*3.1. E*ff*ect of Co Concentration in Electrolyte*

Cobalt content has been reported to increase with increase in Co concentration in the electrolyte bath [12,49]. For Ni–Co deposition, Ni and Co combine to form a solid solution that engulfs the nanoparticles suspended in the electrolyte. As such, the higher the concentration of Co element in the electrolyte, the more formation of this solid solution matrix occurs on the cathode surface. Variation of Co2<sup>+</sup> in the electrolyte has significant effects on grain sizes, and cobalt content of the electrodeposited Ni–Co alloys and nanocomposites. Grain refinement can be explained using two approaches. Firstly, a lattice strain is produced as a result of the difference in atomic sizes of Co and Ni. As the content of Co increases, the lattice distortion becomes aggravated causing vacancy and dislocation defects in the lattice. Grain size refinement is a direct consequence of these defects.

Secondly, as a result of anomalous deposition of Co, the deposited Co content is higher than the corresponding concentration of Co in the electrolyte [49,50]. In this process, the less noble element (Co in this case) is reduced preferentially, resulting in a much higher content. This increase in content reaches a critical value, beyond which the microstructure of the electrodeposited coating changes from single phase (a-phase) face-centered cubic (FCC) to a combination of a-phase and hexagonal close-packed (HCP) e-phase. This combination of two structural phases results in grain refinement of deposited coatings.

Addition of Co2<sup>+</sup> into the electrolyte also enhances transport and deposition of suspended nanoparticles. Transfer rate of nanoparticles through the bulk of the electrolyte is a function of electrophoretic forces existing between charged nanoparticles and the cathode surface which in turn is influenced by the quantity of adsorbed cations on the nanoparticle surfaces (zeta potential). Therefore, the amount and type of adsorbed cations determines the magnitude of electrophoretic force [51]. It was reported that deposition of nanoparticles was enhanced by addition of Co2<sup>+</sup> into the electrolyte, and it can be suggested that zeta potential in Co2<sup>+</sup> containing baths is much more positive, owing to Co2<sup>+</sup> being much more easily adsorbed onto the surfaces of nanoparticles than Ni2<sup>+</sup>. As a result, the electrophoretic forces exerted on nanoparticles are more intense.

#### *3.2. Current Density*

Current density (CE) affects the current efficiency of the process. Current efficiency (%) describes the ratio of electrochemical current density for a specific reaction to total applied current density. It illustrates the transfer efficiency of electrons to the electrochemical system. Current efficiency (η) is calculated by factoring charge passed, weight of deposits given by the difference between the weight of samples before and after deposition, and the chemical composition of the coatings as shown in Equation (13) [52].

$$\eta = \frac{\mathcal{W}}{It} \left( \sum \frac{(F\_{\mathcal{G}i} \mathbf{r}\_i)}{KN\_i} \right) \times 100 \tag{13}$$

where *W* is the weight of the deposit (g), *I* is the current passed (A), *t* is the deposition time (h), *gi* is the weight fraction of the element in the binary alloy deposit, *ei* is the number of electrons transferred in the reduction of 1 mol atoms of that element, *Ni* is the atomic weight of that element (g/mol), *F* is the Faraday constant (96,485.3 C/mol) and *K* is a unit conversion factor (3600 C/A h).

Grain size, microstructure, brightness, thickness distribution, composition, surface morphology, microhardness and tensile strength of PC electrodeposited Ni–Co alloys are significantly influenced by variation of peak current density *Jp* [34,53]. It has been reported that current density has a significant influence on rate of deposition, plating adherence and the quality of plating of Ni–Co coatings. The deposition rate increases with increases in current density [54]. Li et al. [34] used a pulse technique to research the effect of varying pulse frequency in electrodeposition of nanocrystalline Ni–Co deposits. It was found that increase in peak current density resulted in a lower cobalt content, smaller grain size, higher tensile strength and a colony like morphology. A recommendation for a peak current

density range of 100–200 A·dm−<sup>2</sup> was suggested for grain sizes ranging from 15–20 nm, a 7%–8% cobalt content, 590–600 kg mm−<sup>2</sup> microhardness and 118–1200 MPa tensile strength. When using the PC technique to electrodeposit Ni–Co, it was reported that the lowest current densities achieved the most compact layer of alloys [5].

Extremely high current density *Jp* however has been reported to have a drastic effect on microhardness and tensile strength of electrodeposited Ni–Co coatings [34]. This has also held true for Ni–Co alloys electrodeposited using jet electrodeposition technique [4]. It could be suggested that this is because of the decrease in cobalt content with increase in the current density. Li et al. [34] researched the effects of peak current density on mechanical properties of Ni–Co alloys and reported that Co content decreased with peak current density increase. One of the key aspects in electrodeposited Ni–Co alloys and nanocomposite coatings is solid solution strengthening where the Ni and Co in electrolyte combine to form a solid solution. A decrease in Co content will therefore yield less solid solution strengthening for coatings electrodeposited at higher current densities. Figure 3 shows the relationship between peak current density and cobalt content [34].

**Figure 3.** Cobalt content in Ni–Co alloy coatings with varying peak current density [34]. Reprinted from Applied Surface Science, 254, Yundong Li, Hui Jiang, Weihua Huang, Hui Tian, Effects of peak current density on the mechanical properties of nanocrystalline Ni–Co alloys produced by pulse electrodeposition/Pages No. 6865–6869, Copyright (2020), with permission from Elsevier.

In the case of Ni–Co, electrodeposition of the Co element is influenced and controlled by diffusion as compared to that of Ni which is predominantly controlled by activation. In light of this, an increase in cathodic current density results in an increase in cathodic overpotential. Concurrently, activation of the electrode reaction increases and, as a result, the rate of deposition of Ni element into the coating increases significantly [34].

In the case of Ni–Co nanocomposites, it has been reported that thinner coatings resulting from smaller amounts of electricity tend to have a higher content of nanoparticles compared to thicker coatings that are formed with larger amounts of electricity. This suggests that nanoparticles become adsorbed during the early phase of deposition and become unevenly distributed with increases in thickness of coatings [55].
