Research On Preparation Of Ni-Al₂O₃ Nanocomposite Coatings And Its Properties

Corrosion and wear are the main forms which disable the function of materials. As one of the surface defense technique, nano-composite electroplating is widely used in the field of aerial, electronic, mechanical, chemical, metallurgic and nuclear energy industry et al, owing to the advantages including simple scale-up with easily maintained equipment, precisely controlled near ambient temperature operation, rapid deposition rate, operation at a normal pressure, low cost, and capability to handle complex geometries. It can not only improve the corrosion resistance, wear resistance and anti-oxidation at high temperature, but also save materials to decrease the fabrication cost. The amount of nano particles in deposits is one of the key factors determined the properties of composite coatings. How to increase the co-deposition content and improve the distribution of nano particles in composite coatings become to a crucial problem. Attempts to alleviate the above problem, the use of various physical and chemical methods have been reported by many researchers. The Ni-Al₂O₃ nanocomposite coatings were prepared by using conventional electroplating (CEP), sediment co-deposition (SCD) technique and imposition of high magnetic field. The properties of the resulting composite were investigated. The following are the salient conclusions of the present study:The process parameters affecting the nano-Al₂O₃ contents in composite coatings were investigated, the optimum process parameters were chosen: the nano-Al₂O₃ concentration in plating solution: 10g/l; current density: 3A/dm²; stirring rate: 350rpm; solution pH:4.0; bath temperature: 50℃. The surface morphology, structure, surface roughness, bonding intensity and the elements distribution of composite coatings were analyzed. The results show that the nano-Al₂O₃ content in deposits obtained by the SCD technique is much greater than that of the CEP technique. The nano-Al₂O₃ particles take on spherical shape, fine, compact and uniformly dispersion in the Ni matrix. The pure Ni coating exhibits the (200) plane, while the composite coatings show the (111) plane preferentially growth orientation.The hardness and properties of prepared coatings were investigated systematically. The mechanisms of wear, corrosion and high temperature oxidation resistance were discussed. The results show that the hardness of composite coatings fabricated by the SCD technique is higher than that of CEP technique. The improved hardness of coatings can be attributed to the dispersion strengthen and refining of crystal. The wear rate and the coefficient of friction of coatings decrease with the increase of nano-Al₂O₃ content in composite coatings. The corrosion resistance of composite coatings via SCD technique is better than that of CEP technique. In the identical corrosion medium, the corrosion potential of composite coatings increases, while the corrosion current density decreases. The incorporated nano-Al₂O₃ particles decrease the porosity, improve the dense of coating and reduce the diffuse of corrosion product to enter into the inner of coating, therefore, the corrosion resistance of coating will enhance. After high temperature oxidation at 800℃for 120min, the oxidation resistance of SCD composite coating increased 46.4% and 24.7% when compared to pure Ni and the CEP coating, respectively. The products of pure Ni coating and composite coatings after high temperature oxidation consist of NiO and Fe₂O₃. After oxidation, the crystalline grain of pure Ni coating became coarsening. But, the embedded nano-Al₂O₃ in deposits suppressed the growth of Ni grains, the coating became smooth. The composite coatings produced by SCD technique declined the real contact area of Ni matrix under high temperature oxidation, reduced the stress of coating, blocked the generation and reproduction of crack to improve the oxidation resistance.The surface charge status of nano-Al₂O₃ particles was determined, the kinetics mechanism of co-deposition was analyzed, and the co-deposition mechanism of nano-Al₂O₃ particles with metal matrix was discussed. The results show that the nano-Al₂O₃ particles carried positive charge. The reaction active sites at the electrode and electrolyte interfaces will be impeded with the addition of nano-Al₂O₃ particles in plating solution. Consequently, the adsorption of particles on cathode causes an increase of cathode polarization of electrochemistry reaction. The mechanism of nano-particles incorporation with nickel matrix can be explained as follows: under the effect of ultrasonic and stirring of magnetic force, nano-particles suspend uniformly and stably in plating solution, they carry the positive charge due to adsorption the Ni²⁺, H⁺ and Ni[B(0H)₄]⁺ ions; the metal ions and nano-particles adsorbed positive charge are transferred into fluid boundary layer adjacent on the surface of cathode by electric field, gravity and diffusion effect; the particles are adsorbed to the surface of cathode by static attraction force and mechanical impact effect; the metal ions adsorb on the surface of cathode to obtain electrons, and then arrive at the growth sites of crystal lattice by diffusion, when the hydrated layer break off from them, they will embed in the substrate lattice; smaller particles was captured effectively by the deposited metal and rest on the surface of cathode, then they will be embedded by growth metal, to some extent, the particles embed permanently in the growth layer of metal to form nano-composite coatings. While the larger particles or conglomeration will return to the plating solution under the influence of impact effect.The Ni-Al₂O₃ composite coatings were produced successfully by imposition high magnetic field. The surface morphology, microstructure and properties of coatings were investigated in detail and the action mechanism of high magnetic field was discussed. The results show that the high magnetic field effects not only on the structure of composite coating to change the preferred orientation of nickel crystal, but also restrains the evolution of hydrogen to make the coating dense and smooth. The microhardness increased, wear resistance improved, wear rate decreased, weight loss declined and high temperature oxidation resistance enhanced of composite coatings produced in the presence of high magnetic field than that of magnetic force stirring. When magnetic flux density is up to 8T, the hardness of composite coating increase 71.1%, the wear rate enhance 3.7 times, the weight loss improves 41.9%, the weight gain decreases 5.9% in comparison with composite coating prepared by magnetic force stirring, respectively. Eddy motion generates around each particle due to the interaction of an imposed magnetic field and an electrolytic current distorted by the existence of non-conducting particles to make plating solution flowing. This action will enhance the limited current density, improve the dispersing power of plating solution, and thin the thickness of diffuse layer to strengthen the mass transfer of solution, therefore, the high magnetic field can influence the surface morphology and the size of crystals.