Current Density Effect on Nickel Electroplating Using Post Supercritical CO2 Mixed Watts Electrolyte

Abstract

In this study, a nickel film with nano-crystalline grains, high hardness and smooth surface was electrodeposited using a post supercritical carbon dioxide (CO2) mixed Watts electrolyte. Although the hardness was not as high as its Sc-CO2 counterpart, the thin coating contained significantly less number of nano-sized pinholes. By measuring the escape concentration of the dissolved CO2 in post Sc-CO2 mixed electrolyte with the elapsed time, it was believed that the residue of dissolved CO2 bubbles should closely relate to the improvement in hardness and surface roughness over its conventional plating counterpart. Therefore, shortening the duration of electroplating with the raise of current density up to 0.5 A/cm 2 could effectively retain more post Sc-CO2 mixing effect. This study not only confirms the roles of dissolved CO2 bubbles in electrolyte but also provides a potential process to overcome most issues associated with the cost in building high-pressure chamber for large size products and continuous plating using supercritical method. be formed. In the supercritical state, with proper agitation, the CO2 bubbles are introduced into electrolyte forming a mixture that includes both gaseous and liquid phases. With an addition of surfactant, the supercritical CO2 droplets have been observed in nano scale (~10 nm) and in some cases the stability can exist over 24 hrs. (7). Those nano-sized bubbles in aqueous electrolyte are postulated as the key factors in transforming the direct current into a periodic one and assisting desorption of the generated hydrogen simultaneously in the electrodeposition process. Both the size reduction and population increase of CO2 bubbles in Sc-CO2 mixed electrolyte have been reported as the main reasons for refining grains and smoothening coating surface. Nonetheless, the circular marks observed on deposited