*3.3. Particle Content*

The magnetic properties, corrosion resistance, and mechanical properties of electrodeposited Ni–Co nanocomposites are mostly governed by the amount and distribution of nanoparticles. There are several interrelated factors that influence incorporation and distribution of nanoparticles into the Ni–Co coatings. These factors can be broadly classified as (i) electrolyte composition (reagents, pH, additives); (ii) nanoparticle characteristics (size, shape, type); (iii) deposition parameters (current density, bath temperature, concentration of nanoparticles and rate of electrolyte agitation); (iv) cobalt content in the coating where the Co2<sup>+</sup> cations that are adsorbed on the particle surface increase the incorporation of nano particles into the coating deposits, causing an increase in nanoparticle deposition with increase in cobalt content [28,51,56]; and (v) electrode orientation, which determines the incorporation efficiency of the nanoparticles. Increase in nano particle content on Ni–Co nano composite coatings can be improved by using the sediment deposition technique (SCD) as opposed to the conventional electrodeposition technique. In the SCD technique, the electrodes immersed in the electrolyte are placed horizontally and parallel to each other. As such, electrodeposition in this technique takes advantage of gravitational pull coupled with the electrophoresis force resulting in better incorporation of nano particles [30,49]. In conventional electrodeposition, only the electrophoresis force is utilized. Figure 4 shows the electrodeposition setups depending on electrode orientation in the electrolyte. Research shows that nanoparticle content in electrodeposited Ni–Co coatings increases steadily with increase in nanoparticle concentration to a given maximum value beyond which the nanoparticle content in the deposit decreases. This increase in nanoparticle content with increase in concentration can be attributed to increased transportation of the nanoparticles to the cathode surface where more and more nanoparticles can be engulfed in the growing Ni–Co matrix. At high concentrations of nanoparticles, the interaction equilibrium between the suspended nanoparticles and the embedded ones is exceeded, beyond which the surface of the cathode becomes covered such that more suspended nanoparticles cannot be embedded into the coatings. Moreover, there is increased mechanical collisions between the nanoparticles and this reduces their transportation efficiency across the electrolyte bulk. The content of nanoparticles in the coatings therefore decreases.

**Figure 4.** Schematic image of the deposition setups: (A) DC power supply, (B) Epoxy cover, (C) plating solution, (D) anode, (E) cathode, (F) magnetic bar and (G) external pH–temperature probe [30].

#### *3.4. Electrolyte Agitation*

High electrodeposition current density translates to high deposition rates, which almost always causes burrs on the cathode surface as well as increasing the coating roughness. Electrolyte agitation causes a distinct reduction of burrs that are formed on the edges of coated substrates and this reduces the coating roughness and improves uniformity [53]. Furthermore, increasing the stirring speed of the electrolyte during the deposition process increases the content of nanoparticles deposited in the Ni–Co coatings up to a given maximum level beyond which the content reduces [26,57]. At low agitation rates, the concentration of nanoparticles surrounding the cathode may reduce, resulting in the feed rate of the nanoparticles being lower than their adsorption into the Ni–Co matrix. Additionally, incomplete dispersion as a result of insufficient convection may cause agglomeration of nanoparticles and gravity settling. The surface energy of nanoparticles is greatly reduced when nanoparticles agglomerate, and this lowers the content in the deposited coatings. At higher agitation rates, the volume of nanoparticles reaching the cathode surface (mass transfer) increases thereby increasing the overall content of nanoparticles in the coatings. Goto et al. [55] reported that for the range of the experiment conducted, the nano diamond (ND) content in the coatings increased with increase in the stirring speed as shown in Figure 5. Where pulse electrodeposition is conducted using sediment deposition technique, agitation is a factor of *TON*/*TOFF* ratio, where *TON* and *TOFF* represent the time intervals when the agitation is on and off. In this technique, the nanoparticles settle on the horizontal cathode with aid from gravitational force. As such, the lower the *TON*/*TOFF* ratio, the higher the rate of nanoparticle incorporation into the growing Ni–Co matrix [57].

**Figure 5.** Relation between stirring speed and nano diamond (ND) content of the coatings [55].

Excessive rotation is however detrimental to coating quality. Vigorous hydrodynamic forces are generated at high agitation rates and these forces pluck out nanoparticles from the cathode surface before they become successfully embedded in the growing matrix surface, leading to lower nanoparticle content in the deposited coatings. This conclusion was reported by [26,58,59] who related the increase in corrosion resistance up to a maximum value to an increase in electrolyte agitation rate, beyond which it decreased with further increase in agitation rate.
