Solid-liquid phase change of choline chloride type deep eutectic solvents towards lubrication regime

Abstract

Inspired by the fact that ice lubrication with solid–liquid phase change might be a good candidate for lubrication regime, we designed a new lubrication strategy based on solid–liquid phase change of deep eutectic solvents (DESs). The melting point controllable character of DESs makes them easy to realize solid–liquid phase change during friction. Here, two choline chloride (ChCl)-type DESs with different physical state (i.e., solid and liquid) were synthesized through regulating their composition. The two DESs were applied to explore the relationship between solid–liquid phase change of DES and friction-wear under sliding and fretting conditions. Results show that ChCl-thiourea DES undergoes solid–liquid phase change at the initial stage of friction (before 500 cycles) under low loads. The solid–liquid phase change under fretting conditions prolongs the running-in period of friction. Most pronounced under ChCl-thiourea DES lubrication at 5 N-0.1 Hz under sliding friction, the friction coefficient is around 0.03, and the wear volume is 54.3 % lower than that of ChCl-urea DES. ChCl-thiourea DES after solid–liquid phase change forms physical adsorption film and tribochemical reaction film under fretting and sliding, respectively, which results in the reduction of friction-wear as compared with ChCl-urea DES.