Effect of anion on micro/nano-tribological properties of ultra-thin imidazolium ionic liquid films on silicon wafer

Four kinds of room temperature ionic liquids (RTILs), as a new kind of lubricant for micro/nano-electromechanical system, with the same imidazolium cation but carrying different anions including hexafluorophosphate, tetrafluoroborate, nitrate and perchlorate were synthesized and these nano-scale films were prepared on single-crystal silicon wafer by dip-coating method. Atomic force microscopy was used to examine the morphologies of the films and evaluate their nano-friction and nano-adhesion properties. Chemical compositions of the films were characterized with a multi-functional X-ray Photoelectron Spectrometer. Micro-tribological properties of RTIL films were evaluated using a ball-on-plate micro-tribometer, and compared to that of perfluoropolyether. Results show that 3-butyl-1-methyl-imidazolium hexafluorophosphate exhibited the best anti-wear ability in comparison with the other four lubricants, so the ionic liquid films could be used as a kind of novel lubricant for application in MEMS/NEMS. Friction-reduction, adhesion resistance and anti-wear properties of RTILs were closely related to their anions. For the friction at nano-scale, the surface energy of the lubricant played a significant role, while the durability and friction-reduction of IL films strongly depend on their anions at micro-scale.