*1.5.3. Current density*

By definition, current density is the current per unit surface area of the cathode. It is expressed in *mA* / *cm*<sup>2</sup> of surface of the electrode.

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‐ site coatings.

The current density can be measured in the following terms:


**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 continue to be entrapped.
