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.

The thermal conductivity of nanoscale nickel particles due to phonon heat transfer is extrapolated from thin film results calculated using nonequilibrium molecular dynamics (NEMD). The electronic contribution to the thermal conductivity is deduced from the electrical conductivity using the Wiedemann-Franz law. Based on the relaxation time approximation, the electrical conductivity is calculated with the Kubo linear-response formalism. At the average temperature of T = 300 K, which is lower than the Debye temperature D = 450 K, the results show that in a particle size range of 1.408-10.56 nm, the calculated thermal conductivity decreases almost linearly with decreasing particle size, exhibiting a remarkable reduction compared with the bulk value. The phonon mean free path is estimated, and the size effect on the thermal conductivity is attributed to the reduction of the phonon mean free path according to the kinetic theory.

Relative motion between artificial cartilages is too slow to form a sufficient lubricating film for hydrodynamic lubrication, and this can result in large friction and wear. Hydration lubrication is effective in reducing friction at low speeds, and the lubricity is mostly determined by surface charges, which can be assessed by isoelectric point (IEP). Therefore, by controlling the IEP of friction pairs, ultralow friction can be achieved even with weak hydrodynamic lubrication. Here, a negatively charged material is synthesized by grafting carboxylate anions into an epoxy resin matrix. Compared with pristine epoxy resin (IEP = 6.0), the modified epoxy resins (IEP = 3.8, IEP = 3.0) show better tribological properties. In particular, at low speeds (4.7 and 1.2 mm s−1), the friction coefficients are as low as 0.02–0.04, and the average contact pressures are ≈10 MPa, when a NaCl solutions are used as the lubricants. Furthermore, the addition of hydroxyapatite to the modified epoxy resin not only increases its bioactivity, but also reduces friction. Superlubricity can be achieved for the speeds higher than 23.5 mm s−1. These results provide a method to design artificial cartilage with good tribological and bearing capacity.

Polyether ether ketone (PEEK) is a widely used material for friction pairs due to its excellent mechanical strength, good wear resistance, and chemical inertness. However, some modifications are necessary when PEEK is used as a water-lubricated friction pair. In this study, a novel sulfonation method was developed to design a water-lubricated friction pair with ultralow friction, good wear resistance, and high loading capacity. PEEK powders were sulfonated using ClSO3H and sintered to form bulk plastic. The sulfonated PEEK (SPEEK) plastic exhibited good tribological properties. At a low sliding speed, the friction coefficient was smaller than 0.02 when a 3 wt% NaCl solution was used as the lubricant. The order of magnitude of the wear rate was as low as 10−8 mm3/(N·m). The mechanism of friction reduction was mainly hydration lubrication. The negatively charged −SO3− groups on the friction pair can adsorb hydrated Na+ cations by electrostatic interactions. These hydrated Na+ cations have a high load capacity and low shearing resistance. The ultralow wear mechanism observed in this study is possibly due to ultralow friction properties of the friction pairs prepared through the proposed sulfonation and thermoforming procedures.