Molecularly thick dicationic ionic liquid films for nanolubrication

Abstract

Ionic liquids (ILs) are attractive as lubricants for micro- and nanoelectromechanical systems due to their superior thermal stability and electrical conductivity compared to conventional lubricants. The adhesion, friction and wear properties of two dicationic ILs based on the imidazolium cation and the triflamide anion were studied and compared to the monocationic IL 1-butyl-3-methyl-1H-imidazolium hexafluorophosphate (BMIM-PF6) using atomic force microscopy. The ionic liquid film removal mechanism was investigated by monitoring the friction force, surface potential, and contact resistance. An IL based on the triflamide anion and two imidazolium cations linked by a pentane chain exhibited the best nanolubrication properties. This is attributed to the presence of hydroxyl groups at its chain ends which can hydrogen bond with the surface, and a hydrophobic linker chain. Another dicationic liquid, with a polyether chain linking the cations, had less desirable adhesion, friction, and wear properties compared to the monocationic IL. Intermolecular hydrogen bonding in this IL reduces the chain ordering on the substrate surface, which accounts for the observed higher adhesive force and coefficient of friction relative to the other ionic liquids investigated.