Solvent-free ionic silica nanofluids: Smart lubrication materials exhibiting remarkable responsiveness to weak electrical stimuli

Solvent-free ionic nanofluids (NFs) are promising smart lubrication materials due to their remarkable stimuli responsiveness to an external electric field. In this study, four kinds of silica (SiO2) NFs with diverse corona molecule structures were successfully synthesized. Effects of NFs’ structures on their physic-chemical properties and lubricity were investigated. In particular, nanostructures of the tribofilms were comprehensively characterized for shedding light on lubricating mechanisms of the NFs. Our results demonstrate that the newly synthesized NFs exhibit excellent lubrication performance and their lubricity displays remarkable electrical responsiveness. Applying even a rather low electrical potential, e.g. 1.5 V, gives rise to significant and immediate friction reduction. Moreover, lubricity and electrical responsiveness of the NFs are closely dependent on structures of the corona layers. Interestingly, our work provides direct evidence that feeding NFs onto the friction zone expedites growth of a robust tribofilm consisting mainly of SiO2 nanoparticles and tribochemical products. This work opens up new opportunities for tuning structures and functionalities of boundary layers of numerous motion components by tailoring architectures of NFs and controlling the electrical stimuli.