Shaped pulse electroforming of nickel

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Shaped pulse electroforming of nickel


Author: Wong, Kam-po
Title: Shaped pulse electroforming of nickel
Degree: Ph.D.
Year: 2002
Subject: Hong Kong Polytechnic University -- Dissertations
Pulse plating
Department: Dept. of Industrial and Systems Engineering
Pages: xv, 208, [28] leaves : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: Although pulse current electroforming has been demonstrated to be a powerful means for the fabrication of many micro-devices, there are still some key issues such as surface finish and grain size that needed to be tackled before this technique can be widely used for micro-electroforming of premium products. As far as pulse current electroforming is concerned, most of the published works have focused on conventional rectangular waveform, there is only a limited amount of work reported on the use of non-conventional shaped waveforms. Since nickel coatings enhance the value and usefulness of industrial equipment and components, investigations on improving the properties of nickel electroforms are of high significance. The present study therefore aims to investigate, both theoretically and experimentally, the effect of different types of shaped waveforms on surface finishing and grain size development of electroformed nickel. A mathematical model is established for formulating the effects of different types of waveform on surface finishing in pulse current electroforming of nickel. The model describes the change of concentration profile of electroactive ions, micro-current distribution and the rate of protrusion growth at cathodic surfaces. The theoretical predictions are compared with the experimental results: within the range of the study, a maximum discrepancy of 10% on surface roughness values between the two was found. This discrepancy could be due to the evolution of hydrogen gases at the cathodic surface during electrodeposition. Both theoretical predictions and experimental results show that the quality of the electroforms, in terms of surface roughness improvement, intluenced by the types of waveform is of the order of ramp-down waveform > triangular waveform > ramp-up waveform > rectangular waveform, all with relaxation time. This is also supported by the study of the surface morphology of the electroforms by scanning electron microscopy. Another mathematical model is also established for. formulating the effects of different types of waveform on 3-dimensional electrocrystallization of nickel electroforms. The model describes how the different types of waveform influence the rate of 3-dimensional nucleation, J, the rate of 3-dimensional step growth via surface diffusion path, JSD, and the rate of 3-dimensional step growth via direct transfer path, JDT, at the cathodic surface. Moreover, the dynamic electrocrystallization process including J, JSD and JDT was simulated. When grain size and hardness among waveforms were compared, both the most fine-grained structure and the highest hardness value were found when a ramp-down waveform was employed. The experimental results show that, compared with the conventional rectangular waveform, the hardness value can be improved by about 28% when a ramp-down waveform is used. These results are also supported by theoretical predictions and the study of the surface morphology of the electroforms by scanning electron microscopy.

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