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.

Den globala klimatförändringen är uppenbar och dess konsekvenser är synliga för människor runt om i världen. FN har därför enats om målet att begränsa ökningen av den globala temperaturen. Detta mål ska uppnås genom flera åtgärder, bland annat minskad energiförbrukning och energislöseri. Ett sätt att uppnå detta genom att välja ett lämpligt smörjmedel för dess applicering för att minimera friktionsförluster. Därför är joniska vätskor (IL), särskilt icke-halogenerade IL, lovande kandidater. Förutom deras inneboende joniska egenskaper, så som lågt ångtryck och hög termisk stabilitet, som är önskvärda för smörjmedel. Dessutom är icke-halogenerade IL:er gynnsamma jämfört med halogenerade IL:er, eftersom de senare är föremål för hydrolysprocesser och kan producera frätande föreningar så som halogenider. Icke-halogenerade IL som relativt nya typer av smörjmedel kräver systematiska studier för att öka deras tekniska beredskapsnivå förutom att förstå deras funktion.

Denna doktorsavhandling fokuserar på smörjprestanda hos ortoborat-fosfonium och imidazolium IL. Olika tribologiska kontaktförhållanden undersöktes i ett temperaturområde från 80 ◦ C till 140 ◦ C och för glidavstånd på 200 m, 2000 m och 4300 m, användes kula-på-tre plattor och fram- och återgående kula-på-skiva tribo-tester. Dessutom bestämdes viskositeten och densiteten för dessa ortoborat icke-halogenhaltiga IL för att möjliggöra deras jämförbarhet med vanliga smörjmedel. Bortsett från dessa fysikaliska egenskaper mättes deras jonledningsförmåga för att jämföras med andra IL.

Resultaten visar att den elementära sammansättningen och strukturen hos IL spelar en stor roll på deras smörjbeteende så väl som på deras fysikaliska egenskaper. Alkylkedjelängden och strukturella egenskaper, så som aromatiska ringar, påverkar huvudsakligen fysikaliska egenskaper. Smörjprestandan hos ILs beror dock på deras elementära sammansättning, deras förmåga att självmontera ytan och nedbrytningen av joner. Imidazoliumbaserade IL:er överträffar de fosfoniumbaserade IL:er när det gäller friktion, men motsatsen observeras för slitaget. Detta beteende induceras av nedbrytningsprodukter från IL som orsakas av smörjtester. Till och med produktionen, d.v.s. syntesen, har en effekt på ILs smörjprestanda. Närvaron av ejfullständigt reagerade reaktanter kan främja smörjprestandan på grund av en accelererad nedbrytning av IL. Balansen mellan nedbrytningsprodukter och deras inverkan på friktion och slitage är dock känslig. Märkbar är inverkan av spår av reaktanter på fysikaliska egenskaper hos IL. Beviset på konceptet att ILs är avsevärda gränsfilm-bildande smörjmedel har visats genom smörjtester med IL som tillsatser till polära och icke-polära biologiskt nedbrytbara basoljor. Tillsatsen av ortoboratfosfonium-IL som tillsats minskar slitaget för blandningar med båda genom att skapa slitagereducerande filmer vid gränsytor. Men när det gäller den polära oljan är minskningen liten på grund av konkurrensen mellan joner och de polära basoljemolekylerna om ytarea. Enbart jonerna är kapabla att bilda nötningsreducerande filmer vid gränsytor.

Sammanfattningsvis bidrar detta arbete avsevärt till utforskningen av IL som nya smörjmedel genom att utöka förståelsen för inverkan av strukturella och elementära variationer på deras smörjande prestanda och fysikaliska egenskaper. Som en konsekvens är IL kandidater för mer energieffektiv smörjning.

Lubrication properties of imidazolium and phosphonium bis(oxalato)borate ionic liquids (ILs) are compared in a reciprocating sliding contact at 80 degrees C and 140 degrees C. Both the influence of the alkyl chain length and the cation architecture on friction, wear and lubricant breakdown are investigated. Imidazolium ILs showed lower friction than phosphonium ILs though only phosphonium-based ILs reduced wear. A longer alkyl chain reduced friction only in the case of the imidazolium-based ILs. Analysis of the wear scars was consistent with chemical breakdown solely of the anion. Chemical changes in the ILs after the tribotests were more pronounced for imidazolium-based ILs, and comparison of breakdown and tribofilm formation implicated catalysis by the imidazolium center, which, in turn, had a strong dependence on the surface self-assembly.

The lubricating performance of trihexyl(tetradecyl)phosphonium bisoxalatoorthoborate (P-BOB) ionic liquid is analysed at 80 degrees C and 140 degrees C, and compared to an ionic liquid containing a partially hydrated version of the anion. The reduction of the anions produces oxalate complexes that contribute synergistically to lower friction. The role of oxalate in enhancing lubricity was indicated by the fact that the partially hydrated anion is a precursor orthoborate anion complexed with oxalic acid. It consequently showed the lowest friction at 80 degrees C. Upon heating, the precursor was converted into [BOB]- and displayed the same friction at 140 degrees C as the fully synthesised species. The mechanisms of the breakdown of the [BOB]- anion and formation of the tribofilm are elucidated.

It was found that Li[BOB]center dot nH(2)O salts were not readily suitable for the synthesis of high-purity orthoborate-based tetraalkylphosphonium ionic liquids, as exemplified here for trihexyl(tetradecyl)phosphonium bis(oxalato)borate, [P-6,P-6,P-6,P-14][BOB]; along with [BOB](-), a metastable transition anionic complex (TAC) of dihydroxy(oxalato)borate with oxalic acid, [B(C2O4)(OH)(2)center dot(HOOC-COOH)](-), was also formed and passed into the ionic liquid in the course of the metathesis reaction with trihexyl(tetradecyl)phosphonium chloride. On the contrary, Na[BOB] was found to be a more suitable reagent for the synthesis of this IL, because [BOB](-) anions safely passed into the final IL without hydrolysis, when metathesis reactions were performed using aqueous-free media. Since ultra-pure Na[BOB] is not commercially available, in this work, a preparation protocol for ultra-pure (>99%) Na[BOB] was developed: (i) molar ratios of boric and oxalic acids were optimised to minimise boron-containing impurities, (ii) the Na[BOB] product was thoroughly purified by sequential washing of a fine powder product in hot acetonitrile and ethanol and (iii) characterised using powder X-ray diffraction and solid-state B-11 MAS NMR spectroscopy. The physico-chemical properties of the prepared boron-impurity-free IL, i.e., its density, viscosity, electric conductivity, glass-transition temperature and thermal stability, were found to be significantly different from those of the previously reported [P-6,P-6,P-6,P-14][BOB], containing ca. 45 mol% of TAC, [B(C2O4)(OH)(2)center dot(HOOC-COOH)](-). It was found that a high-purity [P-6,P-6,P-6,P-14][BOB] prepared in this work has a considerably lower viscosity, a higher viscosity index and a wider electro-chemical window (ECW) compared to those of the sample of [P-6,P-6,P-6,P-14][BOB] with ca. 45 mol% of TAC. Interestingly, [B(C2O4)(OH)(2)center dot(HOOC-COOH)](-) in the latter sample almost completely transformed into [BOB](-) anions upon heating of the IL sample at 413 K for 1 hour, as confirmed using both B-11 and C-13 NMR. Therefore, in this work, apart from a well-optimised synthetic protocol for boron-impurity-free [P-6,P-6,P-6,P-14][BOB], implications of boron-containing transition anionic complexes in tetraalkylphosphonium-orthoborate ILs used in different applications were highlighted.

It is demonstrated that a phosphonium orthoborate ionic liquid may serve as a wear reducing additive in biodegradable oils at steel-steel surfaces in the boundary lubrication regime. Tribological tests were performed in a bail-on-three plate configuration. A set of surface characterization techniques-SEM/EDS, FIB and white light interferometry were used to characterize surfaces following the tribotests and to observe the formation of any tribofilms. B-11 NMR was used to follow changes in the composition of the ionic-liquid-oil blends and to identify boron-containing decomposition products after the tribotests. The ionic liquid reduces the wear of steel surfaces by up to 92% compared to the neat oil at 90 degrees C; it is shown that the reduction in wear can be correlated with the formation of boron enriched patches in the boundary films.

We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, silica, and gold. We find that the force response has a monotonic form, from which a characteristic steric decay length can be extracted. The decay length is comparable with the size of the ions, suggesting that a layer is formed on the surface, but that it is diffuse. The long alkyl chains of the cation, the large size of the anion, as well as crowding of the cations at the surface of negatively charged mica, are all factors which are likely to oppose the interfacial stratification which has, hitherto, been considered a characteristic of ionic liquids. The variation in the decay length also reveals differences in the layer composition at different surfaces, which can be related to their surface charge. This, in turn, allows the conclusion that silica has a low surface charge in this aprotic ionic liquid. Furthermore, the effect of temperature has been investigated. Elevating the temperature to 40 °C causes negligible changes in the interaction. At 80 °C and 120 °C, we observe a layering artefact which precludes further analysis, and we present the underlying instrumental origin of this rather universal artefact.

We have studied the rheological behavior of the ionic liquid trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P-66614][BMB], and compared it with that of another ionic liquid, namely trihexyl(tetradecyl)phosphonium chloride, [P-66614][Cl]. The non-halogenated [P-66614][BMB] has been selected as it is known to provide enhanced lubrication performance and is, consequently, of technological importance. The ionic liquid [P-66614][Cl], despite its relatively simple anion, exhibits viscosities very similar to those of [P-66614][BMB], making it an excellent reference fluid for the modeling study. The viscosities of the ionic liquids have been obtained by equilibrium atomistic simulations using the Green-Kubo relation, and by performing nonequilibrium shear flow simulations. The influence of the simulation system size and a reduction of the atomic charges on the viscosities of the ionic liquids are systematically studied. The atomic charges are reduced to mimic the temperature dependent charge transfer and polarization effects. It has been found that scaling the point charges with factors between 0.60 and 0.80 from full ion charges can provide reliable viscosities of [P-66614][BMB], consistent with the experimentally measured viscosities within the studied temperature interval from 373 to 463 K. The viscosities of [P-66614][Cl] have been obtained with scaling factors between 0.80 and 1.0 reflecting the lower polarizability and charge transfer effects of the chloride anion.