Study On Electrodeposition Of Ni-graphite Composite Coatings In Sulfamate Bath

With the development of high-tech industries and the higher requirement of electronic product, the LCD technology is widely used in various areas. However, the LCD materials which are used as LCD cutting tools in the processing and manufacturing process are still imperfect. Most of the cutting tools are obtained by means of electroforming. The electroformed nickel-diamond composite coating is of high hardness and well wear-resistance but poor binding force and cracked morphology which shortens the service life of LCD product. Given the high cost of diamond particles and the same element and similar structure with graphite, graphite particle is used to study the electrodeposition of Ni-graphite composite coating instead of diamond in this work. The dispersion of graphite particle in water and nickel electrolyte is systematically researched. Moreover, the preparation of composite electroplating bath and the influence of process parameters such as current density, bath pH, bath temperature, plating time and graphite concentration etc on the properties of Ni-graphite composite coating are researched in detail. Analysis of the mechanism of the distillation process with electrochemical workstations; X-ray diffraction(XRD) and SEM are used to analyze the structure and surface morphology of the composite coating. The microhardness, binding force, corrosion resistance of composite coating are tested. The main results are as follows:(1) The agglomeration of graphite particle in aqueous solution is studied. Results reveal that the Zeta potential of graphite is zero when the pH is about 6.30; with the addition of dispersant CMC, the Zeta potential changes very little but get higher when pH is about 4.00-4.50, and then the graphite particle disperses very well. Meanwhile, sedimentation experiment shows that CMC has the best dispersion effect among the used kinds of dispersants. The graphite particle has better dispersing ability by thirty-minute ultrasonic waves and adding CMC with the mass fraction of CMC to graphite is 10%. The dispersion of the graphite particle with CMC in nickel electrolyte is also discussed. Results show that it disperses well in nickel electrolyte but not better than in aqueous solution. Graphite particle distributes more uniformly throughout the composite coating with the addition of dispersant agent. (2) Effects of current density, bath pH, stirring speed, bath temperature, plating time, concentration of graphite and main salt are studied to obtain suitable process parameters by single-factor experiment. Results show that the optimum process parameters are as follows: current density 8A/dm2, pH 4.00, temperature 40℃, stirring speed 200 r/min, and deposition time 10min. And the optimum bath composition is as follows:nickel sulfaminate 300g/L, nickel chloride 5g/L, boric acid 35g/L, surfactant 0.2g/L, saccharin lg/L,1,4-butynediol 0.4g/L, AN-160g/L, AN-220g/L, graphite particle 3g/L and CMC 0.36g/L. The Ni-graphite composite coating is grayish white and graphite particle is uniformly distributed throughout the composite coating. The microhardness of the coating is up to 372.6HV, mass fractions of the main elements in the coating are as follows:Ni 85.03%, C 14.86%, it is demonstrated that graphite particle has been deposited with nickel.(3) The surface morphology and structure of Ni-graphite composite coating are characterized by Scanning Electron Microscopy (SEM) and XRD. It can be further confirmed that the graphite particle is embedded into the deposit to form the Ni-graphite composite coating. Graphite particle contained in the coating distributes evenly throughout the coating. The graphite particle that evenly dispersed in the coating can significantly intensify the microstructure of composite coating for the uniform diameter of particle. The micro-hardness of the Ni-graphite composite coating is 372.6 HV, higher than that of the pure nickel coating 175.2HV. XRD analysis indicates that characteristic absorption band appears at the 20 of 27°which belongs to the graphite; this is to say that graphite particle exists in the coating. Analysis of Tafel curves demonstrates that corrosion resistance of the coating prepared from bath containing complex agents is better than that prepared from bath without complex agents in the 5%NaCl and 5%HC1 solution, but worse in the 5%NaOH solution.(4) The electrochemical behavior of Ni-graphite composite electrodeposition are investigated by analyses of potentiodynamic polarization, cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS) and compared with that of the nickel deposition. Results show that the potential of Ni shifts towards to negative side and the cathodic polarization increases with the increasing scanning speed. When the complex agents are added into composite electrolyte, cathodal polarization curve moves toward to the negative side, improving the current efficiency and reducing hydrogen evolution potential; furthermore, the coating has a more flat morphology and compact microstructure since the current density decreasing at the same potential, resulting in better corrosion resistance compared with that of pure nickel coating. The nucleation mechanisms of pure nickel and Ni-graphite composite electrodeposition are discussed by cyclic voltammetry. Results of polarization curves and impedance method reveal that corrosion resistance of composite coating is better than pure nickel coating both in neutral and acidic solution due to the presence of graphite particle, which not only make the metal matrix grains finer, but also improve corrosive potential.