**4.2. Zeta potential**

*3.1.2. Celis model*

258 Electrodeposition of Composite Materials

experimental data.

**4.1. Deposition current density**

their embedment in the metal coating (Fig. 3).

Guglielmi model triggered the development of other codeposition theories by Celis, and his co-workers. They predicted the co-deposited particles content in the matrix. According to the Celis model the inert particles approach towards the cathode in a specific sequence: (1) the adsorption of ionic cloud surrounding the particle surface, (2) the force of convection currents in electrolyte also supports the particles incorporation partly, (3) the diffusion kinetics of particles across the electric double layer, (4) the adsorption of the inert particles and the absorption of ionic cloud onto the cathode, and (5) the decrease in adsorbed ionic content and

**Figure 3.** Electric double layer around the ceramic particle showing the ionic concentration and generated potential

There are various other codeposition models such as for non-Brownian motion of particles in the bath.[52, 53] Although the various developed codeposition models are important tools in the nanocomposite fabrication yet. these models are not perfect for the understanding the influences of process parameters on the codeposition rate in the metal matrix without any

The deposition rate in electrodeposition process plays a vital role in the fabrication of nano‐ composites. The number density of particles approaching the cathode surface depends on the applied electric potential, and the thickness of the growing metal is determined by the deposition current density. The rate determining step in the co-electrodeposition is when the

difference as a function of the distance from the charged particle suspended in electrolyte.[51]

**4. Factors affecting composite co-electroplating**

The zeta potential is a parameter used to measure surface charge of the particle in the solution, and it indicates the stability of the colloidal suspensions. A higher zeta potential means a lower degree of particle agglomeration in the electrolyte.[62] From electrodeposition point of view, the stability of the nanoparticles is an important factor for a better embedded particle density in the matrix. It has been found that for the nickel matrix composites, when the electrostatic repulsion within the dispersed particles increases, particle agglomeration decreases, and the embedded particle density increases uniformly in the nickel matrix.[62] Uniform dispersion of nanoparticles shows higher attraction toward cathode and thus produces strong adsorption. [63] The zeta potential depends on the various factors, i.e., size of particles (nano or micro), addition of surfactants, electrolyte composition, the particle shape, the bath pH, and the energy of hydration.[64–66] Bund and Thiemig reported that negatively charged particles are highly attracted when there is an excess plus charge on the electrical double layer. This observation works for various composite systems. However, in case of electrophoresis, the negatively charged particles have been successfully deposited into the nickel matrix.[68] This might be correlated to not only the zeta potential but the absolute value of the zeta potential too in addition.

#### **4.3. Bath temperature and pH**

Generally, effect of temperature is always to increase the metal grain growth rate. There are different results of the effect of temperature on various codeposition systems. For example, the nickel matrix composite reinforced with Al2O3 shows no increment of the volume fraction of particles with temperature.[60, 69] On the contrary, the Ni–Co alloy matrix composite shows a significant increment in the reinforced TiO2 as a function of temperature up to 45 °C.[70] Similar behavior was shown by the nickel matrix composite reinforced with SiC at 50 °C. [71] Several reports are also available that show a linear decrease in the mass fraction of copper matrix composites as described in Ref. [42].

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

#### **4.4. Bath agitation**

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, but the result of particle incorporation is not reasonable.

#### **4.5. Particle concentration and size**

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 and decreases again. The particle shape affects the adsorption of the particles on the cathode by varying the charge on the particle surface and the suspension stability. Regarding the effect of particle size on codeposition, different results have been reported. Such as for Ni–Co/ SiC[80], an increase in the fraction of embedded particles was reported for micron-sized SiC compared to nano-SiC, while a negligible influence of particle size is observed for Ni–A12O3 composite.[69] It can be explained as when two particles come closer, agglomeration occurs as the force of attraction exceeds the force of repulsion between them. The magnitude of the net forces thus produced depends on the bath parameters and the processing conditions of the system.[41]
