*3.1.6. Particle size*

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

**•** Low pressure gas blowing into the electrolyte or unbalanced air circulation [13];

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‐

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

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]

contaminations [10].

64 Electrodeposition of Composite Materials

These are:

surface.

The coating baths are usually agitated by several methods.

**•** Cathode motion (vertical, horizontal or rotating) [8];

**•** Magnetic or mechanical stirring [13];

**•** Sound or ultrasound vibrations [15-16].

**•** Circulating by pumps [14];

The properties of composite coatings also depend on the particle size and distribution of the second phase. The second phase particles have to be suspended in the electrolyte uniformly. For this purpose, the tendency to settle under gravity must be constantly overturned using stirring, pumping electrolyte flow, ultrasonic agitation or air injection where it is desired to achieve a uniform distribution of the second phase through the matrix.

While the fine particles are suspended in the electrolyte easily, the coarser ones tend to agglomerate and/or settle by the gravitational force. Consequently, because of the aggressive circulation of the electrolyte effect the codeposition conversely, stabilizing the suspension has much more importance in this specific process. In this situation there are two alternatives. One of them is using submicron or nano particles in the electrolyte and the other one is adding surface active chemicals (surfactant) into the electrolyte to solve the stabilization problem totally or partially. This phenomenon is explained in Section 3.1.4.

The properties of electrodeposited metals or alloys can be substantially modified by arranging the codeposition of fine particles with the metal or alloy. Such particles may be inorganic (oxides, carbides, diamond), metallic (chromium) or organic (PTFE). The amount of secondphase incorporated particles will be a function of the deposition which has to be taken into account [11-12].
