*3.1.7. Current type and density*

Until quite recently electrodeposition was carried out using direct current (DC) [1]. DC electrolysis can be represented with the connection of two electrodes immersed in solution to the output of a DC power supply (rectifier). The cathode may itself be a metal or might be a semiconductor or a non-metallic conductor such as graphite. The primary purpose of this is to complete the electrical circuit.

Another type of current used in electrodeposition is pulse current (PC, pulse plating or pulse electrodeposition). In pulse electrodeposition the potential or current is alternated swiftly between two different values. It is possible to control the film properties in an atomic order by regulating the pulse width and amplitude [8].

Huang et al. compared DC and PC on chromium coatings and found that the coatings fabricated using PC are showing less surface cracks than that of the coatings fabricated using DC. Additionally, corrosion resistance of these coatings is higher than that of the coatings obtained under DC conditions [8].

Another research group studied three different electrodeposition methods (direct, pulse, and pulse reverse current). According to their results, nanocomposite coatings' microhardness values significantly improved. When compared to the methods, hardness values were shown higher in the PRC coated materials due to increased reinforcement content (see Figure 5a). Additionally, Ni-Co alloy matrix nanocomposites exhibited better wear resistance as com‐ pared to pure Ni–Co alloy coatings [17].

Similarly, Nemes et al. [18] focused on both current type and frequency which are effective parameters of codeposition process. For all the same conditions oxide (CeO2) addition increase the overall hardness (see Figure 5b). These results are affected by not only the codeposited ceramic content but also by the decrease of the grain size and the increase of deposit compact‐ ness affect [18].

**Figure 5.** Hardness results of two different research due to process conditions

#### **3.2. Property-performance relation**

The properties of electrodeposits are important for several engineering applications. A fundamental concern of materials science is the connections between structure and properties, which is true for both bulk and coated materials. Properties of the electrodeposited composites are defined by the properties of the reinforcements used. Metal matrix is the phase for particles to be embedded. The particles used in applications such as diamond, SiC,and Al2O3 can be applied by vacuum coating techniques [1, 13-14] on metallic substrates. However, for the substrates that have girift geometry it is hard to obtain homogenous coatings with high adhesion. Metals and ceramics show different physical properties, such as thermal expansion coefficient; thus, multilayered structures could be necessary to acqiure better adhesion for codeposition process.

Electrochemically codeposited composite coating applications are devided into three groups. These are: dispersion hardening, wear, and electrochemical activity [13]. Dispersion hardening effect can be seen easily for oxide, nitride, carbide, and boride codeposited composite coatings when compared to pure metallic coatings. Dispersion hardening is defined as the increase against deformation. The main mechanism of deformation is motion of dislocations. The reinforcement in the composite coating structure blocks the dislocation motion and, as a result, strengthens the increase. Although there is no particular study of this phenomenon, grain size reduction can be seen in the matrix for codeposited composite coatings, which is thought to be the reason of increase in hardness. In scientific research, synergistic effect of grain size reduction and particles are given together [13]. Hardness is related to particle size, agglomer‐ ation reduction and volume fraction of the particles in the metal matrix. Similarly, dislocation motion is defined with the distance of particles distributed in the coating layer. Increase in hardness is strongly affected by both the distance between particles and the volume fraction. However, it should be known that there is no limitless hardness increase in dispersion hardening, affected by reinforcement particles [13].

Composite coatings are commonly used in wear applications. Particle reinforcement to high and low frictional materials increases the wear resistance. Applications in equipments and contact surfaces of the motional parts of motors' can significantly extend their lifetime [13]. Composite coatings containing ceramic particles such as BN, diamond, SiC, WC, and Al2O3 show better abrasive wear resistance than the pure metallic coatings. Ramesh [19] found that Ni-TiC composite coatings with the volume ratio of 3% TiC showing four times less wear loss than the metallic Ni coatings.

In another research, open circuit potentials (the OCP curves) clarified that the values for insitu co-deposited samples were shifted toward nobler potentials when compared. In addition, polarization curves of the coated samples were shifted toward the lower current densities. Upon the reversal of scanning, the increase in the zero-current potential also indicated the increased corrosion protection due to the in-situ phase transformations of electro codeposited composite coatings in inert atmospheres [15].

#### *3.2.1. Porosity and grain structure*

Similarly, Nemes et al. [18] focused on both current type and frequency which are effective parameters of codeposition process. For all the same conditions oxide (CeO2) addition increase the overall hardness (see Figure 5b). These results are affected by not only the codeposited ceramic content but also by the decrease of the grain size and the increase of deposit compact‐

The properties of electrodeposits are important for several engineering applications. A fundamental concern of materials science is the connections between structure and properties, which is true for both bulk and coated materials. Properties of the electrodeposited composites are defined by the properties of the reinforcements used. Metal matrix is the phase for particles to be embedded. The particles used in applications such as diamond, SiC,and Al2O3 can be applied by vacuum coating techniques [1, 13-14] on metallic substrates. However, for the substrates that have girift geometry it is hard to obtain homogenous coatings with high adhesion. Metals and ceramics show different physical properties, such as thermal expansion coefficient; thus, multilayered structures could be necessary to acqiure better adhesion for

Electrochemically codeposited composite coating applications are devided into three groups. These are: dispersion hardening, wear, and electrochemical activity [13]. Dispersion hardening effect can be seen easily for oxide, nitride, carbide, and boride codeposited composite coatings when compared to pure metallic coatings. Dispersion hardening is defined as the increase against deformation. The main mechanism of deformation is motion of dislocations. The reinforcement in the composite coating structure blocks the dislocation motion and, as a result, strengthens the increase. Although there is no particular study of this phenomenon, grain size reduction can be seen in the matrix for codeposited composite coatings, which is thought to be the reason of increase in hardness. In scientific research, synergistic effect of grain size reduction and particles are given together [13]. Hardness is related to particle size, agglomer‐

**Figure 5.** Hardness results of two different research due to process conditions

**3.2. Property-performance relation**

codeposition process.

ness affect [18].

66 Electrodeposition of Composite Materials

Porosity is the main sources of discontinuities in electrodeposites. It can noticeably affect corrosion behaviour, mechanical, and electrical properties, and also diffusion characteristics. It is influenced by the substrate, the plating solution and its operating conditions, and posttreatments [15]. An efficient method to minimize porosity is to use an under plate.

Moreover, the grain size of the coating metal affects several properties of the coating structure. The following properties change in size, hardness, surface roughness, brightness, resistance to deformation, stress, corrosion, and several mechanical properties. For a decorative coating, brightness is the major property, whereas the other properties are most important for industrial and engineering applications. If the grains consisting of the coating metal are coarser, coating will be both less hard and mat. On the contrary, metallic coating will be harder, smoother and brighter. Besides, coatings with finer grain size are expected to be less porous when compared to the coatings with coarser grains [10].

It is known that the reinforcement of ceramic particles dispersed in the metal matrix can increase the overall composite coating hardness by two possible hardening mechanisms. One of them is the dispersion of sub-micron sized hard particles in the matrix and another one is the grain size refinement of the metal matrix assisted by dispersed ceramic second phase particles. Therefore, according to the Hall-Petch effect, an increase in the hardness of matrix can be expected [20].

Bahkit & Akbari [21] revealed the synergistic effect of grain size reduction and particle reinforcement on the composite coatings fabricated by sediment codeposition technique (SCD). In Figure 6, both metallic and composite coatings' hardness and grain size measure‐ ments are given, respectively.

**Figure 6.** Hardness (left) and grain size (right) of unalloyed, Ni-45Co, microcomposite and nanocomposite coatings produced by SCD [21]

It is clearly seen that average particle size used in the process also affect the properties under same deposition conditions especially the grain size refinement of the metal matrix.
