Electrodeposition has been widely used to develop micro-mechanical devices because it offers high dimensional accuracy, flexibility, and relatively low cost. In devices such as mechanical watches, micro-components are required to be made from non-magnetic, hard, and wear-resistant materials.
Copper is a common non-magnetic material used in the electrodeposition process, but its intrinsic low strength and soft mechanical behavior limit its use in such applications. These limitations can be overcome by developing copper matrix composite materials via electro-co-deposition, which improves copper's mechanical properties by incorporating particle reinforcements. However, electro-co-deposition is inherently complex, and the mechanisms governing the co-deposition process, as well as the associated hardening mechanisms, are not sufficiently understood.
This thesis addresses these existing scientific gaps by elucidating the influence of key process parameters governing particle incorporation into a copper matrix during the electro-co-deposition process, and assessing the consequent influence on the enhancement of hardness by analysing the different governing hardening mechanisms. To this end, thick (200 micrometer) electro-co-deposited composite coatings are produced with different particle reinforcements (graphite, alumina, cubic boron nitride) using a standard three-electrode setup with a rotating disc electrode (RDE).
A systematic experimental approach was used to investigate how particle mass transport (RDE rotation speed), metal ion reduction kinetics (current density and waveform), particle characteristics (type and concentration), and other process parameters affect particle co-deposition in the metal matrix. Scanning electron microscopy (SEM) and focused ion beam (FIB) cross-sections were performed to determine particle incorporation and grain size. Nanoindentation hardness measurements were performed to determine the hardness of the developed composite material.
The co-deposition mechanism of the particles was governed by the small grain size, where the increase in the number of grain boundaries and triple junctions served as anchoring points, facilitating their incorporation into the metal matrix. The smaller grain size (around 100 nm), increased RDE rotation rate (1600-2500 rpm), and higher concentration of particles in the electrolyte (80 g/L), collectively led to a substantial increase in the incorporation of alumina particles (up to 15% by volume), resulting in a hardness of 304 ± 20 HV, for a 200-micrometer Cu-Al2O3 composite.
Thus, the co-deposition of particles was described as a synergistic coupling between grain boundary pinning/anchoring and particle incorporation. The small grain size achieved with high peak pulse current density (-2 A/cm2), the use of graphite, or tribological sliding not only enabled the incorporation of a large number of particles but also helped understand the hardness of the developed composite, typically t
