| Zinc dialkyl dithiophosphate(ZDDP)has been the most commonly used extreme pressure anti-wear additive in engine oils,gear oils and common lubricants for many years.In addition to excellent wear resistance,it also has oxidation resistance and corrosion resistance and other properties.However,ZDDP as the most common engine oil additive has been difficult to adapt to the increasingly strict environmental requirements.A large number of studies have shown that Zn,P and S elements in ZDDP can reduce the efficiency and life of catalytic converters.This challenges the further development of ZDDP in modern engine lubricants.Graphene is a two-dimensional nanocarbon material composed of a hexagonal honeycomb lattice of carbon atoms with sp~2 hybrid orbitals.Graphene has good chemical stability,excellent antifriction performance and environmental friendliness,which makes it a potential friction improver in the field of lubrication.In this paper,from the perspective of tribological properties,graphene is mainly used as a supplement of ZDDP in engine oil to study the influence of graphene content on the tribological properties of ZDDP.In this paper,the CSM-TRN friction and wear testing machine was used to evaluate the macroscopic tribological properties of graphene and ZDDP under lubrication conditions,and the internal microscopic mechanism was explored by molecular dynamics simulation.The aim is to reduce the amount of ZDDP by adding appropriate amount of graphene,so as to get the lubricating oil with excellent performance.In this paper,GCr15 balls and sheets were selected as auxiliary materials to carry out macroscopic friction and wear tests in lubricants with different graphene contents.The change rules of friction coefficient and wear rate with different graphene content were compared.The surface morphology and chemical composition of the grinding surface of the grinding ball and the grinding plate were characterized by surface analysis and detection equipment,and the influence of graphene content on the macroscopic tribological behavior of ZDDP lubricating oil was studied.The results show that the content of graphene has a significant impact on the tribological properties of ZDDP,adding appropriate amount of graphene will significantly reduce the friction coefficient,too much or too little graphene content will lead to the increase of the friction coefficient.An appropriate amount of graphene can reduce the wear on the spot diameter and friction interface of the grinding ball.At 100?,the change of graphene content did not play a positive role.However,at 50?,the wear rate decreased first and then increased with the increase of graphene content.In this case,graphene had a certain positive effect on the wear resistance of lubricating oil.In this paper,hexadecane molecules were used to construct the base oil model,and together with graphene and ZDDP molecules,a lubricating oil system with multi-layer structure was formed.The iron oxide molecular model was used as the wall surface on the upper and lower sides of the lubricating oil,and the controls of pressure,temperature and shear speed are all accomplished based on the wall surface,so as to complete the shear friction simulation based on molecular dynamics.The lubrication mechanism of graphene and ZDDP lubrication on the surface of iron oxide was studied.The influence of the content,position and molecular size of graphene on the tribological behavior of ZDDP lubricating oil was analyzed,and the changes of molecular posture and configuration under nano scale were analyzed in depth to explore the microscopic mechanism of tribological behavior.The molecular dynamics simulation of shear friction shows that the friction coefficient of the interface can be effectively reduced by the appropriate addition of graphene molecules,and the size and content of graphene molecules also have a certain effect on the results.During the simulation,single-layer graphene self-assembled and formed multi-layer structures with different thickness,which also had a certain influence on the friction properties. |
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