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.

This paper explores the development of a subject called "tribotronics", the science of actively controlled tribological elements, and considers the potential of tribotronics as part of a revolutionary machine element design for the future. It presents a "state-of-the-art" assessment of some devices that can be considered "tribotronic" and are currently in common use, as well as a review of examples of recent progress in research in tribotronics. It also presents a "perspective" on the future challenges and likely developments in the subject, including the integration of tribotronics with developments in other fields of digital technology, such as Industry 4.0, the "Internet of Things (IoT)", and machine learning (ML), potentially leading to "cyber tribotronic systems". In conclusion, tribotronics will contribute significantly to transforming the design of the components and machines of the present day into cyber-physical systems in the future.

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.

An exploration of new space lubricant architectures is motivated by the lack of diversity of heritage space lubricants, their known limitations, increasingly demanding mission objectives, and the emergence of new regulations. In this study, we have benchmarked a non-halogenated orthoborate ionic liquid (IL) lubricant formulation against two space heritage commercial fluids based on multiply alkylated cyclopentane (MAC) and perfluoropolyether (PFPE) using a simple vacuum pin-on-disk test performed at three temperatures (40, 100 and 140°C). The study encompasses friction data, detailed wear quantification, high-quality surface digitalization, and mass spectrometer analysis of gases generated during tribological tests. The results reveal that each one of the different lubricant chemistries is conducive to a drastically different level, and character, of friction and wear behavior. The non-halogenated orthoborate IL formulation shows extraordinary surface protection compared to the heritage fluids in boundary lubrication, with average wear volume reductions in excess of an order of magnitude. This indicates a promising potential to expand the operating conditions of mechanisms before detectable wear occurs. Further research is needed to comprehensively verify and validate the usability of specific IL formulations for space lubrication; however, these results suggest that such efforts should be undertaken

Commercial (protiated) samples of the "green" and biodegradable bioester 2-ethylhexyl laurate (2-EHL) were mixed with D-2-EHL synthesized by hydrothermal deuteration, with the mixtures demonstrating bulk structuring in small-angle neutron scattering measurements. Analysis in a polymer scattering framework yielded a radius of gyration (R (g)) of 6.5 angstrom and a Kuhn length (alternatively described as the persistence length or average segment length) of 11.2 angstrom. Samples of 2-EHL dispersed in acetonitrile formed self-assembled structures exceeding the molecular dimensions of the 2-EHL, with a mean aggregation number (N-agg) of 3.5 +/- 0.2 molecules across the tested concentrations. We therefore present structural evidence that this ester can function as a nonionic (co)-surfactant. The available surfactant-like conformations appear to enable performance beyond the low calculated hydrophilic-lipophilic balance value of 2.9. Overall, our data offer an explanation for 2-EHL's interfacial adsorption properties via self-assembly, resulting in strong emolliency and lubricity for this sustainable ester-based bio-oil.

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.

The paper shows how grease thickener polarity affects performance of the typical powertrain components: gears and rolling element bearings. Greases based on a non-polar polypropylene thickener reduce friction losses (more than 20%) in high-speed deep groove ball bearings and provide a longer service life (more than 2 times) in highly loaded bevel gears, compared to the greases based on polar lithium thickeners. The electrification and sustainability trends have led to additional requirements to be addresses during grease design process: tunable electric conductivity and reduced environment footprint. The grease design challenges caused by the novel requirements and potential solutions are discussed.

As wave and tidal energy generators advance towards full-scale deployment and commercialization, addressing challenges in the marine environment regarding the lubrication of the components integrating the Power- Take-Off (PTO) system become crucial. Environmental acceptable lubricants that can be biodegraded such as watersoluble polymers are being considered as an alternative to control friction and wear in ocean energy generators. However, compared to synthetic lubricants or mineral oil, they have shown limitations in preventing corrosion, wear, and excessive friction, which can result in premature failure of moving parts. This study explores the potential of different water-soluble polymers as environmentallyfriendly lubricants that meet stringent regulations and provide effective protection against wear and corrosion in offshore conditions. The analysis focuses on the polymers ability to form an elastohydrodynamic film, mitigate surface degradation, prevent corrosion, as well as their rheological properties at different concentrations. The findings reveal that large polymers such as PAG when added in low concentrations in the water can form a separating film under high contact pressure in the low-speed region, while viscosity has shown to have less impact on the ability of the polymer to form a full film. Increasing polymer concentration in the aqueous solution negatively affects the corrosion resistance of steel components at the expense of improving the film-build up. These results offer valuable insights for designing lubrication solutions to protect offshore energy devices.