Insights into the tribological behavior of choline chloride–urea and choline chloride–thiourea deep eutectic solvents

Deep eutectic solvents (DESs) have been considered as novel and economic alternatives to traditional lubricants because of their similar physicochemical performance. In this study, choline chloride (ChCl) DESs were successfully synthesized via hydrogen-bonding networks of urea and thiourea as the hydrogen bond donors (HBDs). The as-synthesized ChCl–urea and ChCl–thiourea DESs had excellent thermal stability and displayed good lubrication between steel/steel tribo-pairs. The friction coefficient and wear rate of ChCl– thiourea DES were 50.1% and 80.6%, respectively, lower than those of ChCl–urea DES for GCr15/45 steel tribo-pairs. However, for GCr15/Q45 steel, ChCl–urea DES decreased the wear rate by 85.0% in comparison to ChCl–thiourea DES. Under ChCl–thiourea DES lubrication, the tribo-chemical reaction film composed of FeS formed at the interfaces and contributed to low friction and wear. However, under high von Mises stress, the film could not be stably retained and serious wear was obtained through direct contact of friction pairs. This illustrated that the evolution of the tribo-chemical reaction film was responsible for the anti-friction and anti-wear properties of the DESs.