**5. Effect of process operating parameters**

Electrodeposition of metal matrix nanocomposite coatings is very complex and requires the process operating parameters to be optimized to produce high-quality deposits with improved functional properties. There are several parameters that are very important in fabrication of the composites but only the following will be covered in this review. These include current density, type of current, particle size, particle loading, stirring speed, bath composition, time, and temperature. The typical bath composition and parameters are shown in Table 1.


**Table 1.** Bath composition and plating parameters [35]

#### **5.1. Current**

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

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‐

electrocrystallization and are limited to specific conditions.

**•** Coatings that are just covered by adsorbed particles on the surface.

exhibit superior properties than the other manners of incorporation.

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

**•** Coatings containing particles truly embedded uniformly into the metal matrix.

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

porated in to a metal matrix [18]:

208 Electrodeposition of Composite Materials

**•** Coatings containing entrapped particles and

Current is one of the most important parameters that are used to control the plating process. It is used to reduce dissolved metal cations in solution to form a protective layer on the cathode. The rate in which metal cations are transferred to the cathode for plating determines the amount of second-phase particles that will be incorporated into the metal matrix [14]. Higher current densities enhance deposition rate and thus increasing the chances for the reinforcement of particles to be adsorbed on the cathode. The current density was found to influence the content of alumina particles present in the Ni–P matrix [19]. The increment of current density from 5 to 20 A/dm2 increased the content of Al2O3 particles from 7.75 to 13.65 vol.%. The hardness and phosphorus content of the deposits were also found to be affected by varying the current density. The hardness property of the coatings had direct relationship with the increase in current density while the phosphorus showed an inverse correlation. Ref. [20] obtained similar results when they studied the incorporation of CNT particles into Ni matrix. The CNT content in the coating increased with rising current densities. However, it is not in all cases where increasing current density during plating yields coatings with high micro‐ hardness values and particle content. Ref. [21] established an optimum current density of 0.8 A/dm2 for fabrication of Ni–Cr2O3 nanocomposite coatings with excellent mechanical proper‐ ties. Increasing the current density beyond the optimal conditions had no positive influence on the microhardness values of the resultant deposits. Plating in lower current densities requires time to achieve the desirable thickness and hence gives more time for the particles to be available at the cathode. This increases the chance of the particles to be homogenously incorporated into the matrix, leading to formation of harder surfaces due to dispersion hardening. Higher current densities increase deposition rate but reduce controllability of the co-deposition process. Ref. [22] obtained similar results when nano-titania particles were added into a nickel electrolyte. At constant pH, increasing current density yielded deposits with low TiO2 content and increased mean grain size. Table 2 shows the effect of current density on compositional, structural, and mechanical properties of N–W and Ni–W–Al2O3. Lower current densities favour good incorporation of tungsten in the matrix, high macro-residual stresses, and small crystallite sizes. However, incorporation of alumina also depends on the type of ceramic phase. Sigma Aldrich alumina particles follow the trend of tungsten, and inclusion of Taimicron alumina shows a deviation. Therefore, it can be concluded that optimization of operating current is required for electrodeposition of nickel composite/ nanocomposites to produce coatings with enhanced surface properties.


**Table 2.** The effect of current density on the content of tungsten and alumina in the Ni matrix, macro-residual stresses and crystallite size [23]

Direct current plating had been and is still commonly used for fabrication of thin films. However, this plating technique is associated with slow deposition rates and many defects in Tribological and Corrosion Performance of Electrodeposited Nickel Composite Coatings http://dx.doi.org/10.5772/62170 211

**Figure 2.** SEM micrographs of the plated samples [24]

hardening. Higher current densities increase deposition rate but reduce controllability of the co-deposition process. Ref. [22] obtained similar results when nano-titania particles were added into a nickel electrolyte. At constant pH, increasing current density yielded deposits with low TiO2 content and increased mean grain size. Table 2 shows the effect of current density on compositional, structural, and mechanical properties of N–W and Ni–W–Al2O3. Lower current densities favour good incorporation of tungsten in the matrix, high macro-residual stresses, and small crystallite sizes. However, incorporation of alumina also depends on the type of ceramic phase. Sigma Aldrich alumina particles follow the trend of tungsten, and inclusion of Taimicron alumina shows a deviation. Therefore, it can be concluded that optimization of operating current is required for electrodeposition of nickel composite/

**) W (wt.%) Al2O3 (%) (MPa) k (nm)** 

4 44±0.9 - 447±67 8 5 42.4±0.8 - 217±76 10 7 40.5±1.0 - 114±33 10 8 37.6±0.7 - -209±58 13

4 37.0±0.6 1.7±0.2 - 8 5 35.3±0.7 1.8±0.2 1150±46 9 7 31.0±0.6 1.1±0.1 1044±114 11 8 26.6±0.5 0.5±0.3 710±85 12

4 35.6±0.7 6.4±0.3 - 6 5 34.4±0.7 5.4±0.2 1360±102 7 7 28.5±0.6 5.6±0.2 1460±80 6 8 23.3±0.5 7.0±0.3 781±186 5

**Table 2.** The effect of current density on the content of tungsten and alumina in the Ni matrix, macro-residual stresses

Direct current plating had been and is still commonly used for fabrication of thin films. However, this plating technique is associated with slow deposition rates and many defects in

nanocomposites to produce coatings with enhanced surface properties.

**j (A/dm2**

210 Electrodeposition of Composite Materials

**Ni-W-Al2O3 (SA)** 

**Ni-W-Al2O3 (TA)** 

and crystallite size [23]

**Ni-W** 

the resultant deposits. Pulse current (PC) and pulsed reverse current (PRC) electrodeposition techniques have been developed to address the drawbacks associated with this deposition method. These methods allow better control of the structure and properties of the coatings [25]. The methods have several operating parameters (such as time period in which the pulses are imposed, relaxation time, pulse frequency, and pulse current density) that can be varied and optimized to achieve better deposit with enhanced surface properties as compared to the conventional method. Fine microstructures can be obtained from these plating methods by allowing high overpotential and low surface diffusion which favour the formation of new nuclei and thus perturbing grain growth [9]. Ref. [24] compared the morphologies and mechanical properties of nickel composite coatings produced by different electrodeposition techniques (DC, PC, and PRC). PC and PRC deposits exhibited fine and homogenous micro‐ structures more than DC ones (shown in Figure 1). The surface roughness was also reduced and the content of the reinforcement particles in the coatings increased. The high content and uniform Al2O3, SiC, and ZrO2 particles incorporated into the nickel matrix improved the microhardness and tribological properties of the deposits. Ref. [9] obtained similar results when Ni–Al2O3 composite coatings fabricated from both DC and PC plating at the same current density were compared. The particle content in the coating increased linearly with increasing current density for PC deposits, while the increase of particle content in DC coatings became negligible in current densities higher than 3 A/dm2 .
