Study On Tribological Properties Of Graphene And Carbon Nanotube Based Nanocomposites Synthesized By Supercritical Deposition

Nanoparticles?soft metals and metallic oxides?and carbon nanomaterials?graphene and carbon nanotube?have many special features and show brilliant application prospect in lubrication field.Carbon based nanocomposites constituted by nanoparticles and carbon nanomaterials inherit their excellent properties of the components,even show synergistic effects,and thus have important application values in many fields.Howerver,at present,there has been little research on tribological applications of this kind of nanocomposite.There are lots of approaches for syntheszing carbon based nanocomposites,but the microstructure of prepared nanocomposite usually shows poor regularity and uniformity,which severely restrict its macro performances.This researche is aim to achieve controllable synthesis and structure optimization of carbon based nanocomposite,investigate its tribological properties and the evolution process of friction pair surfaces,and establish lubrication mechanism of the nanocomposite as lubricant additive.The research contents and main conclusions are listed below.?1?Various carbon based nanocomposites,including graphene based nano copper,nickel,silver,gold or CuO nanopaticles and carbon nanotube supporting nickel or silver nanopaticles,are synthesized by in situ decorating nanoparticles on carbon substrate surface with ScCO₂assisted chemical reduction.X-ray diffraction?XRD?,Raman spectroscopy?Raman?,thermal gravimetric analysis?TGA?,X-ray photoelectron spectroscopy?XPS?,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?are used to analyze phase structure,micro morphology and chemical composition of the synthesized nanocomposites.Characterization results prove that ScCO₂ can remarkably improve the microstructure of the nanocompsites,reduce size of supported nanoparticles and also enhance dispersity of the nanoparticles on carbon substrates.For nanocomposite of graphene supporting silver,the silver nanoparticles fabricated by traditional chemical deposition show broad size distribution and inhomogeneous dispersion on graphene nanosheets and form large agglomerations.After ScCO₂ is introduced,the silver nanoparticles are highly dispersed on graphene nanosheet and show spherical shape with small and uniform paricle size.Furthermore,the average diameter of silver nanoparticle is nearly 2-10 nm and silver loading weight is also increased.?2?Surface chemical modification can improve the dispersibility of carbon based nanocomposites in oils and prevent them from precipitating for a long time.The modified nanocomposites as lubricant additives can greatly enhance the friction reducing and antiwear properties and loading carrying capacity of lubricating oils.The improved effects of the nanocomposites for oils are closely related to several factors including their microsutructures,species and the firciton testing conditions.Synergistic friction-reducing and antiwear effect is found between graphene sheets?or carbon nanotubes?and deorated nanoparticles for these carbon based nanocomposites.For instance,for paraffin oil containing the nanocomposite of graphene supporting nickel nanoparticles?Sc-Ni/GO?,resultant friction coefficient decreases firstly and then increases as load or rotating speed goes up,but wear scar diameter increases almost linearly.The optimum adding concentration of Sc-Ni/GO is about 0.06-0.08wt%,which makes friction and wear reduced by 32%and 42%,respectively.The synergistic effect of the carbon based nanocomposites is attributed to their unique microstructure and strength of the effect is determined by multiscale structure optimization.However,the effect cannot be achieved by means of mechanical mixing.The carbon based nanocomposites also have great influences on rheological properties of lubricating oils and oil film in elastohydrodynamic lubrication?EHL?.Typically,the nanocomposite of reduced graphene oxide supporting CuO nanoparticles raises viscosity of 10w40 engine oil nearly by 20%,and thus thickens the oil film in EHL.?3?To explore the synergistic friction-reducing and antiwear mechanism of the carbon based nanocomposites,corresponding friction surfaces are systematically analyzed by using different characterization methods.The results indicate that the carbon based nanocomposites firstly deposit on friction pair surface during sliding process and thus form thick but discontinuous depositon films.These films gradually become thin and continuous in friction process till tribo-chemical reactions occur on friction surfaces and thus form continuous and uniform tribo-transfer films.The transfer films reduce frication resistance,weaken abrasive wear and adhensive wear behaviors and make the friction between friction pairs transform into the friction between transfer films,thus leading to friction reducing and antiwear effect.Moreover,transfer films can reduce roughness of friction pair surfaces and the deposited carbon based nanocomposites can readily conduct friction heat.Accordingly,the lubricating state of lubricant oils is imporved,which is transformed from mixed lubircation?boudary lubrication+dry friction?into good boudary lubrication,further reducing friction and wear.