Superlubricity Behavior with Phosphoric Acid–Water Network Induced by Rubbing

Abstract

In present work, a superlubricity phenomenon of phosphoric acid (H(3)PO(4)) was found under ambient conditions. An ultralow friction coefficient of about 0.004 between glass/Si(3)N(4) and sapphire/sapphire tribopairs was obtained under the lubrication of a phosphoric acid aqueous solution (pH 1.5) at high contact pressure (the maximum pressure can reach about 1.65 GPa) after a running-in period of about 600 s. The experimental results indicate that the superlow friction state was very stable for more than 3 h. In such a state, solidlike films formed on the two sliding surfaces, which are hydrates of phosphoric acid with a hydrogen-bonded network according to the Raman spectrum. The superlubricity mechanism is mainly attributed to the hydrogen bond effect that forms a hydrated water layer with low shearing strength, and the dipole-dipole effects that form an interfacial Coulomb repulsion force also make some contributions to low friction. This work may help us to introduce a new approach to superlubricity and may lead to the wide application of superlubricity in future technological and biomedical areas.

Physicochemistry aspects on frictional interfaces

“…At present, superlubricity materials include solid lubricants (diamond like carbon (DLC), molybdenum disulfide (MoS 2 ), etc.) [88][89][90] and liquid lubricants [91][92][93][94][95][96][97]. The solid lubricants have been studied in detail by molecular dynamics simulations and experimental investigations.…”

“…However, the investigation on liquid lubricants for superlubricity is still at the early stage. Li et al found the novel superlubricity phenomenon of phosphoric acid (H 3 PO 4 ) under ambient conditions [91,98]. An ultralow friction coefficient of about 0.004 between glass/Si 3 N 4 and sapphire/sapphire tribopairs was obtained under the lubrication of a phosphoric acid aqueous solution (pH 1.5) at high contact pressure (the maximum pressure can reach about 1.65 GPa), after a running-in period of about 600 s. The tribological results showed that the whole friction process could be divided into three stages according to the evolution of friction coefficient with time.…”

“…In this stage, the superlubricity would keep constant if a thin film with hydrogen bond network among H 3 PO 4 , H 2 PO 4 + , and H 2 O is adsorbed on the track to bear the load. Therefore, the superlubricity mechanism of phosphoric acid is attributed to the hydrogen bond network between the phosphoric acid and water molecules formed on the stern layer that is induced by the attached hydrogen ions [91,92,98,99].…”

Thin film lubrication in the past 20 years

“…A solution of phosphoric acid was found to be unique, providing an extremely low friction coefficient that was less than 0.004 by Li et al [135], Ma et al [136] and Li et al [137,138], who achieved superlubricity with alcohols and acids solutions successively. They suggested that the superlubricity of acid and glycerol solutions resulted from a shear-induced hydrogen network structure in a nano-gap, as shown in Fig.…”

From ice superlubricity to quantum friction: Electronic repulsivity and phononic elasticity

“…Phosphoric acid solutions exhibit superlubricity effect as a lubricant [103] with a friction coefficient around 0.004 after a short running-in period (Fig. 22).…”