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<p>Ionic liquids have garnered significant interest from the tribological community due to their exceptional physico-chemical properties, such as high thermal stability, tuneable viscosity, and strong surface adsorption. These properties make them promising candidates for advanced lubrication systems, offering potential to reduce friction and wear under diverse operating conditions. Here, the tribological behaviour of two ionic liquids (ILs), [BMIM][PF<sub>6</sub>] and [BMIM][TFSI], was investigated across macro-, micro-, and nano-scales, focusing on lubrication regimes and their influence on friction. At the macro-scale, mixed lubrication dominated, with friction decreasing with sliding velocity due to the formation of a fluid film that partially separates the two surfaces; the friction force in this case is related to the ability of the IL to form thicker films and therefore, the IL viscosity. Conversely, at the nano- and micro-scales, boundary lubrication prevailed under much lower velocities and higher contact pressures (at the nano-scale), with adsorbed ion layers mitigating friction. The different friction coefficients of the two ILs are attributed to their different interaction with the steel surface. To eliminate the influence of the hydrodynamic film formation, the friction force at the macro-scale was extrapolated to 1&#xa0;µm/s, i.e. within the range of velocities probed at nano- and micro-scale. In this regime, a correlation between friction and load across 8 orders of magnitude for each IL was observed. The relevance of plastic deformation and scratching at the level of asperity contacts underlying the tribological performance at the three scales was highlighted and described using the Bowden &amp; Tabor model. Differences between the correlations of the two ILs were attributed to variations in shear strength and the structure of the adsorbed boundary films, with [BMIM][TFSI] forming more cohesive and lubricious layers at low loads. The results emphasize the interplay between lubrication regimes, contact mechanics, and material properties in determining tribological performance across scales, offering insights for advanced lubricant design.</p> Graphical abstract <p></p>

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Multi-scale Study of the Lubricious Behaviour of Two Imidazolium-Based Ionic Liquids, [BMIM][PF6] and [BMIM][TFSI]

  • 期刊论文

  • 发表日期：

    2026年3月6日

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  • Calixe Bénier,
  • Qianlu Zheng,
  • Anna Igual-Muñoz,
  • Rosa Espinosa-Marzal,
  • Stefano Mischler

摘要

Ionic liquids have garnered significant interest from the tribological community due to their exceptional physico-chemical properties, such as high thermal stability, tuneable viscosity, and strong surface adsorption. These properties make them promising candidates for advanced lubrication systems, offering potential to reduce friction and wear under diverse operating conditions. Here, the tribological behaviour of two ionic liquids (ILs), [BMIM][PF6] and [BMIM][TFSI], was investigated across macro-, micro-, and nano-scales, focusing on lubrication regimes and their influence on friction. At the macro-scale, mixed lubrication dominated, with friction decreasing with sliding velocity due to the formation of a fluid film that partially separates the two surfaces; the friction force in this case is related to the ability of the IL to form thicker films and therefore, the IL viscosity. Conversely, at the nano- and micro-scales, boundary lubrication prevailed under much lower velocities and higher contact pressures (at the nano-scale), with adsorbed ion layers mitigating friction. The different friction coefficients of the two ILs are attributed to their different interaction with the steel surface. To eliminate the influence of the hydrodynamic film formation, the friction force at the macro-scale was extrapolated to 1 µm/s, i.e. within the range of velocities probed at nano- and micro-scale. In this regime, a correlation between friction and load across 8 orders of magnitude for each IL was observed. The relevance of plastic deformation and scratching at the level of asperity contacts underlying the tribological performance at the three scales was highlighted and described using the Bowden & Tabor model. Differences between the correlations of the two ILs were attributed to variations in shear strength and the structure of the adsorbed boundary films, with [BMIM][TFSI] forming more cohesive and lubricious layers at low loads. The results emphasize the interplay between lubrication regimes, contact mechanics, and material properties in determining tribological performance across scales, offering insights for advanced lubricant design.

Graphical abstract