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

Abstract

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 Si 3 N 4 ball, which has a smaller elastic modulus of 75 GPa. Through friction experiments and ζ-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 Si 3 N 4 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.

Probing the hydration friction of ionic interfaces at the atomic scale

“…7 To understand the effects of the surrounding ions, interface charge and polarizability on the hydration friction behaviors give deep insights into the mechanism of hydration friction; numerous experiments have been carried out in exploring the origins of hydration friction by employing a surface force balance (SFB) and a ball-on-disk tribometer. 2,4,[8][9][10][11][12][13][14][15] It has been demonstrated that an extremely low sliding friction could be achieved between charged surfaces in high-salt solutions, 2,4,14 which is attributed to the formation of the hydration layers. However, there is little microscopic understanding of the hydration friction mechanism, especially the role of subnanometer hydration shells in the frictional dissipation process, which remains not well understood.…”

Superlubricity achieved with two-dimensional nano-additives to liquid lubricants

“…Hence, the as-formed hydration layer and hydration repulsion, caused by the adsorbed cations on the negatively charged solid surfaces, may directly affect the tribological properties [90]. Since hydration of Li + is much stronger than those of Na + and K + , extremely low shearing resistance and hydration repulsion is provided with a hydration layer on sliding solid surfaces [75,91]. Both the occurrence of tribochemical reaction with anions and the excellent ability to hydrate by cations in ionic liquids can further improve the wear resistance of liquid superlubricant, and thereby increasing the load-bearing capacity to a high level for industrial application.…”

Effects of Abrasive Particles on Liquid Superlubricity and Mechanisms for Their Removal

“…For AFM analysis using a SiO 2 spherical probe, epoxy glue was used to affix the particles ( R = 20 μm) to the end of a rectangular pointed cantilever beam. The elastic modulus and Poisson’s ratio of the SiO 2 particles used in the probe were 50 GPa and 0.17, respectively …”

“…Li et al showed that the synergistic effect between a polymer and salt solution can result in superlubricity between silicon nitride (Si 3 N 4 ) and sapphire (Al 2 O 3 ) surfaces under extreme pressure conditions. Meanwhile, Han et al demonstrated that a mixed solution of polyethylene glycol and KCl exhibited excellent lubricating properties and revealed that the friction-induced SiO 2 layer played a vital role during the running-in process to realize macroscopic superlubricity . Generally, the lubrication state goes through a running-in phase before reaching liquid superlubricity.…”