**5.2. Particle loading**

The concentration of the reinforcement particles in the solution to be co-deposited with the matrix has a significant influence on their adsorption rate at the cathode. According to Guglielmi's two-step adsorption model, high particle content in the plating bath increases the adsorption rate of the particles on the cathode [14]. However, when the particle concentration in the bath reaches saturation, agglomeration occurs leading to reduced incorporation or formation of deposits with surface defects [26]. Inclusion of TiO2 nanoparticles into nickel matrix was found to be dependent on the bath particle loading. The least bath particle loading of 5 g/l of TiO2 yielded the lowest weight of particles of about 1.8 wt.%, while introducing 15 g/l of the particles increased the particle content in the deposit to be about 3.8 wt.% at the constant current density of 40 mA/cm2 [7]. Similar behaviour was obtained throughout the experiments when other current densities were used (see Figure 2).

The optimum particle loading for SiC particles into nickel matrix was found to be 20 g/l by ref. [4]. Addition of more particles beyond the optimal levels led to decrease in incorporation of the particles into the coating. The decrease was ascribed to the agglomeration of the SiC particles due to their poor wettability. As much as high content of particles in the bath increases their availability at the cathode, the capturing capacity of the growing metal remains un‐ changed [27]. Therefore, particle entrapment into the matrix requires conducible conditions to produce high-quality coatings with enhanced surface properties.

#### **5.3. Particle size**

Nanoparticles have gained wide use in fabrication of nanocomposite coatings due to their excellent and attractive properties. Their incorporation into metal matrixes is associated with better surface morphologies, improved corrosion resistance, thermal stability, and excellent mechanical properties as compared to their micron and submicron counterparts. However, co-deposition of these particles is associated with many challenges. According to the study conducted by ref. [27], zeta potential of the particles is one of the major driving forces respon‐ sible for how micron-, submicron-, or nano-sized particles behave differently in the plating bath. The results showed that micron-sized SiC particles exhibited more negative than the nanoparticles. This indicates that the micron-sized SiC particles are easily adsorbed by the nickel cations. The higher the positive or negative zeta potential the particles possess, the more stable they are in solution since they repel each other and thus limiting the formation of aggregates. Due to the high repulsion forces that exist between the micron-sized particles,

**Figure 3.** Correlation between TiO2 content in the deposit with current densities [7]

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

The concentration of the reinforcement particles in the solution to be co-deposited with the matrix has a significant influence on their adsorption rate at the cathode. According to Guglielmi's two-step adsorption model, high particle content in the plating bath increases the adsorption rate of the particles on the cathode [14]. However, when the particle concentration in the bath reaches saturation, agglomeration occurs leading to reduced incorporation or formation of deposits with surface defects [26]. Inclusion of TiO2 nanoparticles into nickel matrix was found to be dependent on the bath particle loading. The least bath particle loading of 5 g/l of TiO2 yielded the lowest weight of particles of about 1.8 wt.%, while introducing 15 g/l of the particles increased the particle content in the deposit to be about 3.8 wt.% at the constant current density of 40 mA/cm2 [7]. Similar behaviour was obtained throughout the

The optimum particle loading for SiC particles into nickel matrix was found to be 20 g/l by ref. [4]. Addition of more particles beyond the optimal levels led to decrease in incorporation of the particles into the coating. The decrease was ascribed to the agglomeration of the SiC particles due to their poor wettability. As much as high content of particles in the bath increases their availability at the cathode, the capturing capacity of the growing metal remains un‐ changed [27]. Therefore, particle entrapment into the matrix requires conducible conditions to

Nanoparticles have gained wide use in fabrication of nanocomposite coatings due to their excellent and attractive properties. Their incorporation into metal matrixes is associated with better surface morphologies, improved corrosion resistance, thermal stability, and excellent mechanical properties as compared to their micron and submicron counterparts. However, co-deposition of these particles is associated with many challenges. According to the study conducted by ref. [27], zeta potential of the particles is one of the major driving forces respon‐ sible for how micron-, submicron-, or nano-sized particles behave differently in the plating bath. The results showed that micron-sized SiC particles exhibited more negative than the nanoparticles. This indicates that the micron-sized SiC particles are easily adsorbed by the nickel cations. The higher the positive or negative zeta potential the particles possess, the more stable they are in solution since they repel each other and thus limiting the formation of aggregates. Due to the high repulsion forces that exist between the micron-sized particles,

.

negligible in current densities higher than 3 A/dm2

experiments when other current densities were used (see Figure 2).

produce high-quality coatings with enhanced surface properties.

**5.2. Particle loading**

212 Electrodeposition of Composite Materials

**5.3. Particle size**

electrodeposition under high particle loading is possible and thus increasing the chance of more particles to be embedded in the matrix. The authors found that the micron-sized particles were uniformly distributed in the coating as compared to agglomerated nanoparticles. However, these particles were not uniformly distributed along the surface in normal direction which can severely affect the fabricated film properties. Ref. [28] found that particles with average size of 5 μm had lower hardness values than their 10 and 50 nm counterparts. This result can easily be explained by the Hall-Pecth equation (4):

$$
\sigma = \sigma\_o + A / d^{1/2} \tag{4}
$$

where *d* is the grain size and σ*o* and *A* are constants. According to the equation, it can be seen that smaller grain size has a positive influence on the yield strength since the relationship is inversely proportional. Nanoparticles also has the ability to fill the microholes, gaps, and other surface defects present on the surface of the matrix than micron- or submicron-sized particles [29]. This results in fine microstructures with minimal surface defects and better functional properties. The size of the particles also affects the amount of particles that will be incorporated as shown in Table 3. Finer particles are difficult to incorporate in the matrix than coarser ones. Therefore, the operating parameters such as stirring speed, pH, and current density are very crucial to be optimized to facilitate better uniform distribution and high incorporation rate of fine particles in the deposit.


**Table 3.** Volume percent and number density of SiC particles in composite coatings with the distance between particles [28]

### **5.4. Bath composition**

Electrolyte ionic strength, bath additives, and pH are some of the operating parameters that affect the zeta potential of second-phase particles as explained before. The pH of the bath is a function of bath composition which includes the electrolyte and chemical additives. Strong acid or bases can be added to adjust the pH to be acidic or alkaline. Particles have different zeta potential at different pH depending on their nature. Therefore, various ceramic particles will have more positive or negative charge on either acidic or alkaline conditions. Titania particles were found to exhibit negative zeta potential in acidic solution of pH= 4.3 and positive zeta potential at alkaline conditions with pH of 9.5 [30]. A maximum particle content of 4.3 wt. % was achieved from alkaline baths while 3.3 wt.% resulted from acidic solutions. This shows that the conducible co-deposition of TiO2 particles with nickel is achievable under high pH values at particle loading of 10 g/l and current density of 1 A/dm2 . A decreasing trend (from 2 to 5) in particle incorporation with increasing pH values was noticed by ref. [22]. At plating conditions of pH=2, particle loading of 100 g/l and current density of 5 A/dm2 , a maximum of 8 vol.% of TiO2 incorporation was achieved. However, the authors did not study further the behaviour of the particle incorporation in alkaline solutions. The results obtained by the different authors show that particle incorporation depends on several factors than pH of the solution. The electrolyte is the carrier of electroactive species which are responsible for the formation of ionic cloud around the particles to enable their transportation to the cathode for entrapment in the matrix [31]. Variation of nickel sulphate in the bath from 200, 250, and 300 g/l showed significant effect on the incorporation rate of reinforcement particles as reported by ref. [28]. The volume of incorporated SiC particles increased with increasing nickel sulphate content upto 250 g/l and decreased when the concentration was further raised. The effect of electroactive species on SiC particles content present in the coating is shown in Figure 3. Electroactive species to adsorb on the strengthening particles is not sufficient at low nickel sulphate concentration, reducing their entrapment in the coating. Rapid reduction of nickel ions occurs at higher nickel sulphate concentration before the particles can be properly codeposited, leading to low adsorption rate. High ionic strength of plating solution has also been reported to promote particle agglomeration causing the formation of deposits with defects and low particle content [26].

Chemical additives are added in the bath to serve different functions. These functions include controlling the final appearance of the deposit, altering crystal growth kinetics, and influencing the zeta potential of the second-phase particles. The addition of SDS (sodium dodecyl sulph‐

**Figure 4.** SiC volume percent for various nickel sulphate concentrations in the bath [26]

onate) and saccharine promote smoothening of the Ni coating surfaces [8]. The coatings fabricated from solutions containing the additives exhibited smaller grain sizes. The addition of ethanol (25 and 50 vol.%) into nickel bath solution induced grain refinement resulting in finer microstructure [32]. The effect of ethanol on the crystal size of nickel and nanocomposite coatings is shown in Table 4. The truncated pyramidal crystals were modified to globular grains as a result of the presence of ethanol in the electrolyte. The crystal size of 85 nm of the Ni–Al2O3 nanocomposite coating produced from additive free bath was reduced to 25 nm when ethanol was added.


**Table 4.** Effect of ethanol on the crystal size of pure nickel and Ni–Al2O3 nanocomposite coatings

#### **5.5. Stirring speed**

**Particle size SiC in coating**

214 Electrodeposition of Composite Materials

particles [28]

**5.4. Bath composition**

low particle content [26].

**(vol%)**

10 nm 15.2 2.89×1017 1.51×101 50 nm 22.02 3.37×1015 6.67×101 5 nm 55.24 8.44×109 4.91×103

values at particle loading of 10 g/l and current density of 1 A/dm2

conditions of pH=2, particle loading of 100 g/l and current density of 5 A/dm2

**Table 3.** Volume percent and number density of SiC particles in composite coatings with the distance between

Electrolyte ionic strength, bath additives, and pH are some of the operating parameters that affect the zeta potential of second-phase particles as explained before. The pH of the bath is a function of bath composition which includes the electrolyte and chemical additives. Strong acid or bases can be added to adjust the pH to be acidic or alkaline. Particles have different zeta potential at different pH depending on their nature. Therefore, various ceramic particles will have more positive or negative charge on either acidic or alkaline conditions. Titania particles were found to exhibit negative zeta potential in acidic solution of pH= 4.3 and positive zeta potential at alkaline conditions with pH of 9.5 [30]. A maximum particle content of 4.3 wt. % was achieved from alkaline baths while 3.3 wt.% resulted from acidic solutions. This shows that the conducible co-deposition of TiO2 particles with nickel is achievable under high pH

to 5) in particle incorporation with increasing pH values was noticed by ref. [22]. At plating

8 vol.% of TiO2 incorporation was achieved. However, the authors did not study further the behaviour of the particle incorporation in alkaline solutions. The results obtained by the different authors show that particle incorporation depends on several factors than pH of the solution. The electrolyte is the carrier of electroactive species which are responsible for the formation of ionic cloud around the particles to enable their transportation to the cathode for entrapment in the matrix [31]. Variation of nickel sulphate in the bath from 200, 250, and 300 g/l showed significant effect on the incorporation rate of reinforcement particles as reported by ref. [28]. The volume of incorporated SiC particles increased with increasing nickel sulphate content upto 250 g/l and decreased when the concentration was further raised. The effect of electroactive species on SiC particles content present in the coating is shown in Figure 3. Electroactive species to adsorb on the strengthening particles is not sufficient at low nickel sulphate concentration, reducing their entrapment in the coating. Rapid reduction of nickel ions occurs at higher nickel sulphate concentration before the particles can be properly codeposited, leading to low adsorption rate. High ionic strength of plating solution has also been reported to promote particle agglomeration causing the formation of deposits with defects and

Chemical additives are added in the bath to serve different functions. These functions include controlling the final appearance of the deposit, altering crystal growth kinetics, and influencing the zeta potential of the second-phase particles. The addition of SDS (sodium dodecyl sulph‐

**Number density (particles cm2**

**)**

**Distant between Particles (nm)**

. A decreasing trend (from 2

, a maximum of

Stirring speed is one of the important parameters that control the mechanism of particle incorporation. Mechanical inclusion of particles into the coating forms part of the three processes that are involved in entrapment of particles into a metal matrix [31]. Mechanical agitation aids in the transportation of particles to the cathode to be readily available for adsorption [33]. However, very high or low agitation can have adverse effect on the incorpo‐ ration rate of the particles. Low stirring speeds offer low energy to break the agglomerate to fine particles and hence reduce their availability for incorporation. High stirring speeds are associated with high impinging velocity of the particles to the cathode and not giving enough retention time for the particles to be adsorbed on the cathode [27]. Particle size has an effect on the required stirring speed since it is easier to keep coarse particles in suspension in solution than the finer ones. Nanoparticles easily agglomerate when they are added in the plating bath. Therefore, fabrication of nanocomposite coatings requires higher stirring speeds than their composite counterparts. Other forms of agitation have been used in literature and these include the use of ultrasonic energy to keep particles in suspension, enhance mass transport, and reduce diffusion layer thickness [34]. A schematic diagram of electrodeposition cell assisted with ultrasonic energy is shown in Figure 4. The use of ultrasound also helps to modify the surface morphologies of coatings fabricated with conventional DC plating technique [10].

**Figure 5.** Typical electrodeposition cell assisted with ultrasound energy [34]

#### **5.6. Time and temperature**

Particles require time to remain around the cathode to increase the chances of their incorpo‐ ration. Ref. [35] reported that longer deposition time allows for the formation of thicker and compact coatings with improved microhardness. The highest microhardness values were achieved at a deposition time of 14 min, and beyond this time no adherent spherical globules were formed. Temperature also plays a significant role in co-deposition process. Increase in temperature enhances reaction kinetics, leading to more nickel ions to be transferred to the cathode. The content of SiC particles has been found to be a function of temperature by ref. [36]. Increasing temperature up to 50o C increased the content of the particles in the coating. Above optimal conditions, a decrease in incorporation rate was observed. Thermodynamic movement of ions improves with temperature and the particulates' kinetic energy also increases. This causes rapid deposition, which poses a risk on the control of crystal growth and uniformity in distribution of particles within the matrix. According to ref. [37], increase in temperature reduces adsorpability of the particles and hence decrease in overpotential cathode and electric field. Ref. [35] obtained similar results when Fe2O3 nanoparticles were co-deposited with nickel.
