*2.1. Direct Current Electrodeposition*

In Direct current (DC) electrodeposition, an electric current is continuously transferred through the system without any interruptions. DC electrodeposition is divided into two types depending on the orientation of the electrodes in electrolyte during the electrodeposition process. These are the conventional electrodeposition, and sediment codeposition (SCD) techniques. For conventional electrodeposition, the electrodes are placed vertically in the electrolyte, but for SCD they are placed horizontally. Adsorption of nanoparticles into the alloy matrix during electrodeposition is greatly influenced by forces acting on the suspended nanoparticles. The kinetics involved can be used to explain this difference. The two main forces at work during SCD electrodeposition are gravitational pull and the electrophoresis force, thereby giving more desirable properties compared to conventional deposition which solely relies on gravitational pull [30]. DC has several advantages over pulse electrodeposition and pulse reversal current (PRC) electrodeposition. These include simplicity, the availability of vast technical knowledge and affordability. Borkar T [31] reported that for all Ni and Ni nanocomposite coatings deposited, DC deposited coatings exhibited much stronger (less random) crystallographic textures compared to coatings deposited using PC and PRC deposition techniques. Furthermore, in DC electrodeposition, the Co content in the deposited Ni–Co coatings is dependent on the composition of the electrolyte, unlike in jet electrodeposition where Co electrodeposition is controlled by diffusion [4].

#### *2.2. Pulse Current Electrodeposition*

Pulse current electrodeposition (PC) has been used extensively over the years in Ni–Co electrodeposition [32]. The three most fundamental parameters that affect the properties of the coatings include in pulse current electrodeposition include: peak current (*Ipeak*), pulse imposition time (ON-time, *TON*) and switch off time (OFF-time, *TOFF*). These parameters relate mathematically to evaluate (1) pulse frequency, (2) duty cycle, and (3) average current density, as shown [32,33]:

$$f = \frac{1}{T\_{OFF} + T\_{ON}}\tag{1}$$

$$\gamma = \frac{T\_{ON}}{T\_{OFF} + T\_{ON}} \ast 100\tag{2}$$

$$I\_{\rm avg} \xrightarrow[T\_{OFF}]{T\_{ON}} \ast I\_{\rm peak} = I\_{\rm peak} \tag{3}$$

where γ is the duty cycle, *f* the frequency, *Ipeak* the peak current density, and *I*avg the average current density.

In pulse electrodeposition, peak current density has a significant effect on microhardness, crystallite size, surface morphology, microstructure, composition, and tensile strength of PC deposited Ni–Co alloys and their nanocomposites [34]. Crystallization and growth in turn determine the microstructure of the nanocomposite. The texture of the coatings is determined by both peak current density and organic surfactants [35]. The quantity of adatoms located at the surface is higher due to the applied higher current density as compared to DC electrodeposition. This results in smaller grain sizes [5]. Padmanabhan [36] reported that pulse electrodeposition is used as an effective method for the reduction of grain sizes to nanoscale [5].

Although it is a relatively low-cost synthesis technique with simple implementation, pulse electrodeposition is ideal for production of full density nanocrystalline materials [33]. Yang and Cheng [32] reported that the morphology of the deposited coatings changed from nodular to acicular, and a finer grain size was observed with increasing pulse frequency and decreasing duty cycle. It has also been reported that at low current densities, smoother surface morphologies are observed [37], but an increase in peak current density produces distinct colony-like morphologies characterized by clearer colony boundaries [34].

Pulse electrodeposition exhibits several advantages over DC electrodeposition, such as improved wear resistance and hardness, particle distribution, structure, morphological structure and the ability to control the grain sizes of the deposits [38]. PC electrodeposition has higher instantaneous current density compared to DC electrodeposition, thereby increasing its effectiveness in agitating the adsorption–desorption processes that occur at the Ni electrolyte interface. This makes it possible to control the electrodeposited Ni coating microstructure [39,40]. It has also been reported that PC deposits contain a higher nanoparticle content compared to coatings deposited by the DC deposition technique [31]. Yang and Cheng [32] concluded that the high microhardness of deposited Ni–Co–SiC coatings was primarily associated with increasing SiC content in the coatings with increasing pulse frequency and decreasing duty cycle. This significantly increased the microhardness as a function of the dispersion strengthening mechanism. Watts type baths used for pulse plating of Ni based coatings have been reported to produce the best coating results [2].

The PC deposition technique suffers from a drawback called the double layer capacitance effect. The double charging layer of the electrodes is subjected to charging and discharging occurrences. In a case where the ON- and OFF-times are much shorter than the charging and discharging times, the PC would revert to DC current. As such, care is taken to select a high frequency thus ensuring the effect of the double layer capacitance effect is negligible.
