Posted in Technology - Functional Fluids & Additives
Application of ionic liquids as lubricants
Due to their marginal vapour pressure and their non flammability ionic liquids are interesting substances for the lubricant technology. In addition, some ionic liquids possess interesting rheological and tribological properties, a high thermal and chemical stability and high surface tension. The tendency of cavitation is for ionic liquids significantly lower than for other materials. Some ionic liquids are also known to be corrosion inhibitors.
A model of the processes between rubbing surfaces is given in Figure 1. The lubricating oil has the function to minimize the friction between the construction parts and to reduce the wearing of the surfaces.
Figure 1: Model of the friction between two surfaces (a: Bond scission by shearing, b: orientation of the molecules, c: activity of nascent surface, d: exo-electron, e: friction heat).
The lubrication performance of lubricating oils is mainly determined by the viscosity and the sliding velocity. The efficiency of the ionic liquid as lubricant depends on the nature of the cation and the anion. Phosphonium-based ionic liquids have better tribological properties than ammonium- and sulfonium-based ionic liquids and alkyl substituents with long chains increase the tribological efficiency. Unfortunately, long alkyl substituents decrease the thermal stability of the ionic liquids. In general, hydrophobic ionic liquids show better tribological performances than hydrophilic ionic liquids.
Interesting applications for ionic liquids are special lubricants for high vacuum or extreme temperature applications. Ionic liquids can i.e. reduce wear and friction in lubricating an engine piston ring against a flat in comparison to 15W40 diesel engine oil.
Text: Dr. Maria Taige, IOLITEC GmbH, 2012.