Nano/Micro-Tribological Properties of Ultrathin Functionalized Imidazolium Ionic Liquid Films on Silicon Wafer

Abstract

Ionic liquids (ILs) are considered as a new kind of lubricant for micro/nanoelectromechanical system (M/NEMS) due to their excellent thermal and electrical conductivity. However, so far, only few reports have investigated the tribological behavior of molecular thin films of various ILs. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick films on a substrate is a critical step for their application in MEMS/NEMS devices. To this end, four kinds of ionic liquid carrying methyl, hydroxyl, nitrite, and carboxyl group were synthesized and these molecular thin films were prepared on single crystal silicon wafer by dip-coating method. Film thickness was determined by ellipsometric method. The chemical composition and morphology were characterized by the means of multi-technique X-ray photoelectron spectrometric analysis, and atomic force microscopic (AFM) analysis, respectively. The nano and micro tribological properties of the ionic liquid films were investigated. The morphologies of wear tracks of IL films were examined using a 3D non-contact interferometric microscope. The influence of temperature on friction and adhesion behavior at nano scale, and the effect of sliding frequency and load on friction coefficient, load bearing capacity and anti-wear durability at micro scale were studied. Corresponding tribological mechanisms of IL films were investigated by AFM and ball-on-plane micro-tribotester. Friction reduction, adhesion resistance and durability of IL films were dependent on their cation chemical structures, wettability and ambient environment.