Abstract
Phosphonium ionic liquids with orthoborate anions have been studied in terms of their interfacial film formation, both physisorbed and sacrificial from chemical breakdown, in sheared contacts of varying harshness. The halogen-free anion architecture was varied through (i) the heteronuclear ring size, (ii) the hybridisation of the constituent atoms, and (iii) the addition of aryl functionalities. Time of Flight-Secondary Ion Mass Spectrometry analysis revealed the extent of sacrificial tribofilm formation allowing the relative stability of the ionic liquids under tribological conditions to be determined and their breakdown mechanisms to be compared to simple thermal decomposition. Overall, ionic liquids outperformed reference oils as lubricants; in some cases, sacrificial films were formed (with anion breakdown a necessary precursor to phosphonium cation decomposition) while in other cases, a protective, self-assembly lubricant layer or hybrid film was formed. The salicylate-based anion was the most chemically stable and decomposed only slightly even under the harshest conditions. It was further found that surface topography influenced the degree of breakdown through enhanced material transport and replenishment. This work thus unveils the relationship between ionic liquid composition and structure, and the ensuing inter- and intra-molecular interactions and chemical stability, and demonstrates the intrinsic tuneability of an ionic liquid lubrication technology.
Highlights
Phosphonium ionic liquids with orthoborate anions have been studied in terms of their interfacial film formation, both physisorbed and sacrificial from chemical breakdown, in sheared contacts of varying harshness
The work described has considered the effect of the structure of the orthoborate anion of phosphonium ionic liquids on their tribology
The orthoborate core was shown to be important in determining the chemical stability of the ionic liquids, with the size of the heteronuclear ring and the hybridisation of the constituent atoms being significant
Summary
Phosphonium ionic liquids with orthoborate anions have been studied in terms of their interfacial film formation, both physisorbed and sacrificial from chemical breakdown, in sheared contacts of varying harshness. Ionic liquids are a broad family of compounds based on ionic species that are liquid at accessible temperatures (generally below an arbitrary specific value, such as the boiling point of w ater1,2) though the definition of the term is dynamic[3]. With such fluidity in the definition, it is challenging to identify the first discovery of ionic liquids, though many cite Walden’s preparation of ethylammonium nitrate[4]; in that work, the properties of a salt with a melting temperature of ca. There are numerous studies, showing that ionic liquids are competitive with conventional oil-based formulations[15] in efficiently reducing friction and/or wear
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