Study On The Electrochemical Fabrication And The Related Properties Of Nanostructured Ni-Si₃N₄/TiN Composite Film

Compared to the traditional polycrystal and amorphous materials, metal matrixnanocomposite thin films exhibit many excellent performances, such as uniquemagnetic, optical, physical chemic properties and so on. Consequently, it has aexpansive applied foreground in many fields, so extensive research efforts withregard to nano-composites have been done. It is well known that deposition ofelectrochemical composite coatings (ECC) is not a newly developed technique andhas been in continuous development since the 1970s. Meanwhile, when comparedwith other techniques for nanocomposite synthesis, electroplating possesses manyadvantages including: low cost and industrial applicability, simple operation,versatility, high production rates and so on. Therefore, the ECC technique has arosedattention of many investigators in the world.Based on the theories of "epitaxial growth" and "derivational nucleation", thisdissertation tries to adopt a modification method based on traditional compositeelectroplating technique and a few of nano-SiC particle predecessors has beendeveloped to fabricate the holistically nanostructured Ni-SiC films. The research notonly has value in theory, but also has an expansive applied foreground.Firstly, the influences of principal technological parameters on themicrostructure of eletrodeposited Ni-Si₃N₄/TiN nanocomposite film were studied.Such as: the concentration of NSi₃N₄/TiN, current density, pH value, temperature,solution agitation speed and so on. We optimized a steady technics ultimately.Secondly, the microstructure of the optimized nanostuctured Ni-Si₃N₄/TiNcomposite film was characterized by scanning transmission electron microscopy(SEM), atomic force microscope (AFM) and transmission electron microscope (TEM), It can be clearly seen that the optimized nanostuctured Ni-Si₃N₄/TiN composite film is composed of the nanoparticles with the average grain sizes in the nanometer range, which proves that the as-obtained nanostuctured Ni-Si₃N₄/TiN composite film is substantially nanostructured in character.Finally, the mechanism was stuied, especially the influence of nano-Si₃N₄/TiN particles. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) at cathodic potential were performed. In the case of high deposition overpotential, the electroplating of pure nickel film and nanostuctured Ni-Si₃N₄/TiN coating followed 3D nucleating and subsequent instantaneous grain growth mechanism. The addition of nano-Si₃N₄/TiN particles into electroplating solutions, the Si₃N₄/TiN particles will arrive at the cathode surface, and compete the active sites on the cathodic surface with nickel crystallites, which will simultaneously result in the incorporation of Si₃N₄/TiN particles into nickel matrix and the increase the cathodic polarization. Meanwhile, the engulfed nanoparticles will stop the growth and congregation of existing crystalline grains mechanically due to their electric semiconductor properties, and accelerate the nucleation rate of new grains synchronously due to their high surface energy. Accordingly, the formation velocity of crystal grains will exceed the growing velocity, which results in the refining of the crystallites of Ni-Si₃N₄/TiN electrodeposits and finally in nanostructured composites.In addition, the corrosion behavior of nanostructured Ni-Si₃N₄/TiN composite films has been investigated in 3.5.wt.ï¼…NaCl solution by electrochemical impedance spectroscopy (EIS) and Tafel plots measurements. The results shown that, the corrosion behavior nanostructured Ni-Si₃N₄/TiN composite films were divided into two sequential stages according to the characteristics of EIS data. At the first stage, two time constants involved; and at the second stage, there were three time contents while the low frequency one from the diffusion. Otherwise, the nanocomposite films have good corrosion resistance compared with the pure nickel film.