Powder/Sol-enhanced Au–Ni Electroplated Composite Coatings: Structures, Properties and Mechanisms

Abstract

The aim of this thesis research is to study and develop Au–Ni based composite coatings with improved mechanical properties. Noble Au coatings have a wide range of applications including various electrical contacts, decorative coating and jewelry. However, the low hardness and poor wear resistance of the coatings have largely shortened their service life and limited their applications. This project is based on sol-enhanced nano-composite coating technology, which is a novel technique recently developed in our group. Highly dispersed oxide particles such as TiO2 and ZrO2 are introduced into the Au–Ni coating matrix, resulting in much improved mechanical properties. In this research project, electroplating was used to make Au–Ni based coatings. TiO2 (or ZrO2) solid particles and Ti (or Zr) contained sol were added into the standard Au–Ni electroplating solution as the source of the ceramic reinforcement to form different types of composite coatings. At the same time, a pulse power supply was also employed to study the effects of both the processing parameters and the second phase on the microstructure and properties of coatings. A number of metal-oxide combinations, including direct and pulse current power supply produced Au–Ni, Au–Ni–TiO2 and Au–Ni–ZrO2 coatings were prepared and characterized. The addition of solid particle or sol influenced the microstructure and crystallization direction strength of the coatings. Consequently, the nano-hardness, scratch resistance and wear resistance of the composite coatings were improved compared with the standard direct current (DC) plated Au–Ni coatings. The effects of composition and the processing parameters of the deposition on the microstructure and properties of the coatings were studied, and the underlying strengthening mechanism was investigated. The color, reflection, electrical resistivity and corrosion resistance of the improved coatings were also investigated in the research. It was found that the addition of solid particle or sol has no influence on the above properties. This makes it feasible to apply the enhanced coatings in various purposes with longer service life. Scaling-up experiments were conducted in the factory. Au ion concentration decreased significantly and Ti ion concentration dropped gradually during the electroplating process. The sol-enhanced coatings presented improved mechanical properties compared to standard Au–Ni coatings. Sol-enhanced Au–Ni–TiO2 bracelets were manufactured in the factory for a long-term macro-wear test. At the same time, an application-specific modified soft wear test system was established especially for soft coatings providing a visible quantification method to characterize the wear property.