Lubricants with a functional water portion have demonstrated a drastic reduction in friction under elastohydrodynamic lubrication conditions. With water-containing polyalkylene glycols, superlubricity with coefficients of friction <0.01 have been measured in model and gear contacts. In addition to the low friction, their calorimetric properties make them particularly interesting for application in electrified vehicles because the liquid can simultaneously serve as lubricant for the gearbox and coolant for the electric motors and the power electronics. In this study, the influence of water content between 8 wt% and 40 wt% of water-soluble polyalkylene glycols on friction and film thickness in elastohydrodynamically lubricated rolling-sliding contacts such as in gears and bearings is investigated. A polyalphaolefine oil is used as a reference. Friction has been measured on a ball-on-disk tribometer and film thickness on an optical tribometer. For a water content of 40 wt%, superlubricity with coefficients of friction down to 0.004 are found. The decrease in friction is up to 95% compared to the polyalphaolefine reference. The measured film thickness decreases with increasing water content. For a water content of 8 wt%, the film thickness is similar to that of the polyalphaolefine reference while at the same time friction is still reduced by 81%. Depending on the friction and film thickness requirements of a specific tribosystem, the water content of a water-containing polyalkylene glycol can be chosen accordingly.

The reduction of frictional power losses in power transmitting gears takes a crucial role in the design of energy- and resource-efficient drivetrains. Water-containing lubricants like glycerol and polyalkylene glycols have shown great potential in achieving friction within the superlubricity regime with coefficients of friction lower than 0.01 under elastohydrodynamic lubrication. Additionally, a bio-based production of the base stocks can lead to the development of green lubricants. However, one challenge associated with the application of water-containing lubricants to gearboxes is the evaporation of water and its impact on the lubricant properties. In this study, the influence of water evaporation on elastohydrodynamic friction and film thickness was investigated for three water-containing polyalkylene glycols. Two nominal water contents of 20 wt% and 40 wt% and two viscosities were considered. The results show that the friction increases continuously with higher evaporated water content, while the overall friction level remains low in nearly water-free states. A similar trend is observed for film thickness, where the strong increase in viscosity results in a notable increase in film thickness. Nevertheless, the sensitivity of friction and film thickness to water evaporation is low for small amounts of evaporated water. This allows generous thresholds for permissible variations in water content.

Water-soluble polyalkylene glycols have become a growing subject of research to achieve liquid superlubricity in elastohydrodynamically lubricated contacts. While the influence of various factors, including water content and viscosity, has been extensively studied, the underlying mechanisms responsible for liquid superlubricity under elastohydrodynamic lubrication remain poorly understood. In this study, aqueous polyethylene glycols of varying average chain length with the same viscosity but different water content or average chain length distribution are examined in relation to elastohydrodynamic friction and film thickness. The results indicate, that under fluid film lubrication, the low pressure–viscosity coefficient is the primary factor leading to liquid superlubricity. No running-in period is required, allowing for stable and persistent ultra-low friction level immediately. Additionally, at a constant water content, an optimum average chain length distribution was identified, resulting in reduced friction while the film thickness remained largely unaffected. This enables the targeted design of aqueous lubricants based on polyethylene glycol. Graphical abstract

Aqueous polyalkylene glycols have demonstrated superlubricity in model and gear tests. One challenge for its application in gearboxes is water evaporation and absorption. This study investigated the influence of water evaporation and absorption of aqueous polyalkylene glycols on the power loss behavior of cylindrical gears. To this end, a back-to-back gear efficiency test rig was used. Two aqueous polyalkylene glycols with water contents of 20 and 40 wt% and ISO viscosity grade 22 were considered. The results show that the power losses increase with increasing evaporated water, depending on the circumferential speed. The sensitivity of the power loss to water evaporation is greater at higher initial water contents. Even though the sensitivity of the load-dependent power loss factor is a maximum of +0.060 1/wt%, the load-dependent power loss remains low compared to conventional lubricants. The increase in power loss due to water evaporation is reversible by adding the evaporated water. Monitoring and control of the water content is favorable, especially for water contents which differ significantly from the hygroscopic equilibrium. The accumulated energy dissipation is strongly influenced by the no-load power loss. In order to exploit the superefficiency potential of aqueous polyalkylene glycols, the no-load accumulated energy dissipation has to be reduced.

The influence of the complex material behavior of thermoplastic polymers in lubricated contacts is poorly understood. It affects the optimal design of power-transmitting thermoplastic machine elements and the exploitation of its potential, e.g., lightweight design, low-noise operation, and cost-effective manufacturing when injection-molded. This study applies the in situ thin-film sensor technology on a twin-disk tribometer in order to study the elastohydrodynamic lubrication of rolling–sliding contacts with the thermoplastic polymer polyetheretherketone. The results provide insights into the effects and relevance of its thermoplastic material properties. Pressure measurements reveal a typical hydrodynamic profile in combination with a large deformation of the contact zone. The influence of speed and slip ratio is thereby negligible. The temperature rise is low compared to elastohydrodynamically lubricated contacts with steel and is mainly influenced by the slip ratio as well as the load, whereas speed plays a subordinated role. In general, the heat generation is governed by shearing and backflow in the contact inlet zone at low slip ratios and shearing in the contact zone at high slip ratios. No effects attributed to viscoelasticity or loading frequency were observed at the operating conditions considered.