**4.6. Bath types and surfactants**

The bath pH is directly related to the surface charge density of the nanoparticles in the electrolyte. The surface charge density is generally related to the parameter termed as zeta potential. Zeta potential is a measure of the colloidal stability. The nickel matrix reinforced with the Al2O3 shows no effect on incorporation rate when bath pH >2. When the bath pH is increased beyond 2, the incorporation rate declines.[60] This result is consistent with those of Verelst and his co-workers.[69] Similarly, Wang et al. observed that nickel matrix reinforced with SiC nanoparticles shows an improvement in particles incorporation rate beyond a bath pH of 5.[72] Park and his co-workers also investigated the effect of pH on nickel matrix composites reinforced with SiO2 and TiO2 nanoparticles. They found that the dispersion of SiO2 is better at alkaline pH (~ 8) values rather than in acidic pH values. Moreover, for the dispersion of TiO2, the acidic pH (~3.5) provided better results.[73] The particle content in the composite also behaved in the similar way. They correlated this to the fact that SiO2 has a negative zeta potential from pH 2–11. It is not clear if these effects are accompanied by a decrease of particle content below pH 2, because the SiC content of the deposits is not inves‐ tigated. However, in the case of Ni–TiO2, the TiO2 particles experience a point of zero charge

Electrolyte agitation is always required to avoid the settling of the particles in electrolyte and to improve their movement toward the cathode. Vaezi and his co-workers observed that increasing the stirring rate up to 120 rpm increases the amount of SiC nanoparticles in the matrix but falls at a higher stirring rate. At a higher stirring rate, the flow is turbulent and not only the metal ions but the SiC particles are washed away on the cathode surface quickly.[74] A similar trend was observed by Baghery *et al*.[75] in the electrodeposition of Ni–TiO2 nanocomposite. Sen. *et al*.[76] also studied the effect of stirring rate on the microstructure of Ni–CeO2 nanocomposite and found that fraction of CeO2 particles increases up to stirring rate of 450 rpm, whereas at higher stirring rates the incorporation of CeO2 decreases. The coelec‐ trodeposition of ultrafine WC into Ni matrix on a rotating disk electrode with various rotation velocities in the range of 200–1200 rpm under pulse and direct current (DC) conditions is performed in Ref. [77]. This study also verified the fact that the increase in rotation speed has a beneficial effect up to a certain limit. In some studies, the ultrasound waves have been utilized in an attempt to avoid the formation of agglomerated nanoparticles in the plating bath due to the generation of large pressure causing breakdown of agglomerates. Kuo *et al*.[63] reported that the diameter of the agglomerated alumina particles may be refined by ultrasound energy,

Generally, the smaller particles possess higher van der Waals force of attraction or repulsion. It has been reported that the volume fraction of nanoparticles in nickel matrix increases with increasing particle concentration in the electrolyte.[60] The dependence of particle content in the deposit and consequently on the microstructure and surface properties has been studied by many authors.[78–80] In all studies, the volume fraction of the incorporated particles in the coatings with increase in the particle concentration in the plating bath up to an optimum value

around pH<5 in the plating bath and particle content increases.

but the result of particle incorporation is not reasonable.

**4.5. Particle concentration and size**

**4.4. Bath agitation**

260 Electrodeposition of Composite Materials

The type of plating baths also affects the co-electrodeposition process. For example, few studies have been conducted on non-aqueous plating baths and organic solvents. In some cases, water has been used partially or completely mixed with organics to avoid hydrogen embrittlement and to obtain a wider processing window.[82] Shrestha *et al*. studied the codeposition of nickel matrix composite using ethyl alcohol base in nickel bath and succeeded to achieve maximum amount of particle incorporation in the Ni matrix. They also found superior wear properties of the composite coatings prepared from ethanol electrolyte compared to those obtained from Watts's type plating bath.[83] Singh *et al*. also investigated the electrodeposition of Ni–TiC composite in acetate bath using *n*-methyl formamide as non-aqueous solvent and found better composite properties.[78]

Surfactants also enhance the rate of particle incorporation in composite electrodeposition. The choice of a given surfactant depends on their charge or polarity. Cationic surfactants are widely used for increasing the particle incorporation rate in the metal matrix. They impart positive charge to the particle surface and prevent the agglomeration in the plating bath.[84] It has been observed that the cationic adsorption is more effective if the particle size is in the nanoscale. [85] According to one report, cationic surfactants can increase the particle content up to 5 times for nickel matrix composites.[57] In another study, the content can be SiC content increased up to 50 volume percent with a fluorocarbon surfactant.[42] Similar study shows that azocationic surfactant improved the incorporation rate up to 62.4 volume percent.[86] However, it is always recommended to use surfactants in a minute concentration in the plating bath to avoid carbon compounds in the coating.
