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Lubrication mechanisms and properties of non-halogenated orthoborate ionic liquids

Time: Fri 2023-06-09 13.00

Location: Gladan, Brinellvägen 85, Stockholm

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Language: English

Subject area: Machine Design

Doctoral student: Patrick Rohlmann , System- och komponentdesign

Opponent: Professor Ali Erdemir, Texas A&M University

Supervisor: Professor Sergei Glavatskih, Maskinkonstruktion; Professor Mark W. Rutland, Yt- och korrosionsvetenskap; Professor Oleg N Antzutkin, Luleå tekniska universitet

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One way to reduce energy consumption and waste - a necessity to achieve the UN-mandated limitation of average global temperature in-creases - is to select suitable lubricants so that energy-losses through friction and wear are minimised. In this context, non-halogenated ionicliquids (ILs) are promising candidates to achieve this aim due to their intrinsic ionic properties, low vapour pressure and high thermal stability. These are properties desirable for lubricants. Non-halogenated ILs area relatively new type of chemical compounds in tribology. Therefore, systematic studies are required in order to elaborate understanding oftheir functional properties and to increase their readiness level for use innew and emerging technologies such as battery electric vehicles.

This doctoral thesis focuses on the lubrication performance of ILs consisting of phosphonium or imidazolium cations and orthoborate anions. Tribological tests were performed with two different tribological configurations (continuous sliding ball-on-three plates tests and reciprocatingsliding ball-on-disc tests) under different tribological contact conditions(temperature, from 80◦ C to 140 ◦ C; sliding distances of 200 m, 2000 mand 4300 m). In addition, dynamic viscosity, density and ionic conductivity of these ILs were determined for comparative purposes e.g. with commonly used lubricants.

The elemental composition and the structure of the anionic and cationic constituents of the ILs determine their lubrication behaviour and their physical properties. The latter are mainly affected by the alkyl chain length of cations and other structural features, such as the presence of aromatic rings in anions. However, the lubricating performance of ILs depends on their elemental composition, their propensity to surface self-assembly and the effects of the breakdown products of these ILs induced by the thermo-mechanical conditions of the tribo-contact. Imidazolium-based ILs outperform phosphonium based ILs in terms of friction-reduction, but the opposite is observed for wear-prevention. This behaviour is attributable to the breakdown products of the imidazolium ILs caused by lubrication tests. Furthermore, contaminants/impurities present in the ILs (such as trace remnants of the precursor moleculesused in the synthesis of ILs) can affect lubrication, either through improved physical properties or through an accelerated breakdown of the ILs under tribo-contact conditions. This is demonstrated with a phosphonium bis(oxalato)-borate IL and the traces of its transition anionic complex (TAC). It is revealed, that there is a delicate balance to be struck between decomposition of the ILs/TAC and the beneficial effects of the decomposition products on friction and wear. This thesis also shows that the non-halogenated phosphonium IL, added to polar and non-polar biodegradable oils, can work as an anti-wear additive by forming boundary films. This effect was only minor in the case of the polar biodegradable oil due to the competition between the ions of the IL and the polar base oil molecules for surface area.

In conclusion, this work contributes significantly to the characterisation of non-halogenated ILs as novel lubricants. It does so by expanding the understanding of the influence of structural and elemental variationson their lubricating performance and physical properties. Furthermore, lubrication mechanisms for phosphonium and imidazolium ILs are suggested. As a consequence, non-halogenated ILs appear to be excellent candidates for more energy-efficient lubrication.