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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 expe...