**3. Basics of Electroplating of Nickel**

Electroplating of nickel involves the passage of current between two electrodes (anode and cathode) immersed in an electrolyte containing nickel salts to cause dissolution of Ni2+ ions in the anode to be deposited at the cathode. Equation 1 and 2 show the anodic and cathodic reactions that take place during nickel deposition. The anode is mainly a nickel plate while the cathode is any metal or material that needs to be protected or decorated. The nickel ions deposited at the cathode are replenished by those formed as the result of the dissolution of the anode. According to Faraday's law of electrolysis, the amount of nickel dissolved at the cathode is equal to the amount of nickel deposited at the cathode, which is directly proportional to the product of current and time [13].

Anodic:

such as high hardness, excellent corrosion resistance, thermal stability, wear resistance, and self-lubrication properties [3]. The nanoparticles incorporated into the matrix include those of

Electrodeposition is one of the surface modification techniques that are used to fabricate nickel nanocomposite coatings. This technique has several advantages over the other processing methods which include low cost, simplicity of operation, versatility, high production rates, industrial applicability, and few size and shape limitations [7]. However, co-deposition of nanostructured inert particles using electrodeposition has its own challenges. Agglomeration of particles in the electroplating bath, inhomogeneity in the distribution of particles in the matrix, and low content of particles in the coatings are some of the drawbacks associated with this process [8]. These problems compromise the quality of the coatings and result in poor performance during application. Therefore, proper process development and optimization are

Many researches have been conducted in an attempt to address these limitations. Additions of chemical agents into electrolytic solutions to aid co-deposition of the particles have been found by many researchers to reduce agglomeration of particles and increase their incorpo‐ ration in the matrix. These additives disturb the electrostatic stabilization of the particles and hence promote their suspension in the solution [9]. Pulse current electrodeposition is another method that has been employed to enhance co-deposition and improve uniform distribution of particles. This type of plating has three independent variables for controlling co-deposition as compared to direct current plating which only has one variable [10]. Ultrasonic energy has also been used to improve the inclusion of particles into the metal matrix. It enhances mass transport of particles to the cathode for co-deposition, reduces the thickness of the diffusion

**2. Electrodeposition of nickel composite/nanocomposite coatings**

Electrodeposition of composite coatings has been studied and researched by several scholars over the past few decades. The focus of the research is centred around the development and fabrication of advanced and novel surface coatings that can withstand both physical, chemical, and mechanical deterioration under service conditions. In order to produce these high-quality coatings, the mechanism of co-deposition of reinforcement particles with the matrix, optimi‐ zation of the process operating parameters, and the properties of the resultant deposits need

Electroplating of nickel involves the passage of current between two electrodes (anode and cathode) immersed in an electrolyte containing nickel salts to cause dissolution of Ni2+ ions in the anode to be deposited at the cathode. Equation 1 and 2 show the anodic and cathodic

metal oxides, carbides, nitrides, borides, polymers, and carbon-based materials [4–6].

required to counteract the limitations.

206 Electrodeposition of Composite Materials

to be fully understood.

**3. Basics of Electroplating of Nickel**

layer, and disperses the particles in the electrolyte [11,12].

$$\text{Ni} \rightarrow \text{Ni}^{2+} + \text{ } \text{2e}^{-} \tag{1}$$

Cathodic:

$$\text{Ni}^{2+} + \text{2e}^{-} \rightarrow \text{Ni} \tag{2}$$

There are many bath solutions that have been developed for producing different types of nickel deposits. The ones that have gained more usage include the Watts and sulphamate baths. These plating baths are reinforced with surfactants and other additives to improve the quality of the deposits such as brightness, surface morphology, and other functional properties.

### **4. Mechanism of Co-deposition**

Many models have been developed to understand the mechanism of co-deposition of particles into metallic matrix. These models predict the processes that are involved during particle codeposition and their adsorption rate into the coatings. The processes include electrophoresis, mechanical inclusion, adsorption, and convective-diffusion. One of the most used and accepted model by scholars is Guglielmi's two-step adsorption model. In the first step of particle incorporation, the particles are loosely adsorbed on the cathode covered by a cloud of metal ions. Strong adsorption of particles follows in the second step with current density playing a key role in the particles to be strongly adsorbed on the cathode. The strongly adsorbed particles are embedded into the growing metallic layer [14]. The author related the volume fraction of co-deposited particles (α) to the volume percent of particles suspended in the plating bath (*C*) with the Langmuir adsorption isotherm as shown in equation 3

$$\frac{C}{a} = \frac{M i\_{\ o}}{n F \rho\_{\text{m}} v\_{\ o}} \exp(A - B) \eta \left(\frac{1}{k} + C\right) \tag{3}$$

where is the atomic weight of the electrodeposited metal *i*o the exchanging current density, the valence of the electrodeposited metal, *F* the Faraday constant, ρ*m* the density of electrodepos‐ ited metal, the overpotential of electrode reaction, *i* = *io* exp(*Aɳ)* and *k* the Langmuir isotherm constant, mainly determined by the intensity of interaction between particles and cathode. The parameters *vo* and *B* are related to particle deposition, and both play a symmetrical role with the parameters *io* and *A* related to metal deposition [15]. Ref. [16] improved Guglielmi's model by using three modes of current density to differentiate the reduction of adsorbed ion on the particles. This new model involved three steps, where in the first step particles are convectively forced to the surface followed by loose adsorption and then irreversible incorporation of particles into the matrix by reduction of adsorbed ions. Ref. [17] incorporated a third-order polynomial equation to further improve Guglielmi's model. This corrective factor will help account for the effects of adsorption and hydrodynamic conditions. Many other models have been developed which involved statistical approach to predict the chances of particles being included into the deposit. However, all these models cannot predict the effect of particles on electrocrystallization and are limited to specific conditions.

The manner of incorporation of particles into metal matrix depends mainly on the electrode‐ position process parameters. Some of the most important parameters include the speed at which the bath is stirred, the applied current density, and electrolyte composition. Bath agitation serves as a medium that assists particles to be transported to the cathode, while applied current density and electrolyte composition are responsible for the formation of ionic cloud around the introduced particles. There are three possibilities for particles to be incor‐ porated in to a metal matrix [18]:


A schematic diagram of particle incorporation into a metal deposit is shown in Figure 1. The manner in which particles are incorporated into the coating determines the quality of the resulting deposits. Coatings containing uniformly distributed and truly embedded particles exhibit superior properties than the other manners of incorporation.

**Figure 1.** Schematic representation of different co-deposition possibilities
