Inquiry On The Tribological Behavior Of Graphene Nanosheets As Lubricant Additive In Oil-Film Bearing

The lubricity has great influence on the normal operation and service life of machinery,in which oil-film bearing oil can guarantee the safe running of bearing system due to the advantages of large bearing capacity,small friction coefficient and strong impact resistance.The normal operation of oil-film bearing depends on the lubricant film located between the working area of bushing and taper sleeve.However,the phenomenon of bearing wear or even failure of oilfilm bearing often occurs due to the instability formation of lubricating oil film in the actual operation.Therefore,it is important to ensure the stable formation of lubricant film on the surface of the friction pair.Particularly,enhancing the properties of anti-friction and anti-wear of lubricating oil is also necessary.The current studies mainly focus on improving the lubrication system of bearing to maintain the integrity and stability of the lubricant film.For example,the using of magnetic fluid as lubricant instead of conventional bearing lubricants and the research on the influence of bushing materials on oil film.Adding specific anti-wear agents in the base oil to enhance its lubricating property is a common research method in the field of tribology,which also has the potential to improve the lubricating performance of oil-film bearing.Traditional additives(e.g.sulfur,phosphorus and boron)are increasingly limited in their application due to environmental hazards.In contrast,as promising environmental candidate additives,the tribological properties of carbon materials in bearing oil are worth studying and exploring.Carbon materials of different shapes and dimensions can exhibit excellent chemical stability and self-lubrication due to their unique molecular structure and atomic interface.However,in view of the enhancement effects of nanoparticles affected obviously by their geometrical shapes,it is necessary to explore the advantages and disadvantages of lubricating effects for pristine carbon materials with different dimensions and shapes.In this paper,we study the tribological behavior of pristine two-dimensional graphene nanosheet additive in commercial 220 oil-film bearing oil,which is compared with one-dimensional multi-walled carbon nanotubes(MWNTs)and amorphous activated carbon powder.Besides,the lubricating property differences of graphene nanosheets,activated carbon and MWNTs in oil-film bearing oil are further investigated.The microstructure testing results show that activated carbon powders present irregular shapes,and the particles are relatively large and not uniform.The MWNTs represent tubular structures and intertwine with each other under the influence of Van der Waals force,which have many defects.Graphene nanosheets present lamellar structures with overlapping between sheets and fewer defects.Calculated by the Bragg equation,the lattice spacing of graphene used in this work is d = 0.335 nm,which is beneficial for the inter-layer sliding of graphene nanosheets.The tribological testing results show that among the three kinds of carbon materials,graphene nanosheets have the best and most stable lubrication effect.The optimal average friction coefficient and wear rate of graphene nanosheets are reduced by 35.2% and 21.3% respectively under the load of 5 N,resulting from its excellent interlayer slip performance and self-regulation performance.Furthermore,the optimal component of graphene nanosheets changes according to various degree of stacking under low loads of 5-15 N,and its anti-wear performance shows a significant increase with the increase of the load.It is worth mentioning that its anti-wear property has a better stability in the low concentrations between 0.01 to 0.4 wt%.This study is expected to provide a certain reference value for the basic research of carbon material additives in improving the lubricating property of oil-film bearing.