Contribution of a Tribo-Induced Silica Layer to Macroscale Superlubricity of Hydrated Ions

Hydrated ions (Li+, Na+, and K+ are capable of achieving macroscale superlubricity under high contact pressures and high normal loads, which mainly originates from the hydration effect and tribochemical reaction related to the in situ formation of an interfacial nanostructured shear layer, namely, a silica-like tribolayer. Nevertheless, the mechanisms governing this macroscale superlubricity especially the growth activities and the specific contribution of such a silica layer formed through the tribochemical reaction to macroscale hydration superlubricity remains unclear. Here, using transmission electron microscopy and the X-ray photoelectron spectroscopy depth profile technique, we resolved the amorphous structure on the atomic scale and determined the thickness of the tribo-induced silica layer. Using atomic force microscope nanoindentation, we reveal the mechanical properties of the 6 nm-thick silica layer generated on a Si3N4 ball, which has a smaller elastic modulus of 75 GPa. Through friction experiments and zeta-potential analyses, we report on two main effects of the silica layer on achieving superlubricity. First, the silica layer can significantly reduce the friction resistance between ceramic surfaces under boundary lubrication at both the macroscale and microscale. Second, the Si3N4 surface exhibits a larger negative potential and better hydrophilicity due to the presence of the silica layer, thereby adsorbing more hydrated cations. The observations show that the superlubricity of hydrated ions can be obtained not only between two mica surfaces but also for ceramic surface pairs with lower surface charge density, higher elastic modulus, and even larger surface roughness. These findings demonstrate that a tribochemical pretreatment of surfaces allows the hydration effect to be effective to the macroscopic regime, thereby promoting the realization of hydration superlubricity.