*8.1. E*ff*ect of Al2O3 Nanoparticles*

Al2O3 nanoparticles have inherent high corrosion resistance, and in the nano-scale range, can efficiently contribute to porosity reduction, thereby further boosting the corrosion resistance of deposited coatings [132,133]. Incorporation of Al2O3 nanoparticles into the Ni–Co matrix produces nanocomposite coatings characterized by nodular, uniform, and compact morphology [26]. The content of Al2O3 in the coatings increases with increases in Al2O3 concentration in the electrolyte. The same trend was reported by Borkar [31], with a maximum Al2O3 content being achieved with 40 g L−<sup>1</sup> nanoparticle concentration, beyond which the content decreased. Ni–Co alloy coatings exhibit a face centered cubic (FCC) crystal structure and the same has been reported for Ni–Co/Al2O3. However, the crystal orientation of the resulting Ni–Co/Al2O3 nanocomposite coating undergoes transformation from crystal face (200) lattice to (111) lattice [40].

Like with most Ni–Co nanocomposite coatings, adsorption of Al2O3 nanoparticles into the Ni–Co matrix results in a subsequent increase in the coating's hardness and wear to a certain maximum, beyond which the coating becomes brittle and spalls off. Tian [26] concluded that an increase in Al2O3 nanoparticle concentration in the bath caused a subsequent increase in the corrosion resistance of the Ni–Co/Al2O3 nanocomposite coatings up to a certain limiting value, suggesting that optimal operating conditions and parameters are key to corrosion resistance maximization. This may be attributed to uniform dispersion of Al2O3 nanoparticles in the Ni–Co matrix.

It has been suggested that Al2O3 nanoparticles may also improve deposition of Ni and Co elements in the coatings [26]. Wear resistance is also higher for Ni–Co/Al2O3 compared to their alloy counterparts and this too increases with increases in nanoparticle content.

### *8.2. E*ff*ect of SiC Nanoparticles*

Ni–Co/SiC nanocomposite coatings exhibit a porous free and dense microstructure characterized by uniformly distributed SiC nanoparticles throughout the deposited coating surface [126]. The phase structure of Ni–Co/SiC nanocomposites is predominantly face centered cubic. Silicon carbide (SiC) is a chemically inert semi-conductor material [134]. Embedding of SiC nanoparticles causes a significant improvement in the corrosion resistance, wear resistance and microhardness of the deposited coatings [31]. Babak [126] reported the same findings, with the highest corrosion resistance achieved with coatings containing 8.1 vol.% SiC nanoparticles.

In the case of Ni–Co/SiC nanocomposite coatings, it has been observed that the concentration of Co element in the electrolyte has a significant effect on SiC content, whereby SiC content increases considerably with increasing Co element concentration. Babak [135] reported that SiC content increased from 2.0 vol.% to 8.1 vol.% with increases in the concentration of Co in the electrolyte. This phenomenon was attributed to ease of Co2<sup>+</sup> cations adsorbing on nanoparticle surfaces compared to Ni2<sup>+</sup> cations. Therefore, there was an increase in the adsorbed positive charge on the surface of the SiC nanoparticles as the Co element concentration increased. Generally, mass transfer of positively charged SiC nanoparticles towards the cathode surface is enhanced since there is an increase in the electrophoresis force exerted on them [28]. However, when the metallic cations become saturated on the SiC nanoparticle surface, a decrease in SiC content with increasing Co element concentration is observed.

Wear rate has been reported to increase with increases in SiC nanoparticle content in Ni–Co matrices [60]. As more content of SiC nanoparticles is added into the coatings, the grain refining and dispersion strengthening effects become magnified thereby improving the wear resistance of the nanocomposite coatings.
