Tribological Behavior of the Ionic Liquid of 1,1’-(hexane-1,6-diyl)bis(3-ethyl-1H-imidazolium-1-yl) Diphosphonate as a Lubricant

The dicationic ionic liquid of 1,1’-(hexane-1,6-diyl)bis(3-ethyl-1H-imidazolium -1-yl) diphosphonate was prepared and characterized by FT-IR and 1H-NMR. The viscosity and density of the synthetic DIL were measured. Tribological properties of this dicationic ionic liquid as a new kind of promising lubricant for a steel／steel contact were investigated on an Optimal SRV friction and wear tester. The morphology and element analysis of the worn scar surface were characterized by scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS), respectively. The lubricating mechanism of the ionic liquids was proposed. The experimental data showed that this dicationic ionic liquid possessed some advantages, e.g. relatively high viscosity index and low pour point. And the synthetic dicationic ionic liquid showed excellent tribological behavior and was superior to PAO4 in terms of friction-reduction and load-carrying capacity. A boundary lubricating film of ionic liquid was formed on the steel worn surface by physical adsorption, while tribochemical reaction of the ionic liquid with the steel surface was involved at an elevated temperature to generate a boundary lubricating film containing FePO4 and Fe4(P2O7)3. This contributed to reduced friction and wear.