Lubrication mechanism of a strong tribofilm by imidazolium ionic liquid

Friction modifiers (FMs) are surface-active additives added to base fluids to reduce friction between rubbing surfaces. Their effectiveness depends on their interactions with rubbing surfaces and may be mitigated by the choice of the base fluid. In this work, the performance of an imidazolium ionic liquid (ImIL) additive in polyethylene-glycol (PEG) and 1,4-butanediol for lubricating steel/steel and diamond-like-carbon/diamond-like carbon (DLC—DLC) contacts were investigated. ImIL-containing PEG reduces friction more effectively in steel—steel than DLC—DLC contacts. In contrast, adding ImIL in 1,4-butanediol results in an increase in friction in steel—steel contacts. Results from the Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and focused ion beam-transmission electron microscopy (FIB-TEM) reveal that a surface film is formed on steel during rubbing in ImIL-containing PEG. This film consists of two layers. The top layer is composed of amorphous carbon and are easily removed during rubbing. The bottom layer, which contains iron oxide and nitride compound, adheres strongly on the steel surface. This film maintains its effectiveness in a steel—steel contact even after ImIL additives are depleted. Such film is not observed in 1,4-butanediol where the adsorption of ImIL is hindered, as suggested by the quartz crystal microbalance (QCM) measurements. No benefit is observed when the base fluid on its own is sufficiently lubricious, as in the case of DLC surfaces. This work provides fundamental insights on how compatibilities among base fluid, FM, and rubbing surface affect the performance of IL as surface active additives. It reveals the structure of an ionic liquid (IL) surface film, which is effective and durable. The knowledge is useful for guiding future IL additive development.