Study On Process And Properties Of Pulse Electroformed Ni-CeO₂ Nanocomposite Deposits In An Ultrasonic Field

Nanocomposite deposit, which exhibits many special functional properties, can be obtained by using traditional composite electroforming process. However, the nanoparticles were apt to agglomerate during electroforming process; as a result, the composite deposit shows unstable properties with a non-uniform microstructure. Therefore, the application of the composite electroforming deposits is limited to a certain extent. Ultrasound not only could effectively restrain the agglomeration of nanoparticles, promote the uniform distribution of nanoparticles in the deposit and refine the crystals of the deposit, but also strengthen the electroforming process, enhance the electrodeposition velocity and current efficiency.Ni-CeO₂ nanocomposite deposits have been prepared by pulse electroforming under ultrasonic field in this dissertation. Effects of the electroforming methods, pulse process parameters and ultrasound on the CeO₂ content in the nanocomposite deposits were investigated. The surface morphology, microstructure and properties of the Ni-CeO₂ nanocomposite deposits were also observed and examined. The improvement action and mechanism of the ultrasound and the nanoparticles on the microstructure and properties of the deposits were further analyzed. The main works completed and conclusions are presented as follows:Firstly, effects of the electroforming methods, pulse process parameters and ultrasound on the CeO₂ content in the nanocomposite deposits were investigated. Moreover, the process parameters in the composite electroforming experiment were analyzed and optimized by orthogonal experimental method, and the optimum process parameters of ultrasound-pulse electroformed Ni-CeO₂ nanocomposite deposits are obtained: CeO₂ nanoparticles concentration 40g/L, average current density 2A/dm2, pulse duty cycle 0.4, pulse frequency 500Hz, ultrasonic frequency 100kHz, ultrasonic power 180W, magnetic stirring velocity 1000rpm.Secondly, the cathodic polarization curve and electrochemical impedance spectra of co-deposition of CeO₂ nanoparticles and metal nickel were studied. CeO₂ nanoparticles in the electrolyte not only cause negative transfer of the reduction potential and increase of the cathodic polarization, but also present two inductive loops at low frequency of the impedance spectra due to the adsorption of nanoparticles in the electrode surface.Thirdly, the surface morphologies of the deposits prepared by different electroforming methods were analyzed. CeO₂ nanoparticles, pulse current and ultrasound can markedly result in refining of the grain size of matrix metal Ni. The nanocomposite deposits prepared by ultrasound-pulse electroforming had smooth morphology, refined crystal grain and compact microstructure.Fourthly, the effects of different electroforming methods on the crystal preferential orientation of deposits were investigated. CeO₂ nanoparticles and pulse current can enhance the diffraction intensity of crystal face (1 1 1), (2 2 0), (3 1 1) and (2 2 2). With the introduction of ultrasound in the electroforming process, the diffraction strongest peak of the deposits was changed from crystal face (2 0 0) to (2 2 0).Fifthly, microhardness, corrosion resistance, high-temperature oxidation resistance, friction and wear properties of pure Ni deposits and Ni-CeO₂ nanocomposite deposits prepared by direct, pulse, ultrasound-pulse electroforming were investigated. Ni-CeO₂ nanocomposite deposits prepared by ultrasound-pulse electroforming exhibits higher microhardness, better corrosion resistance, high-temperature oxidation resistance and friction and wear properties than those of other deposits. It is suggested that the deposits had excellent properties owing to the cooperative effects of CeO₂ nanoparticles, pulse current and ultrasound.Sixthly, the main actions of ultrasound on the enhancement of properties of the nanocomposite deposits can be attributed to: the improvement of the uniform dispersion of the nanoparticles in electrolyte and in nanocomposite deposits, refinement of the grain of nanocomposite deposits and variation of the crystal orientation of matrix metal.The ultrasound-pulse electroforming process of Ni-CeO₂ nanocomposite deposits with excellent properties has been obtained through experimental research in this dissertation. This research will be scientific guiding significance for the development of electroforming products with excellent properties.