z.D. acknowledges support under the Cooperative Research Agreement between the University of Maryland and the National Institute of Standards and Technology Center for Nanoscale Science and Technology, Award 70NANB10H193, through the University of Maryland. This research was performed while A.S. held a National Research Council Postdoctoral Research Associateship at NIST. Q.L. and X-Q.F. acknowledge support from the National Natural Science Foundation of China (11272177), the Thousand Young Talents Program (20121770071) and the 973 Program 2010CB631005, 2012CB934101 and 2013CB933003.
The authors declare no competing financial interests.
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Deng, Z., Smolyanitsky, A., Li, Q. et al. Adhesion-dependent negative friction coefficient on chemically modified graphite at the nanoscale. Nature Mater 11, 1032–1037 (2012). https://doi.org/10.1038/nmat3452
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