Research On Solid-Liquid Composite Lubrication System Of Two Choline Chloride Based Deep Eutectic Solvents

Solid-liquid composite lubrication system has attracted much interest for low friction and wear.Nevertheless,it is still not well comprehended that the effect of the properties of solid lubricating materials and liquid lubricating materials on the tribological performance of solid-liquid composite lubrication system.The thesis designed solid-liquid composite lubrication systems based on two choline chloride-based deep eutectic solvents.The solid-liquid composite lubrication systems constructed by inorganic and organic coatings with choline chloride-based deep eutectic solvents were studied,respectively.The main research results are as follows:（1）The effect on tribological properties of W-DLC by two deep eutectic solvents was evaluated.Additionally,the effect of W elements on DLC structure,mechanical properties and wettability were systematically investigated.The results show that the doping of W elements evolved the DLC from a porous and loose columnar structure to a dense amorphous structure.Besides,the mechanical properties of W-DLC increase with W concentration.The contact angles of the choline chloride-urea/thiourea on W6DLC（W content is 50.3 at.%）are much higher than other coatings.Under dry friction,friction reduction is contributed by both formation of WO_x and graphitization.The increase in WC carbides contents,which is conducive to the occurrence of abrasive wear.Under choline chloride-urea/thiourea lubrication,the excellent tribological properties of the system is almost no relationship with the mechanical properties of coating,it is more ascribed to the interface wettability at the friction interface or the generation of a tribo-film on the sliding interface.（2）The lubrication mechanism of choline chloride-urea and epoxy coating solid-liquid composite lubrication systems was explored.By comparing with conventional lubricant（PAO）,it was found that the lubrication mechanism of choline chloride-urea was different from PAO.In the PAO and epoxy coating solid-liquid composite lubrication system,the shearing location is inside the PAO liquid.However,in the choline chloride-urea and epoxy coating solid-liquid composite lubrication system,the shearing occurs at the interface of the choline chloride-urea and epoxy coating.Therefore,the choline chloride-urea and epoxy coating solid-liquid composite lubrication system have the characteristics of long life,high load-bearing and low friction.The tribological properties of solid-liquid composite lubrication system are less influenced by viscosity.The water will disrupt the hydrogen bonding network structure of the deep eutectic solvent,which in turn affects its viscosity and reduces the bearing capacity.This result leads to direct contact of tribo-pairs and make the friction and wear increase.（3）The effect of filler（MoS₂ and graphite）content on mechanical and tribological properties of the epoxy coating was investigated.Furthermore,the tribological behaviors of these coatings and choline chloride-urea solid-liquid composite lubrication were explored.The results show that the addition of filler resulted in significant effects on epoxy coating mechanical properties,but elicited little effect on wettability.Under dry friction,the improvement of coating’s mechanical properties and the formation of tribo-film is the key to anti-friction and anti-wear.Under choline chloride-urea lubrication,the pure epoxy coating is soft,which facilitates stress dispersion and thus has the lowest friction and wear reduction.The addition of filler enhances the mechanical properties of coating,but is unfavorable for dispersing contact stresses.This results in a thinner liquid lubrication film and makes tribo-pair easily direct contact,leading to increased friction and wear.However,the tribo-film played a synergistic with choline chloride-urea to anti-friction and anti-wear effects.