| Solid lubricating coatings are widely used in extreme environmental conditions such as high temperature,high pressure,and vacuum,which is one of the leading topics in the field of friction and wear.Since its discovery,graphene has received widespread attention due to its excellent properties such as high mechanical strength,low surface energy,and low shear strength,as well as its application potential in solid lubricant coatings.Electrodeposition is one of the important methods for preparing metal-based composite coatings due to its irreplaceable advantages,such as the deposition of complex surfaces and internal holes.At present,electrodeposition of graphene-reinforced metal-based composite coatings has carried out.Among them,the outstanding performance of nickel-based graphene composite coatings in terms of solid lubricating properties is of particular concern.In electrodeposition,the uniform dispersion of the introduced particles is the key to obtain a composite coating with excellent performance.Graphene is very easy to agglomerate in the electrolyte due to its hydrophobicity,which has been an urgent problem in electrodeposition applications.The agglomeration of graphene results in a rough surface morphology and the poor structural properties of the obtained coating.The high hardness,high wear resistance and excellent lubricating properties expected from doping with graphene cannot be reflected.How to obtain a nickel-graphene solid lubricating coating with flat surface and excellent lubrication characteristics is of great significance.Therefore,this paper proposes“two-step”and“one-step”electrodeposition method to prepare nickel-graphene composite coatings.It hope that the agglomeration of graphene can be reduced to obtain nickel-graphene composite coatings with excellent properties.In this paper,nickel-graphene composite coatings were prepared by two-step and one-step electrodeposition,respectively.In the two-step method,nickel was used as the anode,304 stainless steel was used as the cathode,and Watts plating solution was used as the electrolyte.First,electrochemically exfoliated graphite using a ternary eutectic solvent to obtain exfoliated graphene,and then the exfoliated graphene was added to the above-mentioned Watts electrolyte.After ultrasonic dispersion,Ni~+induced graphene electrochemical co-deposition was performed to obtain a nickel-graphene composite coating.In the one-step method,graphite was used as the anode,304 stainless steel was used as the cathode,and the ternary eutectic solvent was used as the electrolyte.Various experimental schemes were repeatedly tried,and the experimental parameters were continuously optimized.Exfoliation and deposition were performed simultaneously to prepare the nickel-graphene composite coating.The graphene powder was analyzed with SEM,TEM,and Raman spectrum.The crystal structure,surface morphology,composition elements and hardness of the composite coatings were detected by XRD,SEM,EDS and micro-hardness tester.The cross-section morphology and interface characteristics of coatings were characterized by SEM.Dry friction experiment were carried out at room temperature under 1N load to examine the friction and wear characteristics of the composite coating.The results of the study are as follows:Two-step electrodeposition of nickel-graphene(T-NiG)coating:Adding surfactant and ultrasound can improve the dispersibility of graphene in the electrolyte and reduce the agglomeration of the composite coating to a certain extent.However,when the concentration of graphene in the electrolyte is further increased,surfactants and ultrasound cannot effectively reduce the agglomeration of graphene.As the concentration of graphene in the electrolyte increased,the carbon content in the composite coating also increased,and the agglomeration phenomenon on the surface of the coating became more obvious,but the phase structure of the composite coating had not changed.When the graphene concentration in the electrolyte was 300 mg/L,the carbon content in the composite coating T-NiG300 was 29.8 wt%.The agglomeration phenomenon of T-NiG300 surface was clearly observed,and the nickel particles were coarse.One-step electrodeposition of nickel-graphene(O-NiG)coating:When the surface energy of the liquid phase is close to that of graphene,peeling is most likely to occur.In this study,the surface energy of the deep eutectic solvent is 43 mJ/m~2,and the surface energy of graphene is 46.7 mJ/m~2,so it is easy to obtain graphene by peeling off the graphite anode.The few layers of graphene separated from the liquid phase was transparent and had typical D and G characteristic peaks.The electrolyte had the dual functions of peeling and deposition,and realized the simultaneous exfoliation and deposition of graphene,which effectively reduced the agglomeration of graphene.The final composite coating surface was flat and the graphene was uniform dispersed in the matrix metal.Nickel particles were grown using graphene as a template,which increased the binding force between graphene and the matrix metal.After testing,the carbon content in the composite coating was 23.9 wt%.Although the incorporation of graphene did not change the orientation of the crystal plane of nickel in the composite coating,graphene interrupted the growth of columnar nickel crystal,thereby refined the crystal grains.Tribological characteristics:Introducing graphene into the composite coating can significantly improve the hardness and friction reduction of the composite coating.As the concentration of graphene in the electrolyte increased,the friction coefficient of the composite coating could be reduced.However,too high graphene concentration would increase the agglomeration of graphene and caused the fluctuation of the friction curve of the composite coating.Compared with pure nickel coating,T-NiG300 composite coating effectively reduced the friction coefficient.However,due to the serious agglomeration of graphene,nickel particles continuously grew to form larger nickel particles.In the friction experiment,the nickel particles caused fluctuations in the friction curve,and the agglomeration of graphene weakened the bonding force between graphene and the matrix metal.When the friction distance was less than 200 meters,the composite coating peeled off,and the friction curve showed large fluctuations,resulting in poor friction characteristics of the composite coating.For O-NiG composite coatings,graphene had a load-bearing effect and was uniformly dispersed in the coating.Therefore,even if the coating worn up to 200 meters,the composite coating did not show obvious peeling.After the running-in period,the friction curve had maintained a relatively stable state until the experiment was over,resulting in excellent friction characteristics of the composite coating.The nickel and carbon elements were detected on the surface of grinding parts.During the friction experiment,the composite coating continuously worn and plastic deformation of the coating occured.Nickel and graphene were transferred to the friction interface to form a friction film,which effectively reduced the friction of the composite coating. |
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