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.

The churning phase of a grease-lubricated rolling bearing significantly impacts the aging of the grease, affecting both bearing performance and service life. Knowledge about the influence of the running-in procedure on grease aging is limited. In this article, we studied grease aging in a ball-bearing running-in process with two additive-free greases made with the same base oil but different thickeners: lithium complex and polyurea. Rheological characterization of the greases was performed after two different 48-h running-in procedures. Both procedures employed a stepwise running-in process with either increasing or decreasing speeds. The results reveal that grease aging in the bulk and near the raceway varies significantly depending on the speed procedure used. The level of aging induced by the speed procedures is thickener dependent. The polyurea grease exhibits more severe aging when subjected to a running-in procedure with sudden high speeds (decreasing speed procedure). Conversely, the lithium-complex grease is more susceptible to aging during a running-in procedure with increasing speeds. The implications of the applicability of these speed procedures in industrial and research applications are also discussed.

Oscillating rolling bearings are susceptible to accelerated raceway damage triggered by a lack of proper lubricant retention and replenishment within the ball-raceway contacts. A low-consistency grease with high oil release from the thickener matrix is desirable for wear attenuation under these challenging operating conditions. However, these physical grease parameters cannot be taken to extreme values in view of other practical considerations. To address this freedom-of-design limitation, greases formulated with added ionic liquids are experimentally probed at the component level under oscillating bearing conditions for the first time. An in-house-built frameless motor test rig allows for precise control of the oscillations of the angular contact bearings and the monitoring of frictional torque. Our results show that ionic liquids, when used as grease additives, can delay the onset of starvation effects and reduce the associated increase in friction torque in harsh operating conditions (high oscillating frequency); while yielding a narrower range of torque amplitude variations in mild conditions (low oscillating frequency). These improvements hinge on the chemical nature of the ionic liquid. By changing only the anion structure, drastic differences in bearing frictional torque are observed. Surface analysis shows that magnetite and hematite, formed in damaged contact tracks on the bearing raceways, make the affected surfaces softer than the bearing steel. Our findings indicate that the use of ionic liquids constitutes a promising pathway for the development of greases for oscillating bearing applications.

With a rapid shift in our society to e-mobility, novel grease formulations for machine efficiency, reliability, and availability are even more important than ever. Greases are expected to provide low friction and noise while extending the machine service life. Whether the grease should be electrically conductive or not remains a subject of debate. The chapter discusses grease architecture and how to tune it in view of the latest and upcoming specification requirements. Selection strategies for base oils, thickeners, and additives are discussed. Integrating these components in the final formulation is a delicate process of balancing required properties that can be contradictory. Grease testing methods are addressed reflecting the need for complementary characterization of grease energy efficiency, conductivity, and protection against fretting. The arguments are made in connection to the representative transmission components.

Performance of Lithium Complex (LiX), Polyurea (PU), and Polypropylene (PP) greases in SKF6208 bearings subjected to driving cycle conditions for 28 days (equivalent to 23,000 km of electric vehicle operation) was studied by continuously measuring bearing friction torque and temperature. The energy dissipation was correlated to the differences in oil bleeding and rheology for the three greases. Evolution of the friction torque, friction torque hysteresis, and changes in grease rheology were dominated by the oil release property. The latter was determined by the thickener system and its particular response to the conditions imposed by the driving cycle. A quantitative estimate of the carbon footprint from using these greases to lubricate bearings under driving cycle conditions is also presented.

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.

A statistical, energy-based approach is employed to experimentally characterize the ability of different greases to reduce friction when a point contact is subjected to repeated reciprocal displacements of smaller magnitude than the contact diameter. This approach allows the assessment of lubricant fretting performance with respect to its ability to remain within the contact and also its boundary lubrication properties. The results indicate that composition-dependent boundary lubrication properties of greases loaded with non-halogenated ionic liquids containing bis(oxalato)borate ([BOB]) and bis(mandelato)borate ([BMB]) anions can result in no detectable wear and low friction, even under conditions of moderately high pressures and where the original contact area is never fully uncovered. This discovery paves the way for the development of anti-fretting ionic greases.

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.

Electric vehicle motors in e-drivetrain are equipped with grease-lubricated bearings operating at both low and high speeds with frequent speed changes. The grease-bearing system must secure a long lifespan and low frictional torque to improve efficiency and sustainability. The present paper focuses on the influence of two types of thickener, lithium complex and polypropylene, on the grease lubrication performance under conditions typical for e-motors. The comparison of both thickeners is performed in terms of friction torque and energy consumption in eight long-duration experiments (337 hr). The results show that the polypropylene thickener provides 21.5% lower energy consumption compared to the lithium complex. Changes in grease rheology and degradation in the tests are analysed and correlated with the grease lubrication performance.