Tribological Properties Of Ionic Liquids As The Complex Lithium Grease Additives

Complex lithium grease has excellent mechanical stability,colloidal stability and oxidation stability.It is a widely used grease.In recent years,in order to meet the lubrication requirements of heavy-duty,high-temperature and high-speed equipment,it is urgent to develop a new type of complex lithium grease anti-wear agent.Ionic liquid is a green solvent,which has some excellent properties,such as low volatility,non-flammability,low melting point,high thermal stability,conductivity and other high performance lubricants.Researchers have studied ionic liquids as base oils and lubricating grease additives at room temperature,high temperature and high vacuum.The results show that ionic liquids have excellent antifriction and antiwear properties.In this paper,a series of oil-miscible quaternary phosphonium ionic liquids were synthesized.Environmental friendly quaternary ammonium ionic liquids were synthesized with ibuprofen as anions and quaternary ammonium salts as cations.A series of imidazolium-based ionic liquids were synthesized with 1,4-bis(2-ethylhexyl)sulfosuccinate and theophylline as anions and methylimidazole with different chain lengths as cations.The structures and physical and chemical properties of ionic liquids were characterized by NMR,thermogravimetric analysis,infrared spectroscopy and kinetic viscosity analysis.The ionic liquid was added to the complex lithium grease.The physical and chemical properties of the grease were measured by dropping point tester and lubricating grease cone penetrometer.The friction and wear properties of lubricating grease were studied with a four-ball friction tester.Scanning electron microscope,X-ray photoelectron spectroscopy and element analysis were used to study the worn surfaces morphology of steel balls and and the chemical states of elements on the surface of steel balls.The friction reduction and anti-wear mechanism of ionic liquids as complex lithium-based grease additives was revealed.Taking the phosphate ionic liquid as an example,we studied the interaction between the ionic liquid and the rust inhibitor Octadecylamine N-oleoyl sarcosinate(T711).The friction coefficient of base grease and the wear scar diameter of steel ball can be reduced by the addition of several kinds of ionic liquids.The lubricating mechanism of ionic liquids is that there is a tribochemical reaction between ionic liquids and metal surface to form a protective film,which can futher reduce friction and anti-wear.