Study Of The Tribology Behavior Of Graphene Nanosheets And WS₂/graphene Nanoblend As Lubricating Oil Additives

It is well known that numerous mechanical system failures originate from wear,and most non-renewable energy resource exhaustion results from friction.The most effective approach to control or reduce friction and wear is to use a lubricant.Therefore,the progress in lubrication technology will have a huge and far-reaching positive impact on the production and life of human society,such a view might seem self-evident.Graphene,a single-atom-thick carbon,shows a broad application prospect in tribology,because of its 2D structure and unique mechanical property.In the field of lubrication,the current research on graphene is mainly focused on its lubricating performance and corresponding mechanism,but while the research on the synergistic lubrication of graphene and other lubricating factors has rarely been reported.Studies on the synergistic lubrication of graphene,not only enrich the research content on graphene tribology,but also provide theoretical guidance for the lubricating application of graphene.The preparation of lubricating materials with the simultaneous functions of friction reduction,anti-wear,and repair by combining micro/nanomaterials and lubrication is currently a hot research topic in the field of lubrication.Additionally,surface texturing technique is another commonly used method in surface engineering,which could improve the tribological property of contact surface.Therefore,maximizing the role of lubricating factors by combining surface texturing and lubrication,has attracted more and more attention.In this paper,the tribology behavior of the graphene as oil additives on textured alloy cast iron surface,and the lubricating behavior of WS2/graphene nanoblend were investigated on sliding steel-to-steel contact,using a UMT-2 ball-on-plate tribotester.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),energy-dispersive X-ray spectroscopy(EDX),X-ray diffractometer(XRD),fourier transform infrared spectrometer(FT-IR)and Raman microscope system were used to characterize the morphologies and microstructures of the nano-additives.The lubricating performance and corresponding mechanism were evaluated and analyzed using 2D profilometer,3D profilometer,optical microscope(OM),SEM,Raman,EDX and X-ray photoelectron spectroscopy(XPS).The main conclusions can be drawn as follows:1.Three various textures of alloy cast iron surface(dimple density of plate sample: 22.1%,19.6% and 44.2%)were prepared and characterized.WS2 nanoparticles and WS2/graphene nanoblend were also synthesized,and the morphologies and microstructures of the nanomaterials,graphene,WS2 nanoparticles and WS2/graphene nanoblend were characterized.Surfactant span-80 was used as a dispersing agent to improve the dispersity of the nano-additives.The WS2/graphene nanoblend dispersion with the added dispersant shows the optimum effect.2.The tribo-test results show that graphene displays good lubricating performance as additives in the base oil,which is maximized at concentration of 0.01 wt% and temperature of 100 ?.The characterization results of EDX,Raman and XPS show that,during the frictional process,the graphene layers sheared at the sliding interfaces and adsorbed attributed to their 2D structure,a protective film may be formed,and hence provides low friction and wear.3.The effect of surface texture on increasing and decreasing friction is simultaneously presented.Compared with smooth surface,the roughness of the textured surface increases,which leads to greater resistance at the sliding contact interfaces;Additionally,during the frictional process,the boundary lubricating film rupture caused by the local high stress on the dimple edges,and hence provides high friction and wear.But meanwhile the texture micro-dimples can store the lubricating oil and wear debris and release,which act as friction-reducing and anti-wear agent.In summary,the arrangement,dimple density and dimple spacing of texture are important factors influencing its lubricating properties.4.When both graphene nano-additives and texture were used,a synergistic lubricating effect of these two factors was revealed.Friction and wear decreased significantly under these lubrication conditions,and the lubrication effect is also superior to that caused by graphene nano-additives alone.The characterization results of EDX and Raman show that,texture micro-dimples induce graphitizing transformation of graphene,and can store functionalized graphene and release,which effectively promote the adsorption of graphene,thereby greatly reducing the friction and wear.Overall,the larger the texture dimple density,and the smaller the dimple spacing,the more significant synergistic lubricating effect.5.Excellent lubricating performance of WS2/graphene dispersed oil can be achieved,because of the synergistic lubricating behavior of the WS2/graphene nanoblend,which is maximized at concentrations of 0.02-0.04 wt%.The friction-reducing and anti-wear abilities of WS2/graphene nanoblend are also superior to those of graphene or WS2 nanoparticles alone,as well as to a physical mixture of the two nanomaterials.The characterization results of EDX,Raman and XPS show that,the WS2/graphene nano-additives may deposit on the wear surfaces and form a protective film that improves the contact state of the sliding interfaces.Moreover,a synergistic lubricating action between the graphene and WS2 nanoparticles of the WS2/graphene nanoblend was revealed.We suggest that these two nano-additives work cooperatively,which enhances the adsorption of graphene layers and deposition of WS2 nanoparticles,thereby resulting in an improved protective and repairing effect,and then greatly reducing the friction and wear.This is the dominant friction-reducing and anti-wear mechanism of WS2/graphene nano-additives.Frictional chemical reaction is another reason.