1KTH Royal Institute of Technology, Stockholm, Sweden
The lubricant design process requires availability of a wide range of molecular building blocks. Ionic liquids, materials composed entirely of ions but liquid under the conditions used, significantly expand the range of available species. A great variety of cations and anions and their combinations is possible, opening up the potential for the efficient tuning of the tribochemical reactions in the lubricated contacts. They also bring new properties into lubricant design process that enable additional functionality. Their ionic nature facilitates ion-surface interactions promoting rapid growth of the ionic boundary films.
As lubricant additives, ionic liquids are also expected to primarily follow the surface adsorption mechanism for friction reduction. However, this process is strongly affected by the oil or grease type used to prepare an ionic lubricant. To show the differences, we compare phosphonium ionic liquids with orthoborate and phosphate anions in terms of their interfacial film formation: physisorbed, hybrid and sacrificial from chemical breakdown, in both sliding and rolling contacts. A quaternary phosphonium cation decorated with four long alkyl chains is used as such cations have been shown to support miscibility in oil and exhibit good lubricating properties. The lubrication comparison has been carried out for the ionic liquids added to oil and greases. Analysis of the obtained results reveals how contact conditions and ionic liquid carrier chemistry influence lubricating functionality of the ionic lubricant.