*3.1.4. Zeta potential*

Zeta potential (ζ) of the particles is an essential parameter for the codeposition of composite coatings and provides details about the dispersion mechanisms of the particles. Simply, zeta potential can be defined as the value of attractive and repulsive forces between the particles. The behaviour of the particles in aqueous media is not determined by the surface charge but by zeta potential [10].

Zeta potential is directly related to pH due to the fact that in many aqueous systems, H+ ions are the main ion content which determine potential. The value of zeta potential is measured by zetameter in millivolts, which is important for the explanation of the colloidal suspension stability. The higher the zeta potential, the better the stability of colloidal suspensions [10]. This prevents the suspended particles from bein agglomerated. Stability can be achieved mainly by two ways. These are:

**•** providing the particles electrically charged; and

*3.1.1. Substrate and substrate preparation*

62 Electrodeposition of Composite Materials

steps to remove contaminants from its surface [8].

of elasticity).

*3.1.2. pH*

the coating quality [10].

*3.1.3. Temperature*

*3.1.4. Zeta potential*

by zeta potential [10].

condition of particles in electrolytes.

The main idea of coating processes is to protect the main material, to be specific, substrate, from environmental conditions, and to improve its mechanical properties. Undoubtedly, the perfect choice for such engineering applications is steel due to its unique properties. Even though this material group has a lot of advantages, the surface of the steel substrate has to be protected by several coating techniques. Electrical conductivity of steel substrates allows them to be coated by electrolytic deposition techniques, including electro codeposition. The quality of the substrate should be chosen according to the application and is the important detail; it should be coherent with the coating material properties-side (such as hardness and modulus

The preparation of the substrate before plating is another point to be taken into consideration. The coatings with perfect properties have no meaning without a good adhesion. Adhesion between the coating and the substrate identifies the coating quality and determines the life time of this structure. The preparation generally includes chemical and/or mechanical cleaning

Electrolytes are prepared either acidic (nickel, copper, zinc and tin coating baths) or basic (zinc, cadmium, brass, gold and silver baths). It is important to know the pH in order to control and maintain the electrolyte composition stable for long term use. The electrolytes above or below specified pH values which show the acidic-basic characteristics affect and, generally decrease

In their research, Jia Man et al. [11] studied the zeta potential to describe the surface electrical behavior of nano-particles. Changes in the pH value of electrolyte can influence zeta potential. That means, zeta potential and pH are closely related to each other to determine the dispersive

Temperature has two opposite effects on this process. On the one hand it increases the diffusion, and on the other hand, it also increases the crystal growth rate. It allows the grain size reduction, but decreases the cathode polarisation and because of hydrogen release this effect transforms the coating morphology into the spongy structure. Due to this reason,

Zeta potential (ζ) of the particles is an essential parameter for the codeposition of composite coatings and provides details about the dispersion mechanisms of the particles. Simply, zeta potential can be defined as the value of attractive and repulsive forces between the particles. The behaviour of the particles in aqueous media is not determined by the surface charge but

Zeta potential is directly related to pH due to the fact that in many aqueous systems, H+

are the main ion content which determine potential. The value of zeta potential is measured

temperature should be adjusted and kept stable throughout the process [10].

**•** coating the suspended particles by a protective chemical (surfactant) [12].

J. Man et al. revealed pH and zeta potential effect on the α-Al2O3 particle codeposition. Results showed that H+ absorbsion of particles at low pH can prevent the agglomeration of metallic ions, lower the size of grains and directly affect the particle content in the matrix [11]. Zeta potential and particle content versus pH value are given in Figure 3. It is obvious that zeta potential decreases with the increase of pH.

**Figure 3.** Zeta potential (left) and particle content (right) versus pH value [11]

In the case of ultrafine particles, a suitable surfactant, which, after ionising in solution, adsorbs on the surface of the particles allowing them to orient themselves according to their electric charge, is frequently added. Hence, the repulsive electrostatic forces minimise coagulation of the particles.

Meguno et al. accepted zeta potential as an effective quantitative parameter. According to their research, zeta potential values of both α-SiC and γ-SiC are negative but as a function of decreasing pH, the potential increases and reaches positive values for lower pH. In another study Lee and Wan mentioned that increasing the copper sulphate bath concentration and decreasing pH, zeta potential of α-Al2O3 approaches positive values. On the other hand, it is the oppsite for γ-Al2O3 under the same conditions. This situation explains why α-Al2O3 particles are codeposited much more than γ-Al2O3 particles [13].

#### *3.1.5. Agitation and filtration*

ions

A further factor affecting the coating structure is defined as the electrolyte composition. In other words structure and the stability of electrolyte affect crystal formation rate direcly. With the higher crytal formation rate, fine grained and better coatings coherent to the substrate can be obtained. In order to countervail the local ion concentration decrease near cathode surface, a motion can be given to the substrate. This is supported by the cathode motiont generally but in some coating processes it is satisfied by the circulation of the electrolyte by several methods. In this case, there is a possibility of sticking the precipitated contaminants on the cathode surface. It is better filtering the electrolytes periodically to prevent the coatings from unwanted contaminations [10].

The coating baths are usually agitated by several methods.

These are:


Agitation in an electrolyte which can sometimes be invarious combinations. To give an example, Tudela et al. used both the mechanical stirrer and ultrasonic agitation simultane‐ ously. Figure 4 shows the sketch of the setup belong to their experimental set-up [16].

**Figure 4.** Schematic showing (a) front, (b) lateral, and (c) top views of the set-up [16]

Three main objectives for the agitation use are maintaining the temperature homogenous in the coating bath and carring both the metal ions and the suspended particles to the cathode surface.
