Ionic Liquids: Surface Forces and Tribological Properties

Room-temperature Ionic liquids (RTILs) are salts composed of very large and asymmetric anions and cations. This bulky structure inhibits their crystallization at room temperature. ILs have a unique combination of characteristics, which make them attractive for many applications such as colloidal stabilization, protein crystallization/ stabilization, and lubrication at extreme temperatures and pressures or under vacuum.

Enlarged view: Figure 1. On the right: an example of ionic liquid: 1-­ethyl-‐3-­methylimidazolium Trifluoro tris(pentafluoroethyl) tris(perfluoroalkyl)trifluorophosphate ([EMIM] FAP). On the right: a droplet of the ionic liquid on gold inside a the ultra-high vacuum chamber of a X-ray photoelectron spectrometer.
Figure 1. On the right: an example of ionic liquid: 1-­ethyl-‐3-­methylimidazolium Trifluoro tris(pentafluoroethyl) tris(perfluoroalkyl)trifluorophosphate ([EMIM] FAP). On the right: a droplet of the ionic liquid on gold inside a the ultra-high vacuum chamber of a X-ray photoelectron spectrometer.

While many of these applications rely on interactions between interfaces across ILs, little is known concerning their molecular origin.

 IL-mediated boundary lubrication is an example where a comprehensive understanding of the structure of the interfacial layer formed by these liquids is still lacking. Equally important for the selection and design of ILs as lubricant for real applications is an understanding of the mechanisms of wear, which are responsible for changes in composition, structure and morphology in the contact area during sliding under ionic liquids.

The Laboratory for Surface Science and Technology has been actively involved in the experimental study of different aspects of IL-mediated lubrication.

Structure, dynamics and nano-tribology in confined layers of Ionic liquids

In collaboration with Prof. Rosa Espinosa-Marzal (University of Illinois at Urbana-Champaign, IL, USA, former member of LSST), the structure of the nano-confined films of ILs, as well as their frictional behavior, has been studied both with the extended surface forces apparatus and by colloidal-probe lateral force microscopy ([1], [2]).

Enlarged view: Figure 2. Time-dependence of film-thickness transitions in [EMIM] FAP confined between mica surfaces, under dry conditions and at 37 %RH (wet). Upon confinement (increasing load), the characteristic time of the transitions at 0% RH becomes significantly larger than at 37 %RH. Measurement carried out by SFA (a schematic of the apparatus is reported in the upper part of the figure)
Figure 2. Time-dependence of film-thickness transitions in [EMIM] FAP confined between mica surfaces, under dry conditions and at 37 %RH (wet). Upon confinement (increasing load), the characteristic time of the transitions at 0% RH becomes significantly larger than at 37 %RH. Measurement carried out by SFA (a schematic of the apparatus is reported in the upper part of the figure)

These investigations elucidated several fundamental aspects of surface forces profiles and frictional behavior of thin films of ILs confined between mica sheets.

As an example, the existence of weak, long-range repulsions in pure ILs and water-containing ILs was evidenced for the first time at LSST (Figure 3a). The exponential decay of this repulsion suggested an electrostatic screening origin, a hypothesis that triggered an intense discussion on the exact description of the phenomenon among different research groups (Gebbie, Matthew A., et al. Proceedings of the National Academy of Sciences 112.24 (2015): 7432-7437).

We also evidenced that, in spite of the hydrophobicity of the ILs, the presence of water  due to exposure to environmental humidity was found to have a significant effect on this and several other properties of the IL interfacial layer. A further example is reported in Figure 2b: upon sliding of mica sheets separated by molecularly thick films of ILs, the friction response is clearly affected by the presence of water; under certain conditions, a complex stick-slip behavior characterized by cumulative microslips as precursors to collective “avalanches” were observed.

Enlarged view: Figure 3: (a) Force isotherms between mica surfaces across anhydrous and water-containing [HMIM] FAP (exposed to 37 % RH (blue) exhibiting long-range repulsion. The lines give the calculated surface force according to the DLVO theory. (b) Friction force between mica surfaces in [EMIM] FAP and [HMIM] FAP under dry conditions (top) and at ambient RH measured (bottom).
Figure 3: (a) Force isotherms between mica surfaces across anhydrous and water-containing [HMIM] FAP (exposed to 37 % RH (blue) exhibiting long-range repulsion. The lines give the calculated surface force according to the DLVO theory. (b) Friction force between mica surfaces in [EMIM] FAP and [HMIM] FAP under dry conditions (top) and at ambient RH measured (bottom).

Ionic-liquid-mediated lubrication at the macroscale

At the macroscale, the interest in ILs as potential new lubricants lubricant emerged in 2001 (Ye, Chengfeng, et al. "Room-temperature ionic liquids: a novel versatile lubricant." Chemical Communications 21 (2001): 2244-2245), mostly driven by the favorable properties commonly exhibited by these fluids—negligible vapor pressure, high thermal stability and non-flammability, among the others— and the observation that these liquids might provide wear-prevention and friction reduction by tribochemical processes and adsorption of ions on polar surfaces.

Despite the significant number of publication produced after these initial findings, it emerges that the current knowledge of the mechanisms of boundary lubrication with ILs is still rather fragmentary. As a result, it is rather difficult to judge the performance of ILs for specific tribopairs and conditions.

In this field, the research activity of our laboratory is aimed at providing insights on the mechanisms of wear and lubrication observed when using ionic liquids as lubricants, taking into account the specific characteristics of the tribosystems (materials, lubricant, environment and operating conditions).

Enlarged view: Figure 4: experimental methods employed for macrotribological investigations
Figure 4: experimental methods employed for macrotribological investigations

To this purpose, the tribological experiments are complemented by a comprehensive characterization of the tribostressed materials by several characterization techniques, such as X‐ray photoelectron spectroscopy and μ-Raman spectroscopy, optical and electron microscopy, and profilometry, atomic force microscopy.

Comparing the results of tribological tests and the related characterization of worn counterparts revealed that largely different behaviors were observed depending on the ionic liquid used as a lubricant and the operating conditions. In addition, as in the case o nanotribological studies, the presence of water dissolved in the liquid as a result of environmental humidity was found to play a very important role.

As an example, some striking differences in the wear mechanism existing when carrying out experiments in the presence of ILs differing in terms of the anion or the water content are illustrated in Figure 5: the occurrence of extended or surface-localized damage are well evidenced by the analysis of the surface composition and morphology, as well as by the analysis of near-surface region. Following this a approach allows to conclude whether a mechanical or a tribochemical mode of wear prevails under different conditions ([3], [4])

Enlarged view: Figure 5: Secondary‐electron micrographs of FIB cross‐section obtained in the direction orthogonal to the wear track o silicon disk lubricated with  [EMIM] EtSO4 or [EMIM] FAP and in the presence of either humid air (45-55% RH) or a nitrogen atmosphere. (b) Survey and (c) High resolution X-ray photoelectron spectra of samples described in (a). Normal load: 4.5 N sliding speed: 50 mm/min, duration: 200 turns, radius: 3.2 mm.
Figure 5: Secondary‐electron micrographs of FIB cross‐section obtained in the direction orthogonal to the wear track o silicon disk lubricated with  [EMIM] EtSO4 or [EMIM] FAP and in the presence of either humid air (45-55% RH) or a nitrogen atmosphere. (b) Survey and (c) High resolution X-ray photoelectron spectra of samples described in (a). Normal load: 4.5 N sliding speed: 50 mm/min, duration: 200 turns, radius: 3.2 mm.

The current interest in the area of IL-mediated lubrication is directed towards a deeper understanding of the structural (micro-fracturing and stress-induced phase transformation and amorphization) and compositional changes (debris formation and mechanical mixing) caused by sliding and their relation with the severity of the wear process.

In addition, the investigation has been recently extended to other silicon-based materials of high technological significance, such as silicon nitride and carbide.  These materials are expected to exhibit several characteristic of the tribochemical reactivity of silica with water, being at the same time not significantly susceptible to stress corrosion cracking.

Another area of interest concerns the analysis of the onset of wear occurring at the transition between a full-fluid film to a mixed regime of lubrication for the various ILs. The study of this phenomenon, for nanometer rough tribopair, might reveal information on the relative roles of specific ions in preventing asperity-asperity contact.

References

 [1]       Espinosa-Marzal, R. M., Arcifa, A., Rossi, A., & Spencer, N. D. (2013). Microslips to “avalanches” in confined, molecular layers of ionic liquids. The journal of physical chemistry letters5(1), 179-184.

[2]        Espinosa-Marzal, R. M., Arcifa, A., Rossi, A., & Spencer, N. D. (2014). Ionic liquids confined in hydrophilic nanocontacts: structure and lubricity in the presence of water. The Journal of Physical Chemistry C118(12), 6491-6503.

[3]        Arcifa, A., Rossi, A., Espinosa-Marzal, R. M., & Spencer, N. D. (2014). Environmental influence on the surface chemistry of ionic-liquid-mediated lubrication in a silica/silicon Tribopair. The Journal of Physical Chemistry C118(50), 29389-29400.

[4]        Arcifa, A., Rossi, A., Espinosa-Marzal, R. M., & Spencer, N. D. (2016). Influence of environmental humidity on the wear and friction of a silica/silicon tribopair lubricated with a hydrophilic ionic liquid. ACS applied materials & interfaces8(5), 2961-2973.

 

JavaScript has been disabled in your browser