*7.1. Microstructure*

As stated earlier, Ni–Co coatings deposition is anomalous. This can be explained as a function of local pH increase resulting in creation and adsorption of metallic hydroxides, as well as a faster rate of cobalt hydroxide adsorption [104], deposition of Co cations (first step) followed by Ni deposition (second step) in a two-step process [105], and preferential Co element deposition which causes the diffusion layer to become depleted [4]. Ni–Co alloys with up to 58 wt% cobalt content exhibit the single phase of Ni matrix with FCC type of phase structure. When the cobalt content is in the range between 64 wt% to 80 wt% the phase structure becomes a combination of FCC and HCP as shown in Figure 7 [68].

**Figure 7.** X-ray diffraction (XRD) patterns of Ni–Co films deposited from the electrolytes containing different Co concentrations [68]. Reprinted from Applied Surface Science, 258, Ali Karpuz, Hakan Kockar, Mursel Alper, Oznur Karaagac, Murside Haciismailoglu, Electrodeposited Ni–Co films from electrolytes with different Co contents/Pages No. 4005–4010, Copyright (2020), with permission from Elsevier.

Ni–Co alloys with Co content above 80 wt% exhibit complete HCP phase structure [16,40,68]. Ni–Co alloy surface morphologies are significantly influenced by the coating's chemical composition. Rafailovic et al. [106] researched the mechanical properties of Ni–Co alloys deposited on Cu substrates. It was reported that a platelet structured morphology was formed for coatings with a Ni<sup>2</sup>+/Co2<sup>+</sup> ratio of 0.25 at 65 mA cm<sup>−</sup>2. The surface morphology exhibited enhanced dendritic growth when the ratios were 0.5 and 2. At the highest Ni<sup>2</sup>+/Co2<sup>+</sup> ratio of 4, the surface morphology exhibited cauliflower structure as shown in Figure 8.

**Figure 8.** SEM micrographs of Ni–Co deposits obtained at a current density 65 mA cm−<sup>2</sup> from an electrolyte with different Ni<sup>2</sup>+/Co2<sup>+</sup> concentration ratio: (**a**) 0.25, (**b**) 0.5, (**c**) 1, (**d**) 2 and (**e**)4[106]. Reprinted from Materials Chemistry and Physics, 120, L.D. Rafailovic, H.P. Karnthaler, T. Trisovic, D.M. Minic, Microstructure and mechanical properties of disperse Ni–Co alloys electrodeposited on Cu substrates/Pages No. 409–416, Copyright (2020), with permission from Elsevier.

The microstructure of electrodeposited Ni–Co-based coatings is affected by evolution of hydrogen gas at the cathode. Hydrogen is a by-product of the electrodeposition process owing to the breakdown of water molecules in the plating solution during the electrodeposition process. This holds both an advantageous and detrimental effect depending on the desired output of the process. Hydrogen offers a promising versatile, efficient, and clean candidate for use as an energy source to replace commonly used fossil fuels which cause CO2 emissions that are harmful to the environment [107–109]. Hydrogen can be successfully generated using the less efficient (higher operating cost) alkaline water electrolysis [110,111], or using low hydrogen evolution reaction (HER) overpotential electrodes [112]. Ni-based alloys and compounds form such electrode materials owing to their low cost coupled with high catalytic activity and stability [113]. In the case of depositing quality coatings however, the evolution of hydrogen gas is detrimental to the structure, hence the critical need to control its production. The synthesized hydrogen gas attaches to the surface of the base metal creating a blanket of air that inhibits nucleation and deposition of the coatings and this causes poor adherence leading to non-uniform coatings [114]. The hydrogen evolution phenomenon has been reported to be more significant at higher current densities, owing to lower hydrogen overpotential where numerous gas pits are formed on the coating surface as a result of the hydrogen produced [112].

### *7.2. Mechanical Properties*

Microhardness in electrodeposited Ni–Co coatings increases with an increase in Co content in the coatings. Baghal et al. [115] reported similar findings when the microhardness of Ni–SiC coatings was compared to Ni–Co/SiC coatings as a function of increasing current density, as shown in Figure 9.

**Figure 9.** The effect of current density on microhardness of Ni–Co/SiC and Ni/SiC coatings [115]. Reprinted from Surface and Coatings Technology, 206, S.M. Lari Baghal, M. Heydarzadeh Sohi, A. Amadeh, A functionally gradient nano-Ni–Co/SiC composite coating on aluminum and its tribological properties/Pages No. 4032–4039, Copyright (2020), with permission from Elsevier.

Enhancement of micro-hardness in Ni–Co alloy coatings as a function of Co content can be linked to the (i) grain size reduction, (ii) phase composition, where two-phase structures are formed, and (iii) solid solution strengthening [16,20,116]. Babak [12] reported similar observations where coatings up to 45 wt·% Co were characterized solely by FCC lattices. No other phase was observed from the XRD pictographs and it was suggested, therefore, that the effect of phase composition on microhardness of the said coatings was not significant. This was held as evidence that solid solution hardening played a key role in the microhardness of the deposited Ni–Co coatings. The grain size of deposited Ni–Co alloy coatings decreased gradually with increase in content of Co element in the coatings [12].

The wear rate of deposited Ni–Co coatings has been observed to decrease with increase in Co content in the deposits. The phenomena can be associated with increase in microhardness with increase in Co content. A relationship exists between wear rate and microhardness of a coating, called Archard's law, which provides that the sliding wear volume loss is directly proportional to friction coefficient and inversely proportional to the hardness of the material [117]. This relationship can be seen in Figure 10 [60].

**Figure 10.** Microhardness and wear rate of the nanocomposite coating vs. weight percentage of co-deposited SiC particulates in the nanocomposite coating [60]. Reprinted from Applied Surface Science, 252, Lei Shi, Chufeng Sun, Ping Gao, Feng Zhou, Weimin Liu, Mechanical properties and wear and corrosion resistance of electrodeposited Ni–Co/SiC nanocomposite coating/Pages No. 3591–3599, Copyright (2020), with permission from Elsevier.

In some instances, however, the wear rate decreases with decrease in microhardness, a deviation from Archard's Law. This is concurrent with findings reported by Wang et al. [20] where the decrease in wear rate beyond 49 wt% with a concurrent decrease in microhardness from 462 HV to 298 HV was attributed to changes in the phase structure. As Co content increases, the phase structure of the deposited Ni–Co coatings changes from solely FCC structure, to FCC coupled with HCP structure, and when the Co content goes beyond 80 wt% (See Figure 7), the phase structure is transformed to a predominantly HCP structure. This transformation in phase structure to a higher ratio of HCP causes a decrease in the coefficient of friction (COF) of the deposited Co-rich coatings and therefore decreased wear loss [20]. As such, the decreasing wear rate was associated with the decreasing coefficient of friction (COF). This is shown in Figure 11.

Magnetic measurements done on electrodeposited Ni–Co coatings show that Co content has significant influence on the magnetic and structural properties of the coatings. Increase in Co content in the coatings results in a gradual increase in the saturation magnetization. This conclusion is concurrent with Karpuz et al. [68] where the highest in-plane saturation magnetization of 1000 emu/cm3 was achieved at the highest contents of Co (80%). The same trend was also reported by [118].

Coatings improve the performance of the component by isolating the material's structure from the environment. Different substrate materials have been used for deposition of Ni, Ni–Co, and Ni–Co-nanocomposite based coatings ranging from steel, aluminum, to copper [106,115]. Failure of systems is usually associated with substrate–coating interface failure owing to the differences in mechanical and physical properties. Coating adhesion plays a key role in a surface's wear resistance and it is measured through friction testing where the friction abruptly changes when the coating breaks, also known as the critical load point. As such, a larger critical load indicates stronger coating adhesion [119]. Different mechanisms aimed at improving the critical load have been researched over the years. Wei et al. [120] reported that use of the magnetic jet electrodeposition technique (MJE) yielded a higher adhesion compared to traditional jet electrodeposition technique (TJE). At 4 g/L, TJE

technique had a maximum adhesion of 23.58 N compared to 33.20 N under MJE technique as shown in Figure 12.

**Figure 11.** Friction coefficient as function of Co content in the Ni–Co alloy deposit [20]. Reprinted from Applied Surface Science, 242, Liping Wang, Yan Gao, Qunji Xue, Huiwen Liu, Tao Xu, Microstructure and tribological properties of electrodeposited Ni–Co alloy deposits/Pages No. 326–332, Copyright (2020), with permission from Elsevier.

**Figure 12.** Adhesion of the composite coatings [120]. Reprinted from Journal of Alloys and Compounds, 791, Wei Jiang, Lida Shen, Mingyang Xu, Zhanwen Wang, Zongjun Tian, Mechanical properties and corrosion resistance of Ni–Co–SiC composite coatings by magnetic field-induced jet electrodeposition/Pages No. 847–855, Copyright (2020), with permission from Elsevier.

The adhesion of Ni–Co binary alloys can be further improved by incorporating nanoparticles into the matrix such that the effective area of contact between the substrate and coatings is increased, and the dispersion strengthening mechanism of the nanoparticles improves the coating's adhesion [121,122]. Another concept that has been considered for adhesion improvement is functionally graded materials (FGMs), whereby interfacial problems are mitigated by controlling progressive changes in structure and properties [123].

Ni–Co-nanocomposite coatings deposited at high current densities have exhibited higher surface roughness. This can be traced to adsorption of nanoparticles into the coating surface coupled with formation of pits and crevices as a result of an increase in hydrogen evolution rate at high current densities. Similar observations were made by Dheeraj et al. [35] where a surface roughness of 2.31 ± 1.78 μm was reported for sample S3 which represented coatings deposited at higher current densities.
