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Mechanisms of interactions in nano-scale of novel ionic lubricants with functional surfaces

Final Report Summary - MINILUBES (Mechanisms of interactions in nano-scale of novel ionic lubricants with functional surfaces)

See also attached file "216011-Publishable_summary.pdf".

Mechanisms of interactions in nano-scale of novel ionic lubricants with functional surfaces.

In the last decade, ionic liquids have undergone a unique development from the status of exotic substances accessible only to few research groups to the prospect of using ionic liquids in applications such as solvents for chemical synthesis. Till this day, ionic liquids have become a vast and rapidly growing family of chemical compounds, and the object of intense investigations in many fields. Increasing commercial availability has significantly contributed to this rise of attention in the scientific community.
Generally, ionic liquids show no or very low vapour pressures, satisfactory viscosity-temperature behaviour, no inflammability and high thermal stabilities up to 250°C. The choice of cation and anion as well as the design of side chains determine fundamental ionic liquid properties. These properties desirable for lubricants and the possibility to create tailor-made compounds give rise to investigations of ionic liquids in the field of tribology.

Their potential as lubricants was recognised very early starting with ionic liquids involving tetrafluoroborate and hexafluorophosphate as anion. Since then, wear and friction properties of a large number of ionic liquid candidates have been studied, including the elucidation of their tribochemistry, in particular of the bis(trifluoromethylsulfonyl)amide anion. Nowadays, the endeavour to provide halogen-free ionic liquids with comparable tribological performance can be observed.
Although lubricants have to fulfill a multitude of requirements for safe, reliable and long-term use, research on ionic liquid tribology still remains mainly focused on the characterization of wear and friction behavior. The Marie Curie Initial Training Network “Minilubes“ (PITN-GA-2008-216011, 10/2008-09/2012) was initiated as a highly interdisciplinary research group aimed to embrace the complex challenges posed by using ionic liquid as lubricants.

Research fields in Minilubes network were organised in 5 scientific work packages:

Design & Synthesis
Characterisation
Ecological Impact
Modelling & Simulation
Economical Impact

The overall Minilubes objective was to provide the fundament in Europe for the education and employment of young researchers starting their career in a large and cross-disciplinary research group composed of academia, private research institutions and industry. The scientific objectives were defined to provide the knowledge for the implementation of ionic liquids as high performance lubricants.

Research was emphasized on ionic liquids with environmentally benign structures while maintaining tribological performance in terms of low friction and wear as well as meeting economic demands. Selected main results are:
- Synthetic routes for the preparation of monocationic and dicationic ionic liquids with poly(butylene) and poly(ethylene glycol) moieties, respectively, were provided by azide/alkyne “click” reaction.
- Biodegradability and toxicological assays revealed a ranking of cations and anions indispensable for the design of environmentally friendly ionic liquids.
- Structures for thermally stable ionic liquids in long-term use were recommended.
- Miscibility of ionic liquid additives in different base oils, e.g. poly(propyleneglycol)
- Some ionic liquids in lubricant mixtures exhibited lower wear and reduced friction depending on concentration. The effect of anion, alkyl side chain length in the cation, concentration and temperature on tribological properties and corrosion could be shown. For low corrosion and tribocorrosion, lower treat rates are recommended.
- Ionic liquid-surface interactions in terms of tribochemistry are mainly dominated by the anion where exemplarily the formation of beneficial sulphide layers from alkyl sulphonates was revealed and a tribochemical reaction mechanism proposed.
- Halogen-free ionic liquids turned out as potential alternatives to fluorinated ionic liquids for lubrication, in particular the bis(trifluoromethylsulfonyl)amide anion.
- Tetraalkyl ammonium based ionic liquids were studied for their affinity to an iron and carbon surface to provide spatial orientation by modeling and simulation.
- Nano-scale friction properties were obtained by shear “experiments” using non-equilibrium molecular simulations.
- The impact of the type of porosity on the mechanical characteristics of porous materials and their load carrying capacity was demonstrated by mathematical models.
- The benefit of ionic liquids to avoid (undesired) phenomena of fluid-film cavitation was investigated by numerical models applied to lubrication of porous journal bearings.

The systematic research approach defined has revealed structure-property relationships for a multitude of ionic liquid characteristics relevant to establish ionic liquids in tribology, e.g. ease of synthesis, miscibility with base oils, friction and wear performance, biodegradability and toxicity, stability, rheology. This way, appropriate ionic liquids and lubricant formulations for specific tasks can be proposed.
The European research community is at the forefront regarding the development of ionic liquids. For technological exploitation, the findings gained in Minilubes provide substantial support to application-oriented R&D in intense cooperation between private research institutions, academia and in particular industry involved as provider or user of ionic liquids.

Contact to network coordinator:
AC2T research GmbH, Viktor-Kaplan-Str. 2 D, 2700 Wiener Neustadt, Austria;
Tel: +43 2622 81600, e-mail: minilubes@ac2t.at web: www.ac2t.at

More details see Minilubes website: www.minilubes.net