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.
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Wei SONG is supported by China Scholarship Council. The authors would like to thank Dr. Gwilherm KERHERVE of the Advanced Photoelectron Spectroscopy Laboratory, Imperial College London, for his help with XPS. The authors would also like to acknowledge the support from Imperial College Research Computing Service (DOI: 10.14469/hpc/2232).
Declaration of competing interest
The authors have no competing interests to declare that are relevant to the content of this article.
Wei SONG. He received his Ph.D. degree in chemistry in 2021 from Sun Yat-sen University, China. He was a visiting Ph.D. student in Tribology Group in Imperial College London, UK, from November 2019 to October 2021. He recently joined the State Key Laboratory of Tribology in Tsinghua University, China, as a postdoctor. His research interest includes friction modifier (FM) under boundary lubrication.
Jie ZHANG. He received his M.S. degree in tribology and surface engineering from China University of Geosciences (Beijing), China, in 2007. He subsequently joined the Tribology Group in Imperial College London, UK, as a Ph.D. student and completed in 2011. Since 2013, he started to work in the same group as a research associate on different projects. His main research areas cover boundary lubrication, wear, lubricant additives, elasto-hydrodynamic (EHD) lubrication, micro-electro-mechanical system (MEMS), zinc dialkyldithiophosphate (ZDDP), mechanochemistry, and atmospheric tribology.
Sophie CAMPEN. She received her M.S. degree in chemistry and her Ph.D. degree in mechanical engineering from Imperial College London, UK, in 2007 and 2012, respectively. Her current position is the research associate at the University of Cambridge, UK. Her research areas include boundary lubrication, mechanochemistry, and surface adsorption.
Jincan YAN. He received his bachelor’s degree in chemistry and master’s degree in organic chemistry from Huaibei Normal University, China, in 2002 and 2008, respectively, and Ph.D. degree in mechanical engineering from Twente University, the Netherlands, and Ph.D. degree in chemical engineering from Shanghai JiaoTong University, China, in 2014. He is now an associate professor in Shanghai Institute of Technology, China. His research interests include lubricant additives, lubricating oil and grease, cutting fluid, and surface engineering.
Hongbing JI. He received his bachelor’s degree and Ph.D. degree in chemical engineering from South China University of Technology, China, in 1992 and 1997, respectively. His current position is a professor in Sun Yat-sen University, China. His research areas cover biomimetic catalysis, heterogeneous catalysis, homogeneous catalysis, and lubricant additives.
Janet S.S. WONG. She received her Ph.D. degree from University of Illinois at Urbana-Champaign, USA, in 2008. She is now a senior lecturer in the Tribology group, Mechanical Engineering Department, Imperial College London, UK. Her research interest covers phenomena at solid—liquid interface, lubricant rheology, additive technology, mechanochemistry, and in-situ experimentation development.
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Song, W., Zhang, J., Campen, S. et al. Lubrication mechanism of a strong tribofilm by imidazolium ionic liquid. Friction 11, 425–440 (2023). https://doi.org/10.1007/s40544-022-0614-9
- ionic liquid (IL)
- polyethylene-glycol (PEG)
- friction modifiers (FMs)