Title: Ultra-low friction of gold nanocrystals on graphene
Abstract: Any mechanical system in which an object rubs against a surface suffers from friction. Frictional forces convert kinetic energy to internal energy and causes wear, decreasing the energy efficiency and the lifetime of interfacing parts. The economic impact of friction is extraordinary: it is estimated that improved research on friction will save countries up to 1.6 % of their GDP, corresponding to $268 billion for the United States. Even a small improvement can have a significant economical impact! A simple phenomenological formula, F=µN, whereµ is the coefficient of friction and N is the normal force, governs friction. Yet, the fundamental mechanism for the determination of µ remains elusive. Since contacts between sliding surfaces can be modeled as an ensemble of nanoscale contacts, many researchers have focused on friction at nanoscale to enable rational reduction of friction and wear. Recent calculations [Guerra et al, Nature Materials, 9 634 (2010)] have predicted that gold nanocrystals slide on graphite with two radically different friction coefficients depending on their speeds. At low speeds (~µm/sec), nanocrystals on graphite are expected to possess higher friction, consistent with previous studies of thermal diffusion of gold on graphite and on graphene. At high sliding speeds in the range of 100 m/sec, nanocrystals are expected to behave radically differently, with a vanishing drag and, therefore, minimal friction. Such transition to ultra low friction at high-speeds can be used to reduce friction and wear and control motion at the nanoscale. I have measured friction of gold nanocrystals with diameter ranging from 3 to 5 nm on graphene at speeds up to 35.6 cm/sec*. The friction at high speeds is found to be an order of magnitude lower (!) than predicted previously by Guerra et al. I will discuss our measurement technique, the experimental results, the origin of the observed friction (which is even lower than expected!), and possible potential applications of graphene in friction reduction technologies. *Results reported in Scientific Reports, 6, 31837 (2016).