Tang et al. [1] present a thorough literature review on the advances in liquid superlubricity and provide a comprehensive overview of the challenges associated with translating model testing findings into practical applications. The review has led to the formulation of the current requirements for achieving liquid superlubricity, namely (I) a low surface roughness, (II) low viscosity, (III) appropriate contact pressure, and (IV) appropriate velocity. While the review is extensive, we think it lacks a critical examination of the potential application of the reviewed tribosystems in the lubrication of machine elements such as roller bearings and gears. The transfer of superlubricity from model scale to machine elements requires the consideration of boundary conditions of industrial applications. These include material properties, lubricant characteristics, and geometric and kinematic constraints. This comment discusses the work of Tang et al. [1] in the context of superlubricity from the perspective of gears, which are typically characterized by rolling–sliding motion and non-conformal contact geometry. We think that it is imperative to refine the conclusions to avoid misinterpretation of the challenges and potential of superlubricity. We demonstrate that superlubricity can be easily achieved in elastohydrodynamically lubricated steel contacts on engineering surfaces, not limited by the four requirements brought up by Tang et al. [1].

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.