Effect of cation on micro/nano-tribological properties of ultra-thin ionic liquid films

Room temperature ionic liquids (RTILs) have some unique characteristics which meet the requirements as high performance lubricants. In this work, three kinds of RTILs films with the same anion but different cations were prepared on single-crystal silicon wafer by dip-coating method. Thermal stability of the RTILs was evaluated using thermal gravity analysis in a nitrogen atmosphere. The morphology, nano-friction and nano-adhesion properties of the RTILs films were experimentally investigated at nano-scale using AFM/FFM. Chemical compositions of the films were characterized with a multi-functional X-ray photoelectron spectrometer. Micro-tribological properties of RTILs films were investigated using AISI-52100 steel ball in ball-on-plate configuration, and compared with perfluoropolyether. The worn surface morphologies were measured with a 3D optical surface profilometer. Results show that 3-butyl-1-methyl-imidazolium tetrafluoroborate exhibited the best anti-wear ability in comparison with the other three lubricants. RTILs films could be used as a kind of novel lubricant for application in M/NEMS. The corresponding friction-reduction and anti-wear mechanisms of the tested ultra-thin RTILs films under tested condition were proposed based on the experimental observation. The investigation revealed that friction-reduction and anti-wear properties of RTILs were strongly dependent on their chemical structures. For the friction at nano-scale, the flexibility and surface energy of the lubricant played significant role, while for the friction at micro-scale, both the rigid cycle structure and flexible chain of the RTILs played crucial role.