Origins of Superlubricity Promoted by Hydrated Multivalent Ions

Abstract

Strong hydration repulsion exists between two negatively charged surfaces in the alkali metal salt solutions, together with the fluid response to the shear of hydration layers, leading to superlubricity. However, whether the multivalent ions can obtain superlubricity has not been revealed yet. Here, we evaluate the lubrication and adsorption properties of multivalent ions at different concentrations between Si3N4 and sapphire surfaces. The divalent and trivalent ions exhibit extremely low friction coefficients of 0.005–0.006 and 0.002–0.004, respectively, under contact pressures above 0.25 GPa, and three trivalent ions can achieve superlubricity at quite low sliding speeds (3.1 mm/s), which is a significant breakthrough for superlubricity under boundary lubrication. Moreover, compared with monovalent ions, divalent ions can reduce surface potential and lower surface charge density even further, and trivalent ions can neutralize the negatively charged ceramic surfaces and even lead to charge inversion due to excess adsorption of the cations, which ensures strong adsorption of hydrated multivalent ions on friction surfaces.

Macroscale Robust Superlubricity on Metallic NbB2

“…Consequently, the flowing hydration layer exhibits its natural weak shear characteristics to produce the observed small friction. [ 22 , 52 ] According to further calculations by Hamrock‐Dowson theory, [ 52 ] the lubrication regime of NbB 2 film in stable SL state is located in hydrodynamic lubrication (Figure S8 , Supporting Information), which is the result of the synergistic effect of three hydration layers and is directly affected by external friction conditions (Figure S2 , Supporting Information). It is easy to achieve SL when the load does not exceed 2.5 N, and the probability of SL decreases as the load increases.…”

Macroscale robust superlubricity on metallic NbB2

“…Meanwhile, the owing hydration layer contains ample B(OH) 4 ions, leading to an electrical double-layer repulsion between the owing hydration layer and the two xed hydration layers, reducing the contact pressure. Consequently, the owing hydration layer exhibits its natural weak shear characteristics to produce the observed small friction 21,42 . Overall, two factors are key to generating SL at the NbB 2 surface: (i) tribo-products displaying hydrophilic properties to enable the formation of xed hydration layers on the friction surface and (ii) charged agents from tribochemical reactions generating interface and hydration layer repulsion to unleash the weak shear character of the tribo-liquid.…”

Lipids and lipid mixtures in boundary layers: From hydration lubrication to osteoarthritis

“…The above-mentioned strong, short-ranged repulsion is commonly referred to as "hydration repulsion (or force)" [44], which overcomes van der Waal's attraction and so prevents direct contact of PL bilayers. Hydration repulsion is ubiquitous between hydrated species, including trapped hydrated ions, as demonstrated for hydrated sodium ions trapped between mica surfaces [45], divalent cations (≥ 1 M) trapped between mica surfaces [46], divalent and trivalent cations trapped between Si3N4 and sapphire surfaces [47], surfactants-coated surfaces [48][49][50][51], polymer brushes-grafted surfaces [52], as well as biological molecules, such as proteins [53].…”