Research On Rheological And Tribological Performance Of Graphene-based Lubricants

Graphene,a two-dimensional material comprised of sp2 bonded carbon atoms,has been widely applied in many areas for its excellent phisical and chemical properties,as well as the performance of environmental protection.Researches found that graphene nanoplates and the functional graphenes as additives have improved the tribological performance of the base oil.Based on that,graphene is potential to be a sort of lubrication material with high performance applied in the transmission system of space and aerodynamics.In order to achieve the technology breakthrough of the bearing steel protection and long woking life,the diffusion behavior,rheological properties of lubricants and the tribological performance of the representative aviation tribo-pairs,as well as lubrication mechanism were systematically investigated based on graphene as additives.The research results provide the theorotical basis for the chemical modification of graphene and the lubrication technique of additives with high performance.Based on the theory of molecular dynamics,the diffusion behavior of graphenes as additives in lubricating oil was studied by using the equilibrium molecular dynamics.In addition,the effects of graphene size,sorts and number of fuctional group,temperature and pressure on the diffusion coefficient were investigated.According to the study,an approach of evaluating the disper sibility of graphenes in oil by using the diffusion coefficient was propposed for four sorts of graphens,which was validated by mean of the sedimentation experiment,the research results reported by Cha and the simulation discussion of r adial distribution function for graphene-PAO molecules.This method was used to study the effects of graphene size,sorts and number of fuctional groups,temperature and pressure on the dispersibility of graphens.The molecular design integrate with the evaluation of dispersibility of graphenes could be achived by using this method.Nonequilibrium molecular dynamics was employed to study the rheological properties of lubricanting oils containg graphene.The simulation model was established and validated by the experimental data reported by Bair and the simulation conducted by Liu.Herein,the influences of the mass fraction,temperature,pressure and shear rate on rheological properties of blended lubricating oil were investigated.The viscosity of blended lubricating oil was significantly improved by adding graphene nanoplate at high temperature and pressure.The method was benificial to the study on the effect of rheological properties of bubricating oils on the tribological performance.The effects of mass fraction of graphene and temperature on viscosity of blended lubricationg oils were studyed by using the testing method of dynamic viscosity.The viscosities of blended lubricating oils were going down with the decreasing mass fraction of graphene and the increasing temperature;the effect of temperature became more significate at higher mass fraction of graphene.The effects of graphene on the tribological performance of GCr15/GCr15 steel and Si3N4 ceramics/GCr15 steel tribo-pairs were investigated by using the four ball tester.The ability of friction reduction and anti-wear were respectively improved by 25%,39% and 27%,43.4%.The tribological performance of graphene oxide(GO)as additives in aviation lubricating oil was studied by using the tribometer.The effect of GO on the tribological performance of Si3N4 ceramics/GCr15 steel tribo-pairs was more significate at higher applied loads.Furthermore,the effect of graphene on the oil film thickness of mixed lubrication in point contacts was investigated.The evolution model of lubrication mechanism was established and the interlayer shear mechanism in running in stage,surface mending mechanism in oil film formation stage and film formation mechanism in the stage of stable friction was revealed by the analysis of rubbing surfaces integrated with the friction process.