**6.1. Ni composite coatings**

than the finer ones. Nanoparticles easily agglomerate when they are added in the plating bath. Therefore, fabrication of nanocomposite coatings requires higher stirring speeds than their composite counterparts. Other forms of agitation have been used in literature and these include the use of ultrasonic energy to keep particles in suspension, enhance mass transport, and reduce diffusion layer thickness [34]. A schematic diagram of electrodeposition cell assisted with ultrasonic energy is shown in Figure 4. The use of ultrasound also helps to modify the surface morphologies of coatings fabricated with conventional DC plating technique [10].

Particles require time to remain around the cathode to increase the chances of their incorpo‐ ration. Ref. [35] reported that longer deposition time allows for the formation of thicker and compact coatings with improved microhardness. The highest microhardness values were achieved at a deposition time of 14 min, and beyond this time no adherent spherical globules were formed. Temperature also plays a significant role in co-deposition process. Increase in temperature enhances reaction kinetics, leading to more nickel ions to be transferred to the cathode. The content of SiC particles has been found to be a function of temperature by ref. [36].

optimal conditions, a decrease in incorporation rate was observed. Thermodynamic movement of ions improves with temperature and the particulates' kinetic energy also increases. This causes rapid deposition, which poses a risk on the control of crystal growth and uniformity in distribution of particles within the matrix. According to ref. [37], increase in temperature reduces adsorpability of the particles and hence decrease in overpotential cathode and electric field. Ref. [35] obtained similar results when Fe2O3 nanoparticles were co-deposited with

C increased the content of the particles in the coating. Above

**Figure 5.** Typical electrodeposition cell assisted with ultrasound energy [34]

**5.6. Time and temperature**

216 Electrodeposition of Composite Materials

Increasing temperature up to 50o

nickel.

The inclusion of second-phase particles into nickel matrix influences the evolution of the coatings surface morphologies. The nickel matrix characterized by pyramidal crystal structure was changed to spherical structure through the addition of inert titanium nanoparticles [38]. The composite coatings also exhibited smaller crystallite sizes than those of the metallic matrix. Particle loading plays a major part in refinement of grains and the smallest grains can be obtained in optimum conditions. Embedment of GNS–TiO2 nanocomposite into nickel coatings yielded similar results [39]. The reduction in grain size was attributed to the growthinhibiting ability of reinforcement nanoparticles which are adsorbed on the grain boundaries and thus restricting further growth. The presence of uniformly distributed nanoparticulates in the matrix reduces surface defects (pores, microholes, gaps, crevices, etc.) in the deposits [40]. Coatings with minimal surface defects possess few active sites for chemical attack and thus improve their corrosion resistance. Addition of 50 g/l of nano-SiC particles in a nickel plating bath reduced the current density of nickel coatings from 7.09 to 0.03 μA/cm2 [41]. The Tafel plots of pure nickel and Ni–SiC nanocomposite coatings are shown in Figure 5. Im‐ provement in corrosion resistance is depended on the amount of second-phase particles incorporated, and coatings with higher particle content are associated with increased positive potential shift and lower corrosion currents [42]. These nanoparticles also act as inert physical barrier to the initiation and development of defect corrosion.

Ref. [43] incorporated TiCN (titanium carbon nitride) particles into a nickel matrix and the results obtained were similar to those of SiC. The highest volume of 23.9% TiCN yielded coatings which exhibited the highest potential and lowest current density. Addition of ceria particles into electrolytic nickel solution yielded composite coatings which possessed lower potentials, current densities, and corrosion rates [44]. The results obtained by the authors show that these particles cause cathodic protection when incorporated in a nickel matrix. The particles reduce active surface area and cause blockage on the cathode for HER (hydrogen evolution reaction) to occur. Incorporation of ceramic particles into a metallic matrix does not always guarantee improvement in corrosion resistance of the coatings. The inclusion of carbon nanotubes was found to cause negative potential shift and thus increasing the corrosion rate of the matrix on copper substrate [20]. The poor corrosion resistance shown was due to the porous nature of coatings which was dependent on CNT content. However, ref. [45] obtained different results when Ni–CNT coating was electrodeposited on Ti–6Al–4V alloy. The corro‐ sion resistance was notably enhanced by the presence of CNT. These results show that

**Figure 6.** Polarization curves of Ni and Ni–SiC nanocomposite coatings

improvement of functional properties depends not only on incorporated particles but also on different factors such as type of substrate and operating parameters.


**Table 5.** Tribological properties of Ni–SiC, Ni–Si3N4, and Ni–Al2O3 composite coatings [46]

Refined microstructures exhibiting small and uniform grains are also characterized by high hardness and excellent wear resistance. The particles have pinning effect allowing little or no movement of dislocations. Co-deposition of nickel with SnO2 nanoparticles yielded coatings with high hardness, low friction, and better wear resistance [47]. The improvement of me‐ chanical properties was a function of the content of incorporated particles. Samples with the highest ceramic particle content exhibited the highest hardness values, lowest wear volume and friction coefficient. It can be seen from Table 5 that various incorporations yield different results. This is due to the fact that different ceramic materials exhibit their unique properties. Si3N4 particles have been reported to exhibit high hardness and self-lubricating properties; hence, their incorporation into a metallic matrix enhances its tribological characteristics [48]. Other particles that have been reported to possess self-lubricating properties include carbon nanotubes and molybdenum sulphide [45, 49]. The deposits that contain these particles exhibit reduced coefficient of friction and high wear resistance as compared to the matrix.
