Interactions
among antiwear additives (AWs), friction modifiers
(FMs), and dispersant in a lubricating oil are critical for tribological
performance. This study investigates compatibilities of three oil-soluble
ionic liquids (ILs, candidate AWs) with an FM, molybdenum dithiocarbamate
(MoDTC), and a dispersant, polyisobutene succinimide (PIBSI) under
boundary lubrication. Either synergistic or antagonistic effects were
observed depending on the IL’s chemistry. Adding an aprotic
phosphonium–alkylphosphate or phosphonium–alkylphosphinate
IL into the oil containing MoDTC and PIBSI had detrimental impact
on the friction and wear behavior. PIBSI was found to preferably interact/react
with the aprotic IL to lose its ability of suspending MoDTC and to
partially consume or even deplete the IL. In contrast, a protic ammonium–alkylphosphate
IL seemed to be able to coexist with PIBSI and work synergistically
with MoDTC, yielding a sustainable, ultralow boundary friction. A
three-stage tribochemical process is proposed to explain how this
IL + MoDTC pair interacts with the contact surface to form a chemically
reacted, wear-protective tribofilm supporting a physically adsorbed,
friction-reducing film on top. This study provides fundamental insights
of the compatibilities among three common lubricant components, antiwear,
friction modifier, and dispersant, which can be used to guide future
lubricant development.