Controlled Superlubricity in Water-Based Lubrication: The Overlooked Role of Friction Radius

The coefficient of friction (COF) in superlubrication systems exhibits highly strong correlation with experimental parameters. However, previous studies mainly reported the stable superlubrication formed at small sliding radii (2–5 mm), often without clearly specifying the friction radius. This lack of critical parametric not only affects the reproducibility of the experimental results but also hinders the fundamental understanding of superlubrication formation. In this work, the effects of friction radius and linear velocity on lubrication performance were investigated by using a graphene oxide–ethylene glycol (GO-EG) model lubricant in ball-on-disk rotational sliding. The results showed that superlubricity is also achievable within a large radius range (>5 mm). The GO-EG lubricant attained a stable superlubrication state with a minimum COF of 0.0065 under the conditions of a linear velocity of 0.40 m/s and a friction radius of 6 mm. This value was approximately 50% lower than that under a 2 mm radius condition. Even when the friction radius is increased to 8 mm while maintaining the same linear velocity, stable superlubricity can still be retained. Subsequently, a systematic analysis of the tribological experimental data revealed that at different fixed velocity ranges, the average COF showed different radius dependence. At low linear velocity (<0.2 m/s), the COF decreased with increasing radius, whereas at higher velocity (>0.3 m/s), the COF first decreased and then increased with radius. Mechanistic analysis showed that friction radius governed the COF by modulating rotational speed, shear rate, running-in states, and water-film evolution, including evaporation-driven viscosity changes and hydration-layer stability. This study enhances the understanding of superlubrication formation conditions and provides guidance for the evaluation and measurement of lubrication systems.