*8.3. E*ff*ect of ZrO2 Nanoparticles*

ZrO2 nanoparticles are known for their fracture toughness, stress induced transformation, and strength [136–138]. As such, they are an important consideration in nanocomposite coating deposition for wear protection in environments at high pressure and high temperature. Ni–Co/ZrO2 nanocomposite coatings also exhibit high hardness and excellent corrosion resistance.

## **9. Applications**

Electrodeposited Ni–Co alloys and nanocomposites exhibit unique properties and, as such, they are used for a wide variety of industrial applications. Their combined reduced localized corrosion and

microhardness greatly improves protective coating performance. These coatings can therefore be used to protect less wear resistant and softer substrate surfaces for use in industry.

Karpuz et al. [68] reported that Ni–Co coatings electrodeposited from baths containing nickel sulfamate, boric acid and cobalt sulfate have potential for application in magnetic sensors. These magnetic properties of deposited Ni–Co coatings and their nanocomposites offers attractive potential to serve as soft magnets for motors, power supplies and high-efficiency transformers [3].

Ni–Co hydroxide nanosheets have been identified as candidates for pseudocapacitor application to meet the ever-growing demand for new energy storage devices. Pu J, et al. [139] researched on Ni–Co layered double hydroxides (LDHs) nanosheets and reported that the nanosheets exhibited excellent specific capacitance of 1734 F g−<sup>1</sup> at 6 A g−1. The nanosheets also exhibited better stability with a capacitance retention of 86% in the galvanostatic charge–discharge test after 1000 cycles.

### **10. Future Scope and Recommendations**

Key areas that have been identified for future additional research include [140,141]:


Hydrogen evolution affects the coating structure of deposited coatings. Several approaches have been used with different materials to great success. Kannan and Wallipa [114] coated a magnesium alloy with calcium phosphate using constant-potential and pulse-potential methods and analyzed the in vitro corrosion resistance properties. It was reported that the polarization resistance of pulse-potential deposited coatings was three times higher than that exhibited by constant-potential deposited coatings, and this was associated with the calcium phosphate particles being closely packed for pulse-potential coatings. This provides an interesting approach that can be researched on using Ni and Ni–Co based coatings.

Several other hypotheses have been postulated for the mitigation of hydrogen evolution in the electrodeposition process in different coatings. In past research, an organic solvent (ethanol) was added to the electrolyte bath to slow down hydrogen evolution on a magnesium alloy by decreasing conductivity of the plating solution, resulting in decreased hydrogen bubble bursting rate and hence a highly dense coating was deposited [142]. This approach of slowing down hydrogen evolution by decreasing the conductivity, however, results in lower deposition rates. Metal deposition utilizes the OH<sup>−</sup> ions generated during H2O breakdown whereby the metallic ions are reduced to form the metal. The mechanisms for hydrogen generation and metal-hydroxyl ion adsorption are shown in Equations (14)–(18) [143,144].

$$2\text{H}\_2\text{O} + 2\text{e}^- = \text{H}\_2 + 2\text{OH}^- \tag{14}$$

$$\text{2H} + \text{2e}^- = \text{H}\_2\tag{15}$$

$$\text{M}^{2+} + \text{OH}^- = \text{M}(\text{OH})^+ \tag{16}$$

$$\text{M(OH)}^+ \rightarrow \text{M(OH)}\_{\text{ads}}^+ \tag{17}$$

$$\rm{M(OH)}\_{\rm{ads}}^{+} + 2e^{-} = \rm{M} + \rm{OH}^{-} \tag{18}$$

where M can be Ni or Co ions. As such, a balance must be struck between the rate of hydrogen evolution and the deposition rate, and this offers an interesting area for research and application in Ni and Ni–Co based electrodeposited coatings. Other additives used include polyethylene glycol and di-sodium ethylenediamine tetraacetic acid (EDTA) in pulse copper deposition, and it has been reported that this improves the throwing power, current efficiency, and thickness of deposited coatings [145]. These additives can also be considered for electrodeposited Ni and Ni–Co based coatings.

It is advisable to use larger sample sizes because they hinder the manifestation of edge effects that are common in smaller sample sizes. This is especially common where the current density used is high with respect to the substrate's surface area.

The adhesive force that exists between the substrate and the coatings plays a major role in the wear resistance of the material. As such, ensuring a good bond exists between the deposit and substrate surface is key. Copper electrodes tend to exhibit better adhesive properties compared to their steel counterparts, but they are also more costly. Using a pre-treatment step offers the chance to improve this bond, especially where the substrate is made of steel. From personal experience, a triple immersion procedure of electronic cleaning can be used for better adhesion. This comprises of degreasing using electro-hydrostatic fluid, then removal of oxide layer by passing the substrates through a strong activating solution, and finally removal of carbon-black by passing the substrates through a weak activating solution [49]. A similar pre-treatment process has been used in other coatings like Ni–W nanocomposite coatings with good adhesion translating to superb wear resistance achieved [146]. This pre-treatment process provides interesting possibilities for future use in Ni–Co alloys and nanocomposites. De-ionized water should be used to clean the substrate surface after each pre-treatment step.

Research shows that orientation of electrodes in the electrolyte plays a major role in the deposition process. Results obtained from Ni–Co deposition suggest that the sediment deposition technique (SCD) is more favorable compared to conventional deposition technique. Ni–Co alloy and nanocomposite coatings deposited from the SCD technique have exhibited superior properties of higher Co content and higher nanoparticle content which translate to better microhardness, and improved wear and corrosion resistance of the deposited coatings. As such, selection of SCD in DC electrodeposition of Ni–Co composites should be considered for further research. Based on current trends, it can be seen that owing to their exceptional wear resistance and corrosion properties, deposited Ni–Co alloys and their nanocomposites are strong contenders for further application in the aviation industry, for use in jet engine fabrication, automotive engineering, textiles and general engineering. In recent years, a keen interest has developed in specialized engineering where deposited coatings consist of mixed functional properties, as well as deposition of superhydrophobic surface coatings which exhibit excellent wear and corrosion resistance, better self-cleaning and good tribological properties.

In essence, nanoparticles can be selected to match the desired properties of any coating, and with such capacity for discovery coupled with the ever-growing need of better material properties, the possibilities for future applications are endless.

**Author Contributions:** Conceptualization, N.S.M. and M.K.; methodology, N.S.M., and Y.Z.; validation, N.S.M. and M.K.; formal analysis, N.S.M. and L.Y.; investigation, N.S.M., Y.Z., and N.J.N.; resources, M.K.; writing—original draft preparation, N.S.M.; G.V.B., and N.J.N.; writing—review and editing, N.S.M., Y.Z., and M.K.; visualization, N.S.M.; G.V.B., and L.Y.; supervision, M.K.; project administration, M.K.; funding acquisition, M.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Technology development programmer for the Northern Jiangsu area, grant number BN2014019.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
