Investigation On Preparation Of Ni-nano Al₂O₃ Composite Coating And The Electrodeposition Mechanism In The Magnetic Field

In recent years, heavy attention has been paid to the process of Magneto-electrodeposition by scientists majored in electrochemistry, metallurgy and material due to the remarkable interaction between magnetic field and electric field. However, the former's works mostly focused on the metal or alloy deposition in magnetic field, less attention has been devoted to the composite electro-deposition. As we know, the composite electrodeposition is an very important way to obtain metallic composite material with excellent mechanical, physical and chemic properties, however, there still remain a lot of questions to be solved, for example, how to elevate the content of nano ceramic particles or control the distribution of the nano ceramic particles are still key points for preparing nano composite deposit. Considering the lorenz force, magnetic force can be induced in the electrolyte by superimposing an static magnetic field without contact, which will be helpful to influence the mass transport process, the electron transfering process, even the nucleation and growth of the deposit, So, the electro-deposition of Ni-Al₂O₃ composite coating in horizontal magnetic field (<1Tesla) and strong vertical magnetic field (1^-12Tesla) was discussed in this paper.To study the effect of magnetic field, the horizontal magnetic field and vertical magnetic field were used, in which the process of electrodepositing Ni-nano Al₂O₃ composite was conducted with changing magnetic flux density and current density as well as the relationship between the direction of magnetic field and electric field. The main contents and conclusions in this paper were shown as followings:1. Preparing a stable composite deposition solution is the first step in this work. Sedimentation rate method was used to study the influence of dispersion type, arabic gum content and dispersion time on the sedimentation rate of alumina nano particles in the solution. According to experimental results, the best condition to prepare composite solution as following: arabic gum content 1g/L, ultrasonic disperse for one hour.2. After preparing the composite deposition solution, the author considered the effect of current density on the pure Ni film and Ni-Al₂O₃ composite film without magnetic field. The experimental results showed that the surface of pure nickel film showed pyramidal type. The preferred orientation changed from <111> to <100> with current density increased from 1 A·dm^-2 to 4 A·dm^-2. The surface of composite coating was composed of many lumps which consisted by a lot of nanosize crystals. The preferred orientation of composite coating was <111>, which was not affected by current density.3. A horizontal magnetic field was firstly used to examine the applied magnetic field on the deposition of composite coating. In this field, the magnetic flux density and current density as well as relative orientation of magnetic field with current field must be considered. The results showed that alumina particles dispersed uniform in the coating and its content increased with magnetic flux density increased if the Lorentz force was upward. The particle content also affect by applied current density, a maximum value was got when the current density increased to 2 A·dm^-2 and then decreased with the current density increased more higher. In parallel magnetic field, the alumina particles distributed different with that in perpendicular magnetic field. The alumina particles almost were deposited at the boundary of the lumps. It was also found that the preferred orientation was (111) and could not be influence by the magnetic field and current density. The microhardness experiment showed that the composite coating was harder than the pure nickel coating and the values increased with magnetic flux density increased in perpendicular magnetic field, which might due to the dispersion strengthening effect and the refined crystal by included alumina particles.4. For the magnetic flux density in horizontal magnetic field was smaller than 1T, it was necessary to carry experiments out in vertical high magnetic field which was higher than 1T. In the high vertical magnetic field, the influence of magnetic flux density and current density on the surface, micro-texture and composition of composite coating was studied. In the perpendicular magnetic field, the alumina particles dispersed in the composite coating and its content reached a maximum 4.6wt% with the magnetic flux density increased to 8T then decreased. The effect of current density on the particle content in 10T magnetic field was also taken into mind. The results showed that the particle content increased first and then decreased with current density increased, and its value reached the maximum under the current density of 2 A·dm^-2. Different results were got in the parallel magnetic field. The alumina particles dispersed in a network pattern in the film and its content increased with magnetic flux density increased and reached the maximum 24wt% in the magnetic fild of 6T. As same as results in perpendicular magnetic field, the particle content reached the maximum 22wt% under the current density of 2A·dm^-2 in 10T magnetic field. In all the condition, the preferred orientation was (111) and could not be influenced by the magnetic field and current density. Depending on the experimental results, a model was suggested after analyzing the growth process of composite coating to illuminate the network distribution of alumina particles in the film. In this model, it was considered that the surface of cathode became rougher with film grew and a lot of lumps appeared on the cathode. In front of the lumps, the faradic currents were not parallel to the magnetic field strictly, so Lorentz force could be induced. As a result, anticlockwise eddy would be generated in front of lumps and the positive charged particles in the eddy rotated along with the electrolyte. Under the influence of Lorentz force, the charged particles finally be drawn to eddy edge and then deposited on the edge of lumps. All the results showed that a suitable MHD effect was necessary to get higher particle content.5. Electrochemical methods were also used to study the effect of organic addition and alumina nano particles on the deposition process of Ni-Al₂O₃ system. The CV (cyclic voltammetry) results showed that a kind of intermediate was generated before nickel ions or hydrogen ions discharged. Chronoamperometry curve indicated that the nucleation of nickel in Watts type solution had a good agreement of BTF model, but the addition of dispersant and alumina particles changed the nucleation form which agreed with the SH model. The electrochemical results were according with SEM results. The EIS (electrochemical impedance spectroscopy) results showed that the addition of dispersant and alumina particles decreased the active area and shifted the initial deposit potential to more negative for their adsorption on the cathode. In other way, the alumina particles might function as nucleation site and refined the crystal to form nano crystal.6. In order to know the effect of magnetic field on the mass transport progress and electron transfer process of Ni-Al₂O₃ deposition, the electrochemical tests were performed in both horizontal magnetic field and vertical high magnetic field. The main factors concluded magnetic flux density and magnetic field direction. Experiment results indicated that the composite deposition process could be affected by the relative orientation of the magnetic field and the electric field. The effect of magnetic field on the electrodeposition was more evident in perpendicular magnetic field than that in parallel magnetic field. In the perpendicular magnetic field, the Lorentz force was in the same direction with natural convection, the Faraday current was enhanced by applied magnetic field. When the Lorentz force was in the opposite direction with natural convection, the Faraday current was inhibited by applied magnetic field. when the magnetic field was applied in parallel with current, the magnetic field also enhanced the Faraday current, but the trend changed at the potential more negative than -0.98V in antiparallel magnetic field. The EIS results showed the applied magnetic field decreased the charge transfer resistance. A different result was shown in vertical high magnetic field. The MHD flow increased the distance of cations from anode to cathode, which increased the electron transfer resistance. At the same time, the MHD flow was blocked by electromagnetic braking force, which could be helpful to decrease the electron transfer resistance.7. The paper showed that the magnetic field enforced the mass transport process and increased the particle content when the Lorentz force was in the same direction with natural convection. Conversely, the opposite result would be got when the Lorentz force was in the contrary direction with natural convection.To conclude, the application of magnetic field in Ni-Al₂O₃ nano composite plating could enhance the mass transport process and electron transfer process and influence the nucleation and growth of composite film. Especially in the parallel magnetic field, the micro-MHD flow field induced by Lorentz force could affect the reduction process distinctly and control the distribution of alumina nano particles in the nickel matrix, which maybe influenced the properties of composite coatings.