*3.2.7. Wear/friction*

obtained depending on the several test parameters. Details of the test method can be reached

Corrosion is known to be influenced by a variety of factors. It is not possible to separate corrosion from many of the other property issues related to coatings. For the proper selection of a coating, it is necessary to take its position, with respect to its substrate in the galvanic series for the intended application, into consideration [27]. Decorative nickel-chromium coatings developed for automotive industry applications are a suitable example of the application of

There are several examples of electro codepositited composite coatings for corrosion protection [28-30]. According to these results, it is supported that corrosion resistance of composite coatings increase significantly with respect to the pure metallic or alloy coatings [15, 21]. This phenomenon is suggested to be the effect of the inert properties of the reinforcements.

For the Ni-Co alloy matrix SiC reinforced composite coatings, Bahkit & Akbari [21] expressed the corrosion behaviour of these coatings through potentiodynamic polarization curves (Figure 7). Calculated corrosion current densities and measured corrosion potentials are

materials science and electrochemistry for the corrosion protection of materials.

**Figure 7.** The potentiodynamic polarization curves of alloy coating and composite coatings

**βa (mV/dec)**

**Table 1.** Calculated and measured corrosion data for alloy and composite coatings

**βc (mV/dec)**

Ni-Co -346 177.26 120.9 -354 6.570 4.75 Microcomposite -277 71.94 162.1 -257 0.137 158.48 Nanocomposite -201 71.18 176.5 -199 0.050 439.46

**Ecorr (mV)**

**icorr (μA.cm−2)**

**Rp (kΩ.cm2)**

from ASTM-C1624 standard [26].

70 Electrodeposition of Composite Materials

*3.2.6. Corrosion*

summerized in Table 1.

**Coating Eocp**

**(mV)**

To give some examples for the practical wear applications of coatings, one can count chromi‐ um, electroless nickel, precious metals, anodized aluminium and so on [31]. But for engineering applications pure metallic coatings, or the conventional coatings mentioned are not enough to satisfy. Based on this idea, researchers have been focussed on recent advancements include codeposition of dispersed particles with metals. There are two alternatives to improve the wear resistance of a composite coating. The first method is to use hard particles in the metal matrix so as to increase the strenght of the surface against to the other frictive part. The second way includes creating the composite structure by using self lubricant reinforcements such as graphite, MoS2, h-BN and WS2 [16, 32-34].

Due to their specific study [9], as a function of graphite particle content in the electrolyte, reduction in the hardness is appeared but decrease in the wear loss (Figure 8a). It can be expressed by the self lubricant property of graphite. Furthermore, the phenomenon of composite friction/wear for Ni-Al2O3/graphite composite structure are described in Figure 8b as shown.

**Figure 8.** Wear loss versus graphite content in the electrolyte (a) and the schematic illustration of composite friction/ wear phenomenon [9]

Friction coefficient and wear loss of composites initially decrease and then increase slowly back with the graphite particle concentration in the electrolyte. The results show an optimum graphite concentration in the electrolyte as 1.5g/L. Under this condition, the lowest friction coefficient, relatively high microhardness, and less wear loss was obtained.

### *3.2.8. Residual stress*

Stress in coatings also adversely affects properties. Nowadays, a variety of options are available for stress reduction of coatings. These include: choice of substrate plating solution, use of additives, and higher plating temperatures. A number of theories have been assumed regarding the origins of stress, however, none of them covers every single situation. Several methods for stress measurement vary from the simple rigid strip technique to complicated methods. Both phase transformations in the composite and additional post-treatment of the coatings can decrease residual stress value in a composite coating [12].
