A comment on the review paper “Critical advances in superlubricity: From current challenges to sustainable development beyond laboratory” from a gear perspective

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 a low surface roughness (I), low viscosity (II), appropriate contact pressure (III), and appropriate velocity (IV). 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].