*1.5.4. Duty cycle*

As duty cycle increases from 10 to 100%, micro-hardness as well as incorporated particles decrease significantly. Lower duty cycle gives longer OFF time for arrival of nanoparticles at the double layer. Therefore, more nanoparticles are reinforced in composite coatings at lower duty cycle and coatings become harder. The improvement in hardness at lower duty cycle is mainly due to grain refinement at pulse OFF-time longer than ON-time.

**Figure 2.** Showing different phases of current density during deposition (a) Pulse direct current with zero current dur‐ ing off time; (b) Superimposed pulse direct current with cathodic current during off-time; (c) Pulse reverse current with anodic current during off time; (d) Pulsed reverse current with combination of zero and anodic current during off time.

**Figure 2: showing different phases of current density during deposition** 

### *1.5.5. Bath agitation*

frequency, the content of nanoparticles in composite reduced because the shorter OFF time is not enough to remove concentration gradient of nanoparticles adjacent to the cathode. Wear loss and micro-hardness of composites significantly decreases when pulse frequency increases

By definition, current density is the current per unit surface area of the cathode. It is expressed

Current density actively governs metal deposition and co-deposition process. An increase in current density results in more rapid deposition of the metal matrix and fewer particles are embedded in the coating. To obtain uniform deposition, the current density must be minimal, so that the rate of particles' incorporation into the growing metal will exceed the adsorption on the cathode. Reinforcement of nanoparticles into metal matrix not only restrains the grain growth but also reduces the plastic deformation of metal matrix by combined effect of grain refining and dispersion strengthening, resulting in significant increase in hardness of compo‐

**Direct current** technique is based on the concept that the incorporation of nanoparticles occurs simultaneously with the reduction reaction of an ionic species to form the metal surface. **Pulsed direct current** works on the concept of alternating two or more direct cathodic currents for various deposition times. This allows the incorporation of higher concentrations of nanopar‐ ticles as well as producing a wider range of deposit compositions and properties. **Pulsed reverse current** technique, as the name connotes, has similar characteristics but imposes a cathodic current during the ON time and an anodic current during the OFF time. This method has been the most successful for incorporating higher concentrations of nanoparticles because it helps to eliminate a fraction of the electrodeposited metal during the OFF time. Pulse reverse current technique ensures refine surface microstructure, increased incorporation rate of nanoparticles into the metal deposit and uniform size selective entrapment of particles. During the anodic period, larger sizes of nanoparticles dissolve, whereas smaller nano-particles

As duty cycle increases from 10 to 100%, micro-hardness as well as incorporated particles decrease significantly. Lower duty cycle gives longer OFF time for arrival of nanoparticles at the double layer. Therefore, more nanoparticles are reinforced in composite coatings at lower duty cycle and coatings become harder. The improvement in hardness at lower duty cycle is

mainly due to grain refinement at pulse OFF-time longer than ON-time.

from 10Hz to 1000Hz. Significant adhesive wear is observed at higher frequencies.

*1.5.3. Current density*

42 Electrodeposition of Composite Materials

of surface of the electrode.

The current density can be measured in the following terms:

in *mA* / *cm*<sup>2</sup>

site coatings.

**•** Direct current

**•** Pulsed direct current **•** Pulsed reverse current.

continue to be entrapped.

*1.5.4. Duty cycle*

**Electrolyte agitation** increases convection of bath contents and therefore enhances the flux of the particles reaching the cathode surface. Intensive agitation may cause adverse effect by disconnection and removal of the particles by turbulent streams of the electrolyte.

Bath agitation serves the purposes of keeping the particles evenly distributed, well suspended in the electrolyte and effectively transported to the cathode surface. Excessive agitation has tendency of removing particles from cathode surface before they can be embedded in the metal deposit. This is due to turbulence initiated by the electrolyte in the bath. 6

For industrial applications, the popular methods used in open tanks include the overhead blade stirrer, the reciprocating plate plunger or a pumped recycle loop of the electrolyte. For laboratory investigations, magnetic stirrers, rotating disk or cylinder electrodes and parallel plate channel flow can be employed. In contrast, the commonly used plate-in-tank geometry provides poorly defined fluid flow conditions.

#### **1.6. Nano-composite Coatings**

The backbone of composite coating is multi-phase union of matrix and second-phase particles in correct ratio. Matrix plays the role of continuous phase and the solvent to disperse the second phase.


**Table 1.** Different types of electrolytic baths

Composite coating can be classified on the basis of the matrix and the reinforcing co-deposited particles. The matrix phase on a broader scale can be grouped into:


The common metals used as matrices for electrolytic co-deposition are: Silver (Ag), Chromium (Cr), Cobalt (Co), Iron (Fe), Zinc (Zn), Nickel (Ni), Copper (Cu) and Gold (Au).

A variety of composite coatings can be deposited by reinforcing different nanoparticles. Reinforcement particles can be carbides (TiC, SiC, WC,Cr2C3)[2,3,4,5,6,7], borides [8], oxides (ZnO, In2O3, ZrO2, CeO2, Al2O3,Cr2O3,SiO2,TiO2) [9,10,11.12], graphite, diamond, or solid lubricants, such as polyethylene and polytetrafluoroethylene [13,14]. Variable amounts of these particles in the coatings become precipitated to impart special properties to the deposited layers. These properties mainly depend on the microstructure of the matrix phase of a composite coating and the amount and distribution of co-deposited particles (non-metallic inclusions) which are influenced by many process parameters.
