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|Title:||Electrodeposition of Ni-SiC composites by different shaped current waveforms and magnetic field||Authors:||Hu, Fei||Keywords:||Hong Kong Polytechnic University -- Dissertations
|Issue Date:||2006||Publisher:||The Hong Kong Polytechnic University||Abstract:||The electrolytic co-deposition of particles with metals has been a research topic for more than a decade since composite materials have many enhanced properties such as hardness and wear resistance relative to monolithic metals. There are many researchers to improve the deposit quality of electro-composites by adjusting the bath composition and using pulse current. However, there is a need to develop electro-composites with tailored properties and have a better understanding of the deposition behaviour. In the present study, two additional process variables, magnetic field and current waveforms are introduced to manipulate the microstructure and enhance the deposit quality of electro-composite, and their effect on the deposition mechanisms are examined. The electro-codeposition behaviour of the Ni-SiC composite under different shaped waveforms is investigated. The amount of embedded SiC particles is shown to be related to the current waveforms and the average current density. The Guglielmi's model is extended to predict the volume fraction of SiC particles in the codeposition process under waveforms with arbitrary shape. It is found that the amount of SiC embedded in the nickel matrix is related to the peak current density of the waveforms. With the consideration of the influence of SiC particles on the free energy for creation of new interface and the growth of new grains, an analytical equation for average nucleation rates of the four waveforms is also derived, and the results are in consistence with the experimental findings.
Based on the electrochemical impedance spectra of the Ni-SiC electro-codeposition system, an equivalent circuit model (EC) is formulated to examine the charge transfer process and the double layer effect. It is shown that the use of a shaped waveform results in an improvement in morphology and hardness of the composites as compared with those obtained by direct current electrodeposition. Under the same average current densities, the highest instantaneous peak current for charge transfer is obtained under the ramp-up waveform, as compared to those obtained under triangular and ramp-down waveforms. The findings of the model are in agreement with the experimental results. The present study also applies the equivalent circuit model to explain the effect of duty cycle and frequency on deposition behaviour of composite in pulse electro-codeposition. The findings of the model are also in consistence with the experimental results. The effect of magnetic field on surface morphology, SiC content and crystal orientation of the Ni-SiC electro-composite is further examined. It is found that the magnetic field modifies the surface morphology and the orientation of the composites, and significantly increases the SiC content. This phenomenon can be attributed to the change of charge transfer reactions and the mass transport process through the magnetohydrodynamic effect. The findings of the project are of significance in better understanding the electro-codeposition mechanisms and in improving the quality of electro-composites.
|Description:||xx, 183 leaves : ill. ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577P ISE 2006 Hu
|URI:||http://hdl.handle.net/10397/3134||Rights:||All rights reserved.|
|Appears in Collections:||Thesis|
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