Three diprotic ionic liquids (PILs) containing bis(2-hydroxyethyl) ammonium cations and citrate (DCi), lactate (DL), or salycilate (DSa) hydroxy/carboxylate anions were studied as lubricants for Ti6Al4V–sapphire contact. At room temperature, the neat PILs are non-Newtonian fluids, which show up to a 70% friction coefficient reduction with respect to water. New aqueous lubricants were developed using PILs as 1 wt.% additives in water. The new (Water + 1 wt.% PILs) lubricants showed friction reductions of higher than 50% with respect to water at room temperature. The lowest friction coefficients at room temperature were achieved with thin lubricant layers deposited on Ti6Al4V using Water + 1 wt.% PIL after water evaporation. At 100 °C, the best tribological performance, with the lowest friction coefficients and wear rates, was obtained for the PILs containing aliphatic anions: DCi, and DL. The surface layers of the sapphire balls with mild adhesion and abrasion wear mechanisms were observed via scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS).
