Effect Of Graphene Additive On Tribological And Rheological Properties Of Lithium Grease

The application of advanced lubrication technology plays a very important role in reducing energy consumption,prolonging service life of equipment and improving comprehensive benefits.As a common lubricant,grease has many unique advantages,among which lithium grease is the most widely used.Additives,as one of the important components of grease,have an important influence on the anti-wear performance of grease.As a typical two-dimensional material,graphene has good self-lubricating properties,and has the potential to become a "lamellar structure" lubricating additive.It has been widely concerned in oil lubrication,water lubrication and self-lubricating composites,but rarely involved in grease.In this thesis,the effects of multi-layer and few layer graphene on the physical properties,tribological properties and rheological properties of lithium grease were studied.Atomic force microscopy(AFM)and Raman spectroscopy(RS)were used to analyze the multi-layer(few layer)graphene.The results show that the multi-layer graphene is about 14 layers and the few layer graphene is 8 ? 9 layers.Multi layer(few layer)graphene lithium grease with the content of 0.05 wt%,0.1 wt% and 0.2wt% was prepared by physical dispersion.Graphene dispersed well in the grease without obvious agglomeration.By analyzing the cone penetration and dropping point of the prepared grease,it is found that the addition of multi-layer(few layer)graphene can reduce the cone penetration,that is,improve the consistency of the grease,and the dropping point has little change.The results show that the addition of a small amount of multilayer graphene has little effect on the soap fiber structure of lithium grease.The tribological properties of the original lithium grease and the prepared lithium grease containing multi-layer(few layer)graphene were investigated by using a four ball friction and wear tester.Under the same test conditions,with the increase of the content of multi-layer(few layer)graphene,the friction coefficient and wear spot diameter first decrease and then increase.When the content of multilayer(few layer)graphene is 0.1 wt%,it reaches the minimum value,that is,when the content of multi-layer(few layer)graphene is 0.1 wt%,the friction reduction and anti-wear performance is the best.Field emission scanning electron microscopy(FESEM),energy dispersive X-ray spectroscopy(EDX)and X-ray photoelectron spectroscopy(XPS)were used to characterize the surface morphology,chemical element distribution and valence state of the wear spots.The physical deposition film rich in multi-layer(less layer)graphene and chemical adsorption film with Fe2O3 as the main component are formed on the friction surface when the multi-layer(less layer)graphene lithium base grease is lubricated.This is due to the weak adhesion between the layers of graphene,which is easy to dissociate along the interlayer under the action of shear force in the friction process.At the same time,graphene has high surface activity,which can easily interact with the friction surface and adhere to the friction surface to form a physical deposition film.In addition,graphene itself has better lubricity and higher strength,so as to reduce the friction coefficient between the friction pairs,improve the surface strength of the friction pairs,reduce the direct contact between the asperities,and improve the antifriction and anti-wear performance of lithium-based grease.Under the same test conditions,the friction reduction and wear resistance of multi-layer graphene lithium grease with the same content is better than that of few layer graphene lithium grease.With the increase of speed and load,the average friction coefficient decreases,but the average wear spot diameter increases.The influence of speed and load on the anti-wear performance of grease is different,and the influence of load is more significant.The rheological properties of lithium grease containing multi-layer(few layer)graphene were analyzed by rheometer.With the increase of graphene content,when the grease bears more strain,it can change from solid state to flow state,resulting in the increase of strain value corresponding to flow point.At the same content,the flow point of less layer graphene lithium grease is larger than that of multi-layer graphene lithium grease.With the increase of graphene content,the thixotropic ring area of grease increases,and the thixotropic ring area of less layer graphene lithiumbased grease is larger than that of multi-layer graphene lithium-based grease at the same content,which indicates that the energy required to destroy its structure increases,and the soap fiber structure is more difficult to destroy,but the recovery time after structure damage is longer.According to the experimental data,the rheological equation of multi-layer(few layer)graphene lithium grease with different content was obtained.At the same time,by comparing the variation of friction coefficient and apparent viscosity with time,as well as the corresponding relationship between the friction coefficient,flow point and thixotropic ring area of multi-layer and few layer graphene lithium-based grease,the correlation between the tribological properties and rheological properties of multi-layer(few layer)graphene lithium-based grease was qualitatively analyzed.