Molybdenum disulfide (MoS2) is widely regarded as an effective material for reducing friction. It is used in lubricated contacts, either directly as an additive or via an oil-soluble precursor (molybdenum dithiocarbamate, MoDTC) which forms a MoS2 tribofilm during operation. However, specific strategies for implementing its use in lubricating greases, where challenges often occur due to complex systems involving grease thickeners that actively participate in lubrication processes, are rarely discussed. This study evaluates two distinct grease systems, a lithium complex (LiX) grease and a polypropylene (PP) grease, assessing the effectiveness of various MoS2 additivation strategies under high-load rolling–sliding conditions. The tribological performance is assessed through traction tests and surface analyses, including Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The results show that additive performance in greases depends not only on the chemistry of the additive but also on interactions with the thickener matrix and co-additives. In particular, zinc dialkyldithiophosphate (ZDDP) plays a key role, as its tribofilm is essential for the formation of an effective MoS2 tribofilm. For lithium grease, the only way to achieve friction reduction using MoS2 is to incorporate it directly as an additive, i.e. , ex tribo origin. However, with polymer-thickened grease, much lower traction levels can be attained through in tribo MoS2 generation. A further reduction in the traction coefficient can be achieved when both additivation strategies are used together, i.e. , synergy between ex tribo and in tribo . The extent of these gains depends on the thickener system, making a combined strategy almost unnecessary for the polymer-thickened system. Overall, the study emphasizes the need to optimize additivation strategies for each system.

Lithium salts are by far the most common type of thickener in lubricating greases. The battery industry is placing increasing demands on lithium supply and pricing, however, and a probable toxin classification of precursor material looms. It is thus important to explore a transition to a lithium-independent lubricant industry. In addition to providing the consistency properties of grease, a primary function of the thickener is to act as a passive lubricant supply system, releasing oil into the contact and making functional additives available to the surface as required. In this study, the performance of lithium complex grease was stress-tested by subjecting it to highly loaded rolling-sliding contact conditions that are now commonplace in modern machines. Tribological evaluation and Time-of-Flight Secondary Ion Mass Spectrometry analysis of tribofilms together reveal serious limitations in the properties of lithium thickeners; instead, a lithium-free grease matrix system, based on polypropylene, is shown to display better lubricating performance, rendering it a viable and more effective alternative.

Swelling force is a critical factor impacting battery capacity and safety degradation. Battery designers utilize battery pads to mitigate the effects of swelling force within the module by absorbing stress and preventing thermal runaway. Battery pad mechanical properties are essential for accurately predicting swelling forces, as certain models incorporate these characteristics. However, no prior research has explored the evolution of the mechanical properties of pads subjected to cyclic loading from the charging and discharging processes of the battery. This study investigates the evolution of mechanical properties in circular silicone foam battery pads subjected to cyclic compression, simulating charging-discharging cycles, at 20°C and 60°C. Samples were cyclically compressed, and strain energy density and tangent modulus were evaluated. Two-way ANOVA analysis of the test results demonstrates that increasing the number of cycles significantly decreases both strain energy density (p < 0.05) and tangent modulus (p < 0.05). The strain energy density decreased exponentially with cycle count, while temperature primarily influenced the tangent modulus only in the elastic region (p < 0.05). Moreover, there is no interaction between the cyclic loading and temperature factors. These findings provide a basis for enhancing swelling force prediction models by incorporating the evolution of pad mechanical properties with cycling and temperature, leading to more robust battery module designs and improved lifespan predictions.

Ionic liquids (ILs) have emerged as viable solutions for developing new-age lubricants, both as neat lubricants and lubricant additives. Enabled by the presence of discrete ions, ILs have the possibility to render electrically conductive lubricants, which is a feasible strategy for developing lubricant systems compatible with modern e-drive conditions. However, this requires the characterization of the electrical properties of lubricants, which is a bottleneck for developing electrically conductive greases, given their complex architecture. This work introduces an electrochemical impedance spectroscopy measurement methodology to evaluate grease samples’ electrical properties. Compared to the commonly used conductivity meters, this method, through its multi-frequency alternating current (AC) impedance approach, can effectively distinguish the individual contributions of the bulk and the sample-electrode interface to the measured electrical response. Impedance spectra of grease samples are obtained using an electrochemical cell with parallel plate electrodes, mounted on a temperature-controlled cell stand and coupled with a potentiostat. The grease's bulk conductivity is extracted by fitting the impedance data to relevant equivalent electrical circuits. The bulk conductivity of lithium complex grease doped with ILs is evaluated and compared to greases with conventional conductivity additives (copper powder and conductive carbon black). The analysis of temperature-dependent conductivity reveals the rather different conductivity mechanisms for different additives. For greases doped with ILs, a comparison against the electrical conductivity of neat ILs reveals that, in addition to the ion dissociation, the interaction of the ions with the different grease components (base oil, thickener) is crucial in defining the grease conductivity.

The effect of electric potential on the lubrication of a non-halogenated phosphonium orthoborate ionic liquid used as an additive in a biodegradable oil was studied. An in-house tribotronic system was built around an instrument designed to measure lubricant film thickness between a rolling steel ball and a rotating silica-coated glass disc. The application of an electric field between the steel ball and a set of customized counter-electrodes clearly induced changes in the thickness of the lubricant film: a marked decrease at negative potentials and an increase at positive potentials. Complementary neutron reflectivity studies demonstrated the intrinsic electroresponsivity of the adsorbate: this was performed on a gold-coated silicon block and made possible in the same lubricant system by deuterating the oil. The results indicate that the anions, acting as anchors for the adsorbed film on the steel surface, are instrumental in the formation of thick and robust lubricating ionic boundary films. The application of a high positive potential, outside the electrochemical window, resulted in an enormous boost to film thickness, implicating the formation of ionic multi-layers and demonstrating the plausibility of remote control of failing contacts in inaccessible machinery, such as offshore wind and wave power installations.

Four nonhalogenated ionic liquids (ILs) based on the same phosphonium cation are investigated in terms of the anion suitability for enhancing the lubricity of a biodegradable oil. For all test conditions, typical for industrial machine components, the lubrication is shown to be governed by nonsacrificial films formed by the physisorption of ionic species on the tribo-surfaces. The anionic structure appears to have an important role in the formation of friction modifying films. The orthoborate ILs exhibit the formation of robust ionic boundary films, resulting in reduced friction and better wear protection. On the contrary, the surface adsorption of phosphinate and phosphate ILs appears to antagonistically disrupt the intrinsic lubrication properties of the biodegradable oil, resulting in high friction and wear. Through additional investigations, it is postulated that the higher dissociation of orthoborate ILs in the biodegradable oil allows the formation of hierarchical and electrostatically overscreened layer structures with long-range order, whereas the ILs with phosphate and phosphinate anions exhibit low dissociation in biodegradable oil, possibly due to the ion pairs being surrounded by a hydrocarbon halo, which presumably results in weak adsorption to form a mixed interfacial layer with no long-range order.

Neutron reflectivity (NR) measurements have been employed to study the interfacial structuring and composition of electroresponsive boundary layers formed by an ionic liquid (IL) lubricant at an electrified gold interface when dispersed in a polar solvent. The results reveal that both the composition and extent of the IL boundary layers intricately depend on the bulk IL concentration and the applied surface potential. At the lowest concentration (5% w/w), a preferential adsorption of the IL cation at the gold electrode is observed, which hinders the ability to electro-induce changes in the boundary layers. In contrast, at higher IL bulk concentrations (10 and 20% w/w), the NR results reveal a significantly larger concentration of the IL ions at the gold interface that exhibit significantly greater electroresponsivity, with clear changes in the layer composition and layer thickness observed for different potentials. In complementary atomic force microscopy (AFM) measurements on an electrified gold surface, such IL boundary layers are demonstrated to provide excellent friction reduction and electroactive friction (known as tribotronics). In agreement with the NR results obtained, clear concentration effects are also observed. Together such results provide valuable molecular insight into the electroactive structuring of ILs in solvent mixtures, as well as provide mechanistic understanding of their tribotronic behaviours.

Using neutron reflectivity, the electro-responsive structuring of the non-halogenated ionic liquid (IL) trihexyl(tetradecyl)phosphonium-bis(mandelato)borate, [P6,6,6,14][BMB], has been studied at a gold electrode surface in a polar solvent. For a 20% w/w IL mixture, contrast matched to the gold surface, distinct Kiessig fringes were observed for all potentials studied, indicative of a boundary layer of different composition to that of the bulk IL-solvent mixture. With applied potential, the amplitudes of the fringes from the gold-boundary layer interface varied systematically. These changes are attributable to the differing ratios of cations and anions in the boundary layer, leading to a greater or diminished contrast with the gold electrode, depending on the individual ion scattering length densities. Such electro-responsive changes were also evident in the reflectivities measured for the pure IL and a less concentrated (5% w/w) IL-solvent mixture at the same applied potentials, but gave rise to less pronounced changes. These measurements, therefore, demonstrate the enhanced sensitivity achieved by contrast matching the bulk solution and that the structure of the IL boundary layers formed in mixtures is strongly influenced by the bulk concentration. Together these results represent an important step in characterising IL boundary layers in IL-solvent mixtures and provide clear evidence of electro-responsive structuring of IL ions in their solutions with applied potential.