| Due to its excellent electrical/thermal conductivity and good compliance,copper has a wide range of applications in the fields of bearings,friction materials,and electrical materials.However,the high intrinsic coefficient of friction and poor wear resistance of copper limit its further application in the above-mentioned fields.At present,there are two main ways to improve the wear resistance and friction reduction properties of copper:(1)adding a lubricating phase(such as graphite,molybdenum disulfide,hexagonal boron nitride and other layered materials)to the copper;(2)direct deposition of solid lubricating coatings on metallic copper surface.Regardless of the approach used,the rapid formation of dense and continuous lubricating interfaces(eg,friction and/or transfer layers)during friction is the key to improving the wear and friction reduction properties of copper.However,the establishment of the above-mentioned lubricating interface is affected by various factors,including the dispersion of lubricating components in the copper matrix,the lubricating component/copper interface compatibility,the density of solid lubricating coating,the film/substrate binding force of lubricating coatings to copper substrates,tribochemical reaction between lubricating components and the environment,etc.Therefore,it has always been one of the hotspots in tribological research to develop various means to control the solid lubricating interface and reveal their mechanism of action on the friction and wear properties of copper.The research content of this thesis is divided into two parts:(1)the study of the friction and wear properties of molybdenum disulfide/copper composites;(2)the effect of graphene-based solid lubricating coatings on the tribological properties of copper.The former proposes a strategy of alloying with trace amounts of chromium in the copper matrix to promote the rapid formation of a dense and continuous friction layer on the surface of the molybdenum disulfide/copper composite during the friction process;the latter proposes two strategies of combining graphene oxide with molybdenum disulfide nanosheets and cross-linking graphene oxide with copper ions,respectively,to promote the rapid formation of the frictional interface between the friction layer of graphene oxide and the transfer layer of graphene oxide during the friction process.The specific research contents are as follows:A series of molybdenum disulfide/copper composites were prepared by hot pressing and sintering using copper powder with different chromium additions as the matrix and molybdenum disulfide as the lubricating component.The effect of chromium content on the tribological properties of composites in nitrogen atmosphere was investigated.The molybdenum disulfide/copper interface structure,composition of friction layer and subsurface morphology of wear scar were analyzed in detail by means of transmission electron microscopy,X-ray photoelectron spectroscopy and scanning electron microscopy.The results show that adding an appropriate amount of metallic chromium,such as 1.5 wt.%,the friction coefficient of the composite is as low as 0.07,and the wear rate is 2.4×10-4mm~3N-1m-1,which are 64%and 20%of the composite without chromium,respectively.Adding an appropriate amount of metallic chromium can effectively inhibit the formation of intrinsic brittle phase at the molybdenum disulfide/copper interface and reduce the composition of brittle phase in the friction layer.At the same time,a strong subsurface layer is provided for the friction layer as a support to ensure that the friction layer maintains a continuous and dense state during the friction process.Defect-rich molybdenum disulfide nanosheets and graphene oxide are used as raw materials,and a nanocomposite coating of molybdenum disulfide/graphene oxide is deposited on the surface of the brass substrate by spraying process.The tribological properties of the composite coatings in atmospheric and dry nitrogen environments,and the effect of the content ratio of graphene oxide and molybdenum disulfide nanosheets on the tribological properties of the composite coatings were studied.The results show that the composite coating with optimized composition has a coefficient of friction as low as 0.06 in a dry nitrogen atmosphere,and its life is more than 30 times longer than that of pure molybdenum disulfide and graphene oxide coatings;in the air environment,the coefficient of friction is reduced by 14%and the lifetime is increased by 60%compared with the pure graphene oxide coating.The enhanced tribological properties are attributed to the synergistic effect generated during the friction process of defect-rich molybdenum disulfide and graphene oxide,which promotes the formation of a large-area molybdenum disulfide/graphene oxide hetero contact interface.It reduces the interfacial shear strength and at the same time reduces the structural degradation of molybdenum disulfide and graphene oxide caused by friction.Copper ions cross-linking graphene oxide coatings were deposited on copper substrates by applying a low positive voltage to the copper substrates placed in an aqueous graphene oxide dispersion.The tribological properties of the coating in atmospheric environment were studied,and the morphology,composition and microstructure of the friction layer on the substrate and the transfer layer on the grinding ball were analyzed in detail.It was found that even with a maximum Hertzian contact pressure of 1.1 GPa,the coating had a friction coefficient as low as 0.17,and the wear rate of the copper substrate was more than 20 times lower than that of the bare copper substrate.The reason for the improved tribological properties is attributed to the cross-linking reaction of Cu2+with graphene oxide nanosheets.It not only enhances the mechanical properties of the coating and its bonding force with the copper substrate,but also reduces the content of C-O functional groups,which promotes the rapid formation of stable lubrication interface between the graphene oxide friction layer and the graphene oxide transfer layer during the friction process. |
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