**4. Effect of deposition parameters on ECD**

The electrodeposition process and thus the resulted microstructure of composite coating can be affected by many parameters, such as:


**3.2. Particle charging**

8 Electrodeposition of Composite Materials

function.

**3.3. Particle transport**

and uncharged particles.

**3.4. Other interparticle forces**

larger than 1 μm [24].

consideration:

and buoyancy.

The charge on the particles can be developed according to the following mechanisms:

**d.** Electrons are transferred between the solid and liquid phase due to differences in work

Charged particle in a suspension is surrounded by oppositely charged ions. In the so-called boundary layer, the concentration of these ions is higher than their concentration in the bulk electrolyte. These ions and the particle should move in opposite directions when an electric field is applied. At the same time, the ions are also attracted by the particle, and as a result, a fraction of the ions surrounding the particle will not move in the opposite direction but move along with the particle. Accordingly, the speed of a particle is not determined by the surface charge but by the net charge enclosed in the liquid sphere, which moves along with the particle [6, 29]**.**

The particle transfer toward the cathode surface occurs by four mechanisms, namely convec‐

Convection: It includes thermal and stirring effects, which can be increased extensively by

Migration: It is the movement of positive ions and negative ions, or charged particles, through the electrolyte under the effect of applied potential between the electrodes immersed in that

Diffusion: Electrode reaction decreases the concentration of oxidant or reductant at the electrode surface, producing a concentration gradient. Thus, the species movement from the higher to the lower concentration is enhanced. The diffusion process occurs for both charged

Brownian's movement: It is dependent on the particle size and may be ignored for particle size

During the electrodeposition process of nanocomposites, the following forces are taken into

**•** Mechanical forces, resulting from interaction with the fluid flow and other particles, gravity

**•** Molecular forces working on the particle in the vicinity of the cathode electrodes surface [24].

applying vibration, shock, and other stirring types and temperature gradients.

electrolyte. The migration process occurs only for charged particles.

**•** Electrical forces, due to the electric field presented in the electrolyte.

**a.** Ions are selectively adsorbed onto the solid particle from the electrolyte.

**c.** Dipolar molecules are adsorbed or orientated at the particles surface.

**b.** Ions are dissociated from the solid phase into the electrolyte.

tion, migration, diffusion, and Brownian movement.

Current density plays an important role in controlling the deposition rate which will in turn affect the concentration, composition and morphology of incorporated particles in the coatings. It also influences the thickness of the composite films, such that as the current density increases the thickness of the coatings increases. Low current density produces films with large surface irregularities. When the current density is increased, the amount of particle incorporation obtained has been found to increase for the Ni-TiO2 system with a relatively slow agitation, decrease for natural or synthetic diamond in Ni and for Cr particles co-deposited in Ni and to be unaffected when co-depositing alumina in Ni [24].

Particles reinforcement in the composite coatings varies with current density. At first, incor‐ poration increases sharply at the beginning with increase in current density till it reaches maximum value followed by sharp decrease. Therefore, hardness of composite coatings mainly increases due to the combined effect of both grain refining as well as of dispersive strengthening. When electroplating at lower current densities, metal ions dissolved from anode are transported at low rate and hence there is insufficient time for these ions to absorb on particles resulting in weak Coulomb force between anions adsorbed on particles leading to lower concentration of electrodeposited particles in the composite coatings. On the other hand, at higher current densities, metal ions dissolved from anode are transported faster than particles by the mechanical agitation which causes a decrease in co-deposition of particles as well as hardness of composite coatings. Therefore, selection of optimum current density is important to enhance the concentration of particles in the composite coatings [37]. The maximum current density for preparing nanocomposite coatings is limited by the limiting current density.

The DC electrodeposition methods are often associated with slower deposition rates and coating defects such as surface roughness, porosity, poor adhesion, undesirable microstruc‐ ture, etc. Recently, pulse current (PC) and pulse reverse current (PRC). ECD methods have attracted significant attention to improve deposition rates and microstructure of the coatings for improved mechanical and corrosion properties [24, 25].

**b.** Bath Temperature

According to Akarapu [25], two contrary behaviors were observed regarding the effect of temperature on the obtained crystallization size. This discrepancy was due to the two contra‐ dictory effects of temperature increase on the thermodynamic and kinetic driving force of nucleation process. As the electrolyte's temperature increases, the thermodynamic driving force of crystallization decreases and the critical size of the nucleus will increase. This will lead to lower nucleus densities and formation of coarse grain. On the other hand, the increase in temperature leads to enhancement of the kinetic driving force. This results in an increase in the rate of nucleation and thus fine grains formation [25].
