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Friction  2022, Vol. 10 Issue (2): 268-281    doi: 10.1007/s40544-021-0486-4
Research Article     
Probing the nanofriction of non-halogenated phosphonium- based ionic liquid additives in glycol ether oil on titanium surface
Xiuhua QIU1,Linghong LU2,Zhenyu QU1,Jiongtao LIAO1,Qi FAN1,Faiz Ullah SHAH3,Wenling ZHANG4,Rong AN1,*()
1 Herbert Gleiter Institute of Nanoscience, Department of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
2 State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, China
3 Chemistry of Interfaces, Lule? University of Technology, Lule? 97187, Sweden
4 Department of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
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The nanofrictional behavior of non-halogentated phosphonium-based ionic liquids (ILs) mixed with diethylene glycol dibutyl ether in the molar ratios of 1:10 and 1:70 was investigated on the titanium (Ti) substrate using atomic force microscopy (AFM). A significant reduction is observed in the friction coefficient μ for the IL-oil mixtures with a higher IL concentration (1:10, μ ~ 0.05), compared to that for the lower concentration 1:70 (μ ~ 0.1). AFM approaching force-distance curves and number density profiles for IL-oil mixtures with a higher concentration revealed that the IL preferred to accumulate at the surface forming IL-rich layered structures. The ordered IL-rich layers formed on the titanium surface facilitated the reduction of the nanoscale friction by preventing direct surface-to-surface contact. However, the ordered IL layers disappeared in the case of lower concentration, resulting in an incomplete boundary layers, because the ions were displaced by molecules of the oil during sliding and revealed to be less efficient in friction reduction.

Key wordsionic liquid      nanofriction      atomic force microscopy (AFM)      ordering      density     
Received: 10 August 2020      Published: 17 January 2022
Fund:  Natural Science Foundation of Jiangsu Province(BK20191289);National Natural Science Foundation of China(21838004);The Swedish Foundation for Strategic Research(EM16-0013)
Corresponding Authors: Rong AN     E-mail:
About author: Xiuhua QIU. He received his bachelor degree in materials science in 2017 from Henan Polytechnic University, China. He joined Herbert Gleiter Institute of Nanoscience, Department of Materials Science and Engineering, at Nanjing University of Science and Technology, as a master student since 2017. He is now focusing on the studies about the nanotribological behavior of ionic liquids.|Linghong LU. She received her master degree in engineering from Harbin Engineering University, China, in 2000, and Ph.D. degree in Science from Zhejiang University, China, in 2005. She joined the College of Chemical Engineering of Nanjing Tech University from 2005. Her current position is a professor. Her research areas cover molecular simulation, structural property of confined fluid, adsorption, separation, and electrode materials.|Zhenyu QU. He is a senior undergraduate student in Department of Materials Science and Engineering at Nanjing University of Science and Technology, China. His research interests include nanotribological properties of different functional materials and heterogeneously structured surfaces.|Jiongtao LIAO. He is a senior undergraduate student in Department of Materials Science and Engineering at Nanjing University of Science and Technology, China. His research interests include ionic liquids behavior at solid interfaces and nano-tribology.|Qi FAN. He is a senior undergraduate student in Department of Materials Science and Engineering at Nanjing University of Science and Technology, China. His research interests include the quantitative analysis of single molecular forces of charged surfaces with ionic liquids and lithium-sulfur batteries.|Faiz Ullah SHAH. He received his Ph.D. degree in Chemistry of Interfaces, Luleå University of Technology, Sweden, in 2011. He joined Chemistry of Interfaces, Luleå University of Technology, from 2011. His current position is an associate professor. His research areas cover the synthesis of ionic liquids and the applications, e.g., batteries, lubricants, and gas separation.|Wenling ZHANG. She received her Ph.D. degree from Inha University, Korea in 2015 and joined University of Alberta, Canada as a postdoc (2018-2019). She is currently a professor at the School of Mechanical Engineering, Nanjing University of Science and Technology. Her research interests focus on soft matters (electro/ magneto-rheological phenomena) and nanotribology (adhesion, friction, and lubrication).|Rong AN. She received her Ph.D. degree in chemical engineering from Nanjing Tech University, China, in 2013. Then she worked as a postdoc researcher in Chemical and Biomolecular Engineering at North Carolina State University from 2013 to 2015. She joined Herbert Gleiter Institute of Nanoscience, Department of Materials Science and Engineering, at Nanjing University of Science and Technology from 2015. Her current position is an associate professor. Her research interests include the nanotribology of ionic liquids at solid interfaces, liquid-solid interfacial phenomena, and gas separation, etc.
Cite this article:

Xiuhua QIU,Linghong LU,Zhenyu QU,Jiongtao LIAO,Qi FAN,Faiz Ullah SHAH,Wenling ZHANG,Rong AN. Probing the nanofriction of non-halogenated phosphonium- based ionic liquid additives in glycol ether oil on titanium surface. Friction, 2022, 10(2): 268-281.

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Fig. 1 Structural illustration and abbreviations of [BOB]-, [BScB]-, [BMB]-, [DCA]-, [P4,4,4,8]+, [P6,6,6,14]+, as well as the base oil DEGDBE.
Fig. 2 AFM topographic images for (a) pristine bare Ti substrate and neat DEGDBE oil coated Ti, and (b-f) the IL-oil mixtures coated Ti (1:70, 1:10).
Fig. 3 High-resolution spectra of O 1s, P 2p, and B 1s + P 2s scans for [P6,6,6,14][BScB]-oil mixtures on Ti (1:70, 1:10).
Fig. 4 Comparison of ATR-FTIR spectra for the bare Ti substrate, neat oil, neat IL [P6,6,6,14][BScB], and [P6,6,6,14][BScB]-oil mixtures on Ti.
Fig. 5 Friction force measurements for (a) the bare Ti substrate and neat DEGDBE oil on Ti surface, (b) ILs-oil mixtures on Ti surface with varying molar ratio of the IL in the DEGDBE oil with silicon nitride AFM tip. The fitting slope in (a) and (b) is μ. The raw friction data has been listed in Fig. S4 and Table S1 in the ESM.
Molar ratio of IL to oil1:701:10
Bare Ti0.23 ± 0.001
Neat oil0.14 ± 0.005
[P6,6,6,14][BScB]0.11 ± 0.0010.058 ± 0.004
[P6,6,6,14][DCA]0.10 ± 0.0010.052 ± 0.001
[P6,6,6,14][BOB]0.14 ± 0.0010.060 ± 0.001
[P6,6,6,14][BMB]0.12 ± 0.0030.063 ± 0.002
[P4,4,4,8][BScB]0.10 ± 0.0030.056 ± 0.001
Table 1 Nanofriction coefficients of the bare Ti substrate, neat DEGDBE oil on Ti surface, and ILs-oil mixtures (1:70, 1:10) on the Ti surface.
Fig. 6 Representative approaching force-distance curves for ILs-oil mixtures (1:70 and 1:10) coated Ti surfaces with AFM colloidal probes. (a) [P6,6,6,14][BScB], and the left panel is an AFM glass colloidal probe approaching the liquid film of [P6,6,6,14][BScB]-oil mixture (1:70) on the Ti surface (A-D), the probe being retracted after contact (shown in Fig. S3 in the ESM). The inset is a borosilicate glass microsphere attached to a tipless Si3N4 cantilever with a dimension of 20 μm; (b) [P6,6,6,14][DCA]; (c) [P6,6,6,14][BOB]; (d) [P6,6,6,14][BMB]; and (e) [P4,4,4,8][BScB]. The blue and grey arrows correspond to two interfacial layers.
Fig. 7 Number density profiles of cations, anions, as well as the oil in [P6,6,6,14][BScB]-oil mixtures (1:10 molar ratio) confined between the bilayer graphene tip and rutile (110) surface at a slit pore width of 10.7 nm.
Fig. 8 Schematic illustration of the IL-oil mixtures (molar ratio of 1:70, 1:10) distributed at Ti interfaces. The thickness of the ordered IL layers (tL) is composed of dense and loose layers. tLD: the thickness of the near surface denser layer; tLL: the thickness of the upper slightly looser layer.
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