Abstract
There are three unsolved problems in thin film lubrication (TFL) since it was proposed 20 years ago, i.e., the determination of the type of molecules that can enter the contact region efficiently during sliding, the orientation of molecules in the contact region, and the effect of solid surfaces on the liquid molecular orientation in TFL. In order to answer the first two questions, an in situ measurement system comprising a self-designed Raman microscopy and relative optical interference intensity (ROII) system was set up to study the molecular behaviors. A variety of binary mixtures were used as lubricants in the test, and the concentration distribution profile and orientation of the additive molecules in TFL were characterized. The molecular behavior was determined via a combination of shearing, confinement, and surface adsorption. Furthermore, the difference in molecular polarity resulted in different competing effect of surface adsorption and intermolecular interaction, the influence of which on molecular behavior was discussed. Polar additive molecules interacted with the steel surface and exhibited an enrichment effect in the Hertz contact region when added into a nonpolar base oil. No enrichment effect was observed for nonpolar molecules that were added into the nonpolar base oil and polar molecules added into polar base oil. The enrichment of additive molecules enhanced the film-forming ability of the lubricant and resulted in a reduction in the friction coefficient of up to 61%. The orderly arrangement of the additive molecules was another reason for the friction-reducing. A binary multilayer model was proposed to illuminate the molecular behavior in the TFL, and the model was supported by contrary experiment results in elastohydrodynamic lubrication. This research may aid in understanding the nanoscale lubrication mechanism in TFL and the development of novel liquid lubricants.
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The work was financially supported by the National Natural Science Foundation of China (No. 51875303 and No. 51527901).
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Shaohua ZHANG. He obtained his bachelor and Ph.D. degrees from Tsinghua University in 2008 and 2015, respectively. He works as a design engineer since 2015 at Beijing Key Laboratory of Long-life Technology of Precise Rotation and Transmission Mechanisms at Beijing Institute of Control Engineering. His interested research areas include thin film lubrication and tribology of space mechanism.
Yijun QIAO. He obtained his bachelor degree in 2015 from Huazhong University of Science and Technology. After then, he was a Ph.D. student at State Key Laboratory of Tribology of Tsinghua University. His research interests include lubrication mechanism and solid-liquid interfacial structure.
Yuhong LIU. She is an associate professor, received her Ph.D. degree in CAS Key Laboratory of Molecular Nanostructure & Nanotechnology Institute of Chemistry, Chinese Academy of Sciences, Beijing, China in 2005. She joined the State Key Laboratory of Tribology of Tsinghua University from 2005. Her research areas cover nanotribology, nanostructure, and nanotechnology of surface and interface, chemical-mechanical planarization, and water-based lubrication.
Liran MA. She received her BS degree from Tsinghua University in 2005, and received her Ph.D. degree from Tsinghua University in 2010. Following a postdoctoral period at the Weizmann Institute of Science in Israel, she is now working as an associate professor in State Key Laboratory of Tribology, Tsinghua University. Her interests in tribology have ranged from aqueous lubrication and hydration lubrication to the liquid/solid interface properties. She has published over 50 papers. Her work has been cited some 300 times. Her honors include the Hinwin Doctoral Dissertation Award (2011), the Maple leaf award for Outstanding Young Tribologists (2015), and Chang Jiang Scholars Program-Young Professor Award (2015).
Jianbin LUO. He received his BS degree from Northeastern University in 1982, and got his MS degree from Xi’an University of Architecture and Technology in 1988. In 1994, he received his Ph.D. degree from Tsinghua University and then joined the faculty of Tsinghua University. Prof. Jianbin LUO is an Academician of the Chinese Academy of Sciences and a Yangtze River Scholar Distinguished Professor of Tsinghua University, Beijing, China. He was awarded the STLE International Award (2013), the Chinese National Technology Progress Prize (2008), the Chinese National Natural Science Prize (2001), and the Chinese National Invention Prize (1996). Prof. LUO has been engaged in the research of thin film lubrication, superlubricity and tribology in nanomanufacturing. He was invited as a keynote or plenary speaker for more than 20 times on the international conferences.
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Zhang, S., Qiao, Y., Liu, Y. et al. Molecular behaviors in thin film lubrication—Part one: Film formation for different polarities of molecules. Friction 7, 372–387 (2019). https://doi.org/10.1007/s40544-019-0287-1
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DOI: https://doi.org/10.1007/s40544-019-0287-1