Preparation And Mechanism Of Choline-based Ionic Liquid Analogues Superlubricity Materials

How to efficiently reduce friction and wear has always been a key concern in the field of tribology.The effective means to significantly reduce friction and wear in mechanical systems is of great practical value to improve the performance and life of machinery,reduce energy consumption and increase economic efficiency.As one of the emerging research topics in the field of tribology,superlubricity has attracted much attention from scholars.Superlubricity（μ<0.01）is currently divided into two main categories:solid superlubricity and liquid superlubricity;compared to the harsh conditions for achieving solid superlubricity（e.g.,dry nitrogen environment）,liquid superlubricity is more easily obtained under conventional operating conditions（e.g.,macroscopic level,atmospheric conditions,generally flat surfaces）.Typical liquid superlubricity materials include acid-based superlubricity materials,alcohol-based superlubricity materials,ionic liquid/ionic liquid analogues/hydrated alkali metal ionic liquid superlubricity materials,oil-based superlubricity materials and solid-liquid coupled superlubricity materials,etc.These lubrication systems are characterized by two features:1)the existence of very low shear resistance slip interface between the friction pairs;2)good interaction between the friction pairs and the liquid lubricant material（i.e.,easy to form tribo-chemical reaction film,adsorption film,and transfer film,etc.）.From these two points,this thesis constructs two novel liquid superlubrication systems based on choline-based ionic liquids analogues and analyzes the origin of their superlubricity at the molecular/atomic level through experimental and molecular dynamics simulation methods,providing theoretical and experimental bases for the design and development of new ionic liquid analogues superlubricity materials.The main works are as follows:（1）A water-regulated ionic liquid analogues（ILA/H₂O）superlubrication system was constructed based on choline chloride/magnesium chloride hexahydrate.The strong hydrogen bonding in the ionic liquid analogues was weakened by introducing 2-10 wt%of water,thus enabling macroscopic superlubricity(sliding velocity range:0.1-0.2 m s^-1)at a Hertzian contact pressure of～700 MPa.Based on the in-situ Raman results,it is speculated that the superlubricity originates from the incomplete hydration structure formed by the competitive exchange of external water and crystalline water;this structure is prone to form a slip interface with very low shear resistance in the lubricating film,and the strong hydrogen bonding retained in the incomplete hydration structure can also play a load-carrying role for lubrication,thus inducing the superlubricity.Thanks to the hydrodynamic lubrication mechanism of this lubrication system,extremely low friction（μ<0.01）can be obtained with various friction pairs（e.g.,steel/steel,steel/Si O₂,Si₃N₄/Al₂O₃,and Si₃N₄/Si O₂）.（2）Based on work 1),another superlubrication system was constructed based on a water-regulated choline chloride/D-glucose（Ch Cl/Glu-H₂O）.This lubricating material maintains comparable tribological properties with ILA/H₂O(maximum Hertzian contact pressure of～700MPa,sliding velocity range:0.1-0.25 m s^-1)while improving the tolerance to the environmental humidity and being greener.The in-situ Raman results show that it has a similar lubrication mechanism as ILA/H₂O,i.e.,its superlubricity also originated from the internal incomplete hydration structure.（3）The superlubrication mechanism of water-containing ionic liquids analogues is further analyzed in depth by combining molecular dynamics simulation methods with work 2).From the radial distribution function analysis,it can be concluded that an incomplete hydration structure similar to that postulated in Work 1)does exist in Ch Cl/Glu-H₂O（i.e.,the structure formed by simultaneous coordination of D-glucose and water molecules with Cl^-）.Based on the friction simulation results,it can be seen that an"adsorption layer/load-carrying layer/adsorption layer"layered structure was formed between the friction pairs;the adsorption layer and the load-carrying layer formed a low shear resistance slip interface due to the hydration effect,while a large number of strong hydrogen bonds inside the load-carrying layer acted as a cluster of lubricating fluid,thus preventing the liquid from being extruded during the friction process.Based on this synergy between the adsorption layer and the load-carrying layer,the water-containing ionic liquid analogues material is able to achieve superlubricity under macroscopic conditions.