Tribological Properties of the Functionalized Ionic Liquids Containing Ester-group as Lubricants of Steel/Steel Contact

A series of functionalized imidazolium ionic liquids (ILs) containing ester-group are synthesized and their physicochemical properties and thermal stabilities are studied. The tribological properties of the synthesized ionic liquids as lubricants for a steel-on-steel system are evaluated on an SRV friction and wear tester at room temperature and 100 ℃ and compared with nonfunctionalized IL. The worn surfaces are analyzed by using SEM and XPS. It is found that all the functionalized ILs synthesized have high thermal stability. The introduction of ester-group increases the viscosities of ILs, which leads to worse friction-reducing ability at low loads, but it also increases the chemical adsorption of ILs, resulting in a significant increase in the anti-wear ability of the ILs. At high loads, tribochemical reactions take place between the active elements, such as fluorine which is released from the ILs, and fresh metal surfaces of rubbing pairs to form the admixture reaction films. The films are mainly composed of ferric fluoride mixed with ferric oxide, leading to low friction coefficient and good wear resistance. In addition, it is found that the functionalized ILs bearing Tf₂N anions have better anti-wear ability than the ones bearing BF₄ anions, and the ILs containing longer alkyl chains show better friction reducing and anti-wear abilities than the ones containing shorter alkyl chains.