Abstract
In thin-film lubrication (TFL), generally, the viscosity of the lubricant and its coefficient of friction (CoF) increase. Finding a method to reduce the CoF in TFL is a significant challenge for tribologists. In the present work, we report a robust superlubricity attained by using polyalkylene glycols (PAGs, polar molecules) and poly-α-olefins (PAOs, nonpolar molecules) as lubricants on steel/steel friction pairs that have been pre-treated by wearing-in with polyethylene glycol aqueous solution (PEG(aq)). A steady superlubricity state with a CoF of 0.0045 for PAG100 and 0.006 for PAO6 could be maintained for at least 1 h. Various affecting factors, including the sliding velocity, normal load, and viscosity of the lubricants, were investigated. Element analysis proved that composite tribochemical layers were deposited on the worn region after the treatment with PEG(aq). These layers were formed by the tribochemical reactions between PEG and steel and composed of various substances including oxides, iron oxides, FeOOH, and Fe(OH)3, which contributed to the superlubricity. In addition to the tribochemical layers, ordered layers and a fluid layer were formed by the PAGs and PAOs during the superlubricity periods. All the three types of layers contributed to the superlubricity, indicating that it was attained in the TFL regime. Accordingly, a mechanism was proposed for the superlubricity of the PAGs and PAOs in the TFL regime in this work. This study will increase the scientific understanding of the superlubricity in the TFL regime and reveal, in the future, the potential for designing superlubricity systems on steel surfaces for industrial applications.
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This work was financially supported by National Natural Science Foundation of China (Nos. 51775295, 51405256, and 51527901).
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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 PhD 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.
Jinjin LI. He received the BS degree in mechanical engineering from University of Science and Technology of China, Hefei, China, in 2009, and the PhD degree in mechanical engineering from Tsinghua University, Beijing, China in 2014. He is currently an assistant professor at Tsinghua University, Beijing, China. His major research area includes solid and liquid superlubricity, nanotribology and friction theory. He has published more than 30 papers on the international journals (21 papers as the first author). He has been awarded the 4th Hiwin award for outstanding doctoral dissertation, first prize for outstanding doctoral dissertation, and outstanding postdoctor in Tsinghua University.
Xiangyu GE. He received the BS degree in mechanical engineering from North China Electric Power University, Beijing, China, in 2010, and the PhD degree from the same university in 2016. He is currently a postdoctor at Tsinghua University, Beijng, China. His major research areas include two-dimensional materials lubrication and liquid superlubricity.
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Ge, X., Halmans, T., Li, J. et al. Molecular behaviors in thin film lubrication—Part three: Superlubricity attained by polar and nonpolar molecules. Friction 7, 625–636 (2019). https://doi.org/10.1007/s40544-018-0254-2
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DOI: https://doi.org/10.1007/s40544-018-0254-2