Understanding the Friction of Nanometer-Thick Fluorinated Ionic Liquids

Ionic liquids (ILs) show promise as lubricants for nano- and microelectromechanical systems (NEMS/MEMS) and hard disc drives (HDDs). When deposited on solid substrates as nanometer-thick films, these liquids can exhibit a solid-like layering structure, which makes them ideal as lubricants. Moreover, ILs are "designer" materials with many possible molecular structures and tunable properties. In this work, we have experimentally investigated the friction of various nanometer-thick fluorinated ILs on a silica surface using a nanotribometer. We found that surface tension has a significant effect on the friction. When the surface tension of ILs is low, ILs wet the silica surface, and the friction is low. However, when the surface tension is high, ILs form droplets and do not wet the silica surface, which results in higher friction. The effect of moisture at the interface has also been investigated, and the results suggested that water promotes the surface charging of silica, which induces more ordered cation/anion layering and thus reduces the friction. The finding here is critical in designing novel nanometer-thick IL lubricants.