Influence of heat treatment on the micro/nano-tribological properties of ultra-thin ionic liquid films on silicon

The performance of micro- and nano-electromechanical systems (M/NEMS) depends on the surface and interface properties of the substrate, such as chemical composition, roughness, friction, adhesion, and wear. In order to solve these problems and improve the performance of M/NEMS, molecularly thin films of room temperature ionic liquid (RTIL)-1,3-di(2-hydroxyethyl)imidazolium hexafluorophosphate which has two terminal hydroxyl groups were prepared on silicon substrate. Thermal stability of the RTIL was evaluated using thermogravimetric analysis in a nitrogen atmosphere. A multi-functional X-ray photoelectron spectrometer was used to investigate the chemical compositions of the films. The morphology, nano-friction and nano-adhesion properties of RTIL films with different heat treatment were experimentally investigated at nanoscale using atomic force microscopy/friction force microscopy. The wear-resistant property was tested on a ball-on-plate microtribometer. The results revealed that the micro/nano-friction and adhesion properties of RTIL films were significantly improved with appropriate heat treatment. The corresponding friction reduction and anti-adhesion mechanisms of the tested ultra-thin RTIL films under tested condition were proposed based on the experimental observations. For the micro/nano-friction, bonding ratio of the lubricant film had great effect on the RTIL's performance.