The Mechanism Of Tribochemistry-related Liquid Lubrication Performance Depending On Contact Interface Morphology Evolution

With the rapid development of science and technology,higher requirements are put forward for the operation accuracy,reliability and long life of the transmission rotating parts.In traditional lubrication,tribochemical wear leads to spalling or even fracture in the contact area,which seriously affects the service life of transmission rotating parts.The existing emerging lubricating materials such as water-based superlubricity,oil-based superlubricity and solid superlubricity further reduce the friction between interfaces and friction energy consumption.However,the aggravation of interfacial wear caused by tribochemistry has received little attention.Therefore,revealing the influencing mechanism of tribochemistry-related liquid lubrication performance depending on contact interface morphology evolution,and establishing a reliable accelerated life test method are important to improve the service life and reliability of transmission rotating partIn this paper,the change of contact interface morphology on the liquid lubrication performance dominated by tribochemical reactions.Firstly,the influence of morphology evolution on the tribochemistry-related lubrication performance with H₃PO₄ solution was studied,and the influence mechanism of interface morphology evolution on the lubrication performance transformation with H₃PO₄ solution was revealed.Then,the mechanism of tribochemistry induced diketone solution to quickly realize low shear was explored,and the mechanism of improving the lubrication performance of diketone/iron base by improving the surface quality under tribochemical reactions was clarified.At the same time,the degradation mechanism of lubricating performance of oil-based polyimide was studied,and the effect mechanism of surface quality deterioration on the lubrication performance degradation of steel ball/polyimide with consideration of tribochemical reactions was revealed.Finally,based on the evolution of interface morphology induced by tribochemical reactions,a long-life and reliable liquid lubrication system was constructed,and the optimization scheme of lubrication performance of H₃PO₄ system was proposed.The main research contents of this paper are as follows:（1）The effect of morphology evolution on the lubrication performance with H₃PO₄solution under tribochemical reactions was studied,and the mechanism of tribochemistry-induced evolution of the interface morphology and the transformation of lubrication performance with H₃PO₄ solution was revealed.It was found that,after the system realizing superlubricity at the certain experimental conditions,the reduction of load（or speed）will cause the system to temporary failure,but the substrate remains smooth.However,the increase of load（or speed）will cause the permanent failure of the system,corresponding to the formation of serious wear on the substrate surface.The serious wear results in the failure of the stable and continuous H₃PO₄ hydration layer formed at the friction interface.The changes of the micro morphology of the contact area were detected and analyzed by white light interferometer and AFM,respectively.It was revealed that the tribochemistry-induced evolution of the microscale morphology of the substrate contact interface was the main reason for the permanent failure of superlubricity,and the relationship between the lubrication state of the H₃PO₄ system and the contact surface roughness was established.（2）The mechanism of the rapid realization of low shear in the diketone system under oxygen atmosphere was studied,and the mechanism of the lubrication performance of diketone/iron base enhanced by improving the surface quality under tribochemical reactions was clarified.The results show that high oxygen content significantly shortens the running-in time of the system.The system needs two running in stages to achieve low shear.The rapid decrease of the friction coefficient in the first stage is mainly due to the decrease of the contact pressure at the friction interface,the further decrease of the friction coefficient in the second stage may be closely related to the formation of the surface chemisorption film.The high oxygen environment shortened the running-in time of different stages.In the first running-in stage,oxygen promoted the oxidative wear of the sliding interface,producing the loose and porous friction interface,where the materials were easier to be removed and then generated a smoother contact area,so the time required in the first running-in stage was shortened.In the second stage of running-in process,oxygen promotes the rapid formation of a dense and stable chelate adsorption layer at the friction interface between ferric iron and diketone molecules.The chemical adsorption layer has ultra-low shear strength,which eventually leads to stable superlubricity at the interface between steel and steel.（3）Through the comparative study of rolling-sliding complex motion and pure sliding mode,the mechanism of surface quality deterioration affecting on the lubrication performance degradation of steel ball/polyimide was revealed with consideration of tribochemical reaction occurring at the sliding interface.The results show that the rolling-sliding composite motion has a great influence on the tribological properties of the polyimide composites,which closely relate to the lubrication conditions.Under the dry friction conditions,compared with pure sliding,rolling-sliding motion leads to lower friction and lighter surface wear of polyimide composites.However,under PAO4 oil lubrication,the friction coefficients produced by the two motion are close,but more serious surface wear will occur in the rolling-sliding movement,accompanied by the production of black products.The results of X-ray photoelectron spectroscopy show that the increase of wear of polyimide composites was mainly due to the tribochemical reaction of polyimide composites,which leads to the decrease of their mechanical properties.This view was further verified by SEM characterizations of the worn surfaces and the comparison results of heating sliding tests and heat treatments.Finally,the smooth polyimide composite surface was obtained by mechanical polishing.The smooth contact interface reduces the friction temperature heating and obtains better lubrication performance.（4）A long-life and reliable liquid lubrication system was constructed based on tribochemistry-induced interface morphology evolution.A life test method of H₃PO₄ system was established,and the optimization scheme of lubricating performance of H₃PO₄ system was put forward.Finally,using the strong force between H₃PO₄ molecules and water at the friction interface and optimizing the mechanical properties of the base material can lead H₃PO₄system to remain stable and superlubricity after 100 hours of operation under vacuum,which verifies the feasibility of the design scheme of long-life and high reliability liquid lubrication system.