Study On The Tribological Behavior Of Multiply Reinforced Liquid-solid Composite Lubrication System With Carbon-based Film

The studying of regulating and controlling the friction and wear behavior of the interface of moving pairs is the only way and effective means to improve the service life and reliability of mechanical equipments under harsh and complex working conditions.Therefore,the solid-liquid composite lubrication with carbon-based film was designed,and triggered an upsurge of innovative research on the composite lubrication system aiming at stable and reliable lubrication under all working conditions,promoting the lubrication technology to be more energy-saving,low-carbon and green.However,in order to design more and better liquid-solid composite lubrication systems,we should pay more attention to some basic scientific problems in the vigorous development of liquid-solid composite lubrication technology.Therefore,in this thesis,according to the characteristics of carbon based films,nano-materials with different characteristics are designed as additives,and the influences of film structure and test conditions on the tribological properties of nano-lubricant/carbon-based film liquid-solid composite lubrication system are studied,and the mechanisms of the change of friction contact interface and medium material composition and structure affecting liquid-solid composite lubrication are revealed.The tribological performance of the developed lubrication system is tested and verified in the simulated working conditions of piston ring/cylinder liner and gear transmission of fuel engine.The main research contents and conclusions of this paper are as follows:(1)Spherical Cu nanoparticles with a size of less than 100 nm are selected as base oil additives to form liquid-solid composite lubrication systems constituted with carbon based films.The effects of nanoparticle concentration in base oil and friction test conditions on the tribological properties of liquid-solid composite lubrication systems are studied by using a ball-on-disk friction test system.The results show that,under mild test conditions,the oil film is thick enough in the contact area,and the excellent tribological properties are mainly due to the rolling effect of "micro-bearings" of some spherical Cu nanoparticles and the lubrication effect of the friction film with low shear force between the contact surfaces.When the test temperature and load increase,it is difficult for Cu nanoparticles to enter the friction contact interface.It is the graphitized activated C on the uppermost surface of DLC film and some oxidized Cu atoms resulting in the reduction of friction coefficient and the increase of wear rate.Cu nanoparticles can improve the friction reduction and anti-wear properties of liquid-solid composite lubrication system with carbon based film under a wide range of application conditions,which makes them a very promising lubricant additive in the future.(2)Two dimensional nano-graphenes are used as base oil additives to form solid-liquid composite lubrication systems with carbon based films.The effects of load,test temperature,sliding speed and carbon film structure on the tribological properties of the liquid-solid composite lubrication system are studied by using a ball-on-disk friction test system.The results show that the graphene-addivated lubricating oil/DLC film liquid-solid composite lubrication systems show excellent friction reduction and wear resistance under wide load and temperature range and different sliding speeds.In particular,it shows an excellent friction reduction effect under 20 N load with a friction coefficient of 0.054,along with an anti-wear effect under a high sliding speed of 160 mm/s with a wear rate of 1.64×10-10 mm/N·m.The sp2 hybrid carbon comes from the deposited DLC film together with those transfored from sp3 hybrid carbon in the friction process,forming an ordered sp2 hybrid carbon-graphene-carbon material"sandwich"lubrication structure between the friction contact surfaces.This lubrication structure is the main reason for the improvement of the tribological properties of graphene-addivated lubricating oil/DLC film liquid-solid composite lubrication system.(3)Firstly,the effects of two full formula lubricants(0w-20 and 0w-40)with different viscosities on the tribological properties of DLC coatings are studied.Furthermore,Cu nanoparticles and two-dimensional nano-graphene are added into the full formula lubricant 0w-20 as additives,and the liquid-solid composite lubrication performance of the composite system is studied.The results show that DLC coating shows excellent friction reduction,wear resistance and frequent cold start performance in low viscosity lubricating oil 0W-20.The low-viscosity lubricating oil with low viscosity resistance and good fluidity can timely fill into the friction contact interface,at the same time,the synergistic effect of nano adaptive friction film are formed on DLC coating in the friction process,which are the main reasons for the excellent tribological performances of DLC coating.As an additive of 0W-20 lubricating oil,two-dimensional nano graphene can reduce the wear of DLC film.The more wear-resistant interface constituted by the nano adaptive friction film formed by DLC coating and two-dimensional graphene in lubricating oil is the main reason for the wear reduction of DLC coating.(4)The tribological properties of DLC,CrN and Cr-diamond coatings on the surface of piston ring in piston ring/cylinder liner friction pair lubricated by 0W-20 oil containing graphene additive are studied in simulated engine working environment.Taking gear transmission as the research goal,the wear resistance and load-bearing performance of liquid-solid composite lubrication system with DLC coated gears in graphene-addivated transmission oil are studied by simulating the mechanical motion state containing both rolling and sliding in the process of gear transmission.The results show that graphene-addivated lubricating oil can stabilize and reduce the friction coefficient of the coated piston ring/cylinder liner friction pair and greatly reduce the wear rate of cylinder liner materials in the simulated engine working environment.The main reason is the formation of a multi-component hybrid friction film containing graphene and friction reactants and oxides of lubricating oil additive on the wear surface of the cylinder liner.The testing gear with DLC coating failed in both transmission oil and graphene-addivated transmission oil when the loading level reached level 8,which was mainly due to the large-area massive spalling of DLC film caused by original defects in the gear.In conclusion,the study on graphene-addivated lubricating oil/carbon based film liquid-solid composite lubrication system is expected to provide some data and theoretical support for improving the tribological performance of the key motion pairs of fuel engines.