*6.2. Surfactants*

Nanoparticle stabilization in the electrolyte and non-agglomerated nanoparticle dispersion are key elements of producing Ni–Co nanocomposite coatings with excellent properties. Surfactants such as cetyltrimethylammonium bromide (CTAB) [84–86], sodium dodecyl sulfate (SDS) [87,88], and sodium

lauryl sulfate (SLS) [89] are added into the Ni–Co nanocomposite coating baths to prevent nanoparticle agglomeration by reducing the electrolyte's surface tension to create smaller hydrogen bubbles thereby limiting pitting effect [35]. Surfactants of cationic and anionic nature are commonly used owing to their significant influence on ceramic-based nanoparticle dispersibility. Surfactants added into the bath work by changing the cathode polarization potentials, thereby changing the adhesion, smoothness, grain growth rate, and grain size of the deposited coatings [90]. Several researchers have used optimum quantities of different surfactants and reported improvement in corrosion resistance and mechanical properties [91].

Different surfactants have different effects on different nanoparticles. It was reported that CTAB surfactant offered a better alternative over SDS and triton X when they were compared in deposition of SiC nanoparticles using the PRC technique [92] and the PC technique [35]. However, the advantage offered by surfactant addition is not limitless. When the concentration of a surfactant exceeds a certain limit, it becomes counter-productive. Ger M [93] observed that excessive CTAB surfactant increased nanoparticle's adhesive force which resulted in deposition of coarser SiC nanoparticles.

#### *6.3. Saccharin*

Addition of saccharin results in an increase in alloy deposition overvoltage which promotes deposition of Ni while hindering that of Co, and as such the resulting Ni–Co coatings exhibit reduced Co content [4,94]. Research shows that surface morphologies of Ni–Co coatings exhibited colony-like morphologies which consist of grain morphologies where several grain colonies converge to form one larger colony. As such, grain size is greatly reduced owing to the grain refinement phenomena. This is achieved by inhibition of pyramidal growth by saccharin thereby leading to the production of shiny, smooth surfaces [94]. This concurs with Weil and Cook [95] who reported that addition of organic additives such as coumarin and thiourea into the Ni–Co electrolytes hindered the growth of pyramids, caused surface roughness reduction, grain size reduction and increased surface brightness.

Increase in saccharin content in the coatings up to a certain value also improves the microhardness of Ni–Co coatings beyond which the microhardness reduces with increase in saccharin content. Li et al. [94] reported that increase in saccharin content to 3 g/L, 4 g/L and 5 g/L resulted in increased microhardness values of 456 kg/mm2, 507 kg/mm<sup>2</sup> and 554 kg/mm2, respectively. This conclusion was also reached by Wang et al. [96]. The increase in microhardness with increase in saccharin content to a certain value can be attributed to grain refining effect, and the decrease in microhardness beyond that level of saccharin can be attributed to the inverse Hall–Petch relationship when refining of grain size reaches a certain level [97]. The drawback to adding saccharin to the electrolyte is the reduced ductility of the resulting Ni–Co coatings owing to the sulphur and carbon impurities that are usually present in saccharin laden nanocrystalline coatings. The said impurities separate into grain boundaries thereby preventing the efficient sliding of grain boundaries and hence the low ductility [98].

Saccharin has been reported to act as an internal stress reliever in electrodeposited Ni–Co coatings and this has been attributed to grain refinement. Internal stresses are developed within the coating layer during the electrodeposition process and they cause oriented resultant strain in deposited coatings. Hydrogen ion reduction occurs at the cathode, and the small sized H<sup>+</sup> promote favorable conditions for diffusion to the coating's active centers. As the H<sup>+</sup> become transformed into H molecules, internal stresses are developed as the volume changes. They are classified into three main categories [99–102]:


While internal stresses have been known to improve hardness and abrasion resistance of deposited coatings, at high levels, these stresses increase the coating's brittleness. Brittle coatings develop extensive microcracks which expose the substrate surface to corrosive attack and degradation when exposed to a corrosive medium. The saccharin molecules are reversibly adsorbed on active sites thereby hindering growth of crystals and impeding surface diffusion of adatoms. As such the volume of grain boundaries increases and this dissipates the energy created by internal stresses [103].

Saccharin has been employed in tensile research of electrodeposited Ni–Co nanocomposite coatings. Wang et al. [96] reported that PC deposited Ni–Co/Al2O3 nanocomposites containing saccharin exhibited low-temperature superplasticity, where a maximum elongation of 632% was achieved at a strain of 1.67 <sup>×</sup> 10−<sup>3</sup> s−<sup>1</sup> and a temperature of 823 K. The dominant superplastic accommodation process was taken to be dislocation glide.
