Study On Preparation Process Optimization And Properties Of Ni-Co-graphene Composite Coatings

In order to improve the shortcomings of 45#steel surface properties and meet the needs of the work environment and life of the material,in this paper,Ni-Co alloy plating and Ni-Co-graphene composite coating are prepared on the surface of45#steel with a method of composite electrodeposition in the system of sulfamate plating solution to increase the basal material mechanical properties such as hardness,wear resistance and corrosion resistance,The strengthening effect of graphene on the coating was studied by comparison.The orthogonal test method of L₁₆（4⁵）was used to optimize the process parameters.The micro-hardness and friction coefficient were selected as two indexes to investigate the influences of graphene concentration,current density,bath temperature,stirring speed and plating time on the micro-hardness and wear resistance of the composites.According to the result of the orthogonal test,the influences of graphene concentration and current density on hardness,wear resistance and corrosion resistance were investigated by single factor experiments.The wear resistance of the material was evaluated by the means of average friction coefficient and wear quantity,and the wear mechanism was analyzed.The corrosion resistance of the composite coating was evaluated by open-circuit potential,electrochemical impedance and Tafel polarization curves.SEM,EDS,XRD and Raman spectra were used to characterize and analyze the microstructure,element distribution,phase structure and the presence of graphene of the composite coating respectively.The results showed that:（1）The optimized process were as follows:graphene concentration 5 g/L,current density 7 A/dm²,bath temperature 40℃,plating time 60 min,stirring speed 200 r/min.The surface hardness of the composite reached 1154.0 HV,which was 154.2%higher than that of the substrate（454.0 HV）,the friction coefficient of 0.1503 was 71.6%lower than that of the substrate（0.5287）,and the wear amount of 0.00148 mm² was 87.4%lower than that of the substrate（0.01173 mm²）,indicating that the optimized process greatly improved the properties of the composite.（2）With the increase of graphene concentration,the micro-hardness firstly increased and then decreased.The change trend of friction coefficient and wear quantity is consistent that both decrease first and then increase.When the graphene concentration was 3 g/L,the micro-hardness reached the maximum of 957.7 HV.The minimum value of friction coefficient is 0.1915 at 3 g/L.The minimum wear capacity is 0.00079 mm² in 4 g/L.From the point of view of corrosion resistance,the open circuit potential,arc radius of capacitive reactance and Tafel curve all reached the best at 3g/L.The mechanism of action of graphene was analyzed from three aspects of hardness,wear resistance and corrosion resistance.（3）With the increase of current density,the variation trend of micro-hardness is increasing and then decreasing,while the friction co-efficient and wear quantity both decrease first and then increase.The micro-hardness reaches a maximum of 1078.8 HV at 6 A/dm²,the friction coefficient reaches a minimum of 0.2473 at 6 A/dm²,and the wear capacity reaches a minimum of 0.00066 mm² at 5 A/dm².From the point of view of corrosion resistance,the open circuit potential is the most positive at 5A/dm²,the radius of capacitive reactance arc is the largest at 6 A/dm²,and the Tafel curve is the best at 4 A/dm².The mechanism of current density was analyzed from three aspects of hardness,wear resistance and corrosion resistance.（4）The wear mechanism of Ni-Co alloy coating is abrasive wear and delamination wear,which belongs to serious wear.The wear mechanism of Ni-Co-graphene composite coating changes due to the addition of graphene,which changes from abrasive wear to fretting wear,reduces the wear degree,and significantly reduces the friction coefficient and wear amount of composite coatings.（5）SEM analysis showed that the grain size of Ni-Co coating and Ni-Co graphene composite coating was uniform and the adhesion of the coating was excellent.EDS results show that the elements are uniformly dispersed in the coating.According to XRD analysis,the grain size of Ni-Co graphene composite coating is smaller than that of Ni Co alloy coating.The appearance of（002）crystal plane proves the existence of graphene in the composite coating.On the basis of the Raman spectrum analysis,it proves that the existence of graphene in the coating with a very slight damage extent,which is consistent with the result of EDS carbon content.The results show that composite plating can enhance the properties of the composites without destroying the unique structure of graphene.