**3. Cu–Ni MMCs for Enhanced Mechanical Properties**

Particle incorporation can be used to improve mechanical properties for MMC coatings over that of the pure metal matrix. The Cu–Ni matrix has been incorporated with Ni particles, TiO2, Al2O3, SiC, carbon fibers, and montmorillonite to increase the hardness, wear resistance, shear adhesion, and tensile strength [21, 37–39, 60].

Chrobak et al. [21] examined the electrodeposited copper incorporated with Ni powder (~100 nm) at varying current densities to examine the effects to Young's modulus. They experiment‐ ed with current densities ranging from 1–100 mAcm–2 and found that the concentration of Ni in the coating increases linearly with current density but the thickness of the coating decreases because the transport of Cu to the electrode surface is suppressed by the Ni particles. They also discovered that adding 25 vol.% of glycerol to the bath helped reduce agglomeration and decrease the grain size of the MMC coatings. The Young's modulus was found to decrease when the Ni concentration was increasing. This trend was even more drastic as the current density was increased because an increased current density leads to higher concentrations of Ni [21].

Fawzy et al. [37] examined how the current density, concentration of ceramic particles in the electroplating solution, and pH of the plating bath affected the hardness for Cu–Ni, Cu–Ni– αAl2O3, and Cu–Ni–TiO2 composite coatings. By increasing the current density from 0.33 to 1.33 Adm–2 for the Cu–Ni–αAl2O3 and Cu–Ni–TiO2 depositions, an increase in the Ni percent‐ age in the coating and hardness was observed versus the pure Cu–Ni coating, but the per‐ centage of the ceramic particles in the coating decreased. Increasing the pH of the plating solutions from 2.5 to 4.05 lead to an escalation in the Ni percentage in the film, loading percentage of the ceramic particles, and hardness. The hardness increased as the Al2O3 and TiO2 increased in the plating solution from 170 to 248 kgf mm–2 for the addition of αAl2O3 particles with no effect to the Ni concentration in the coating, whereas the addition of TiO2 particles produced an increase from 170 to 231 kgf mm–2 and helped to increase the Ni percentage in the coating. The optimal coating parameters were determined to be 1.33 A dm– <sup>2</sup> for the current density, at pH 4.05, and incorporating 20 g dm–3 of the ceramic particles into the plating bath [37].

Hashemi et al. [38] studied the electrodeposition of Cu–Ni–W incorporated with SiC nano‐ particles. They investigated the effects of different concentrations of SiC ranging from 0 to 25 g/L, stir rates ranging from 100 to 600 rpm, and the change in current density from 10 to 50 mAcm–2. They optimized the plating conditions to obtain the best wear protection and hardness for the coatings. With the change in concentration from 0 to 25 g/L SiC in the plating solution, 15 g/L was found to have the highest incorporation of SiC into the coating. Also, the 15 g/L incorporation of SiC into the Cu–Ni matrix increased hardness and had the lowest weight loss factor and friction factor from the wear results. It was hypothesized that any amount over 15 g/L caused agglomeration of the SiC particles, which produced observable voids on the surface of the coating in the SEM pictures. A 400 rpm stir rate and the 20 mAcm– 2 current density proved to be the optimal conditions for the best wear resistance and hardness. The SEM of the coating incorporated with 15 g/L of SiC, stirred at 400 rpm, and electrodepos‐ ited with a current density of 20 mAcm–2 showed the least amount of visible wear on the surface after testing [38].

**3. Cu–Ni MMCs for Enhanced Mechanical Properties**

shear adhesion, and tensile strength [21, 37–39, 60].

92 Electrodeposition of Composite Materials

Ni [21].

the plating bath [37].

2

Particle incorporation can be used to improve mechanical properties for MMC coatings over that of the pure metal matrix. The Cu–Ni matrix has been incorporated with Ni particles, TiO2, Al2O3, SiC, carbon fibers, and montmorillonite to increase the hardness, wear resistance,

Chrobak et al. [21] examined the electrodeposited copper incorporated with Ni powder (~100 nm) at varying current densities to examine the effects to Young's modulus. They experiment‐ ed with current densities ranging from 1–100 mAcm–2 and found that the concentration of Ni in the coating increases linearly with current density but the thickness of the coating decreases because the transport of Cu to the electrode surface is suppressed by the Ni particles. They also discovered that adding 25 vol.% of glycerol to the bath helped reduce agglomeration and decrease the grain size of the MMC coatings. The Young's modulus was found to decrease when the Ni concentration was increasing. This trend was even more drastic as the current density was increased because an increased current density leads to higher concentrations of

Fawzy et al. [37] examined how the current density, concentration of ceramic particles in the electroplating solution, and pH of the plating bath affected the hardness for Cu–Ni, Cu–Ni– αAl2O3, and Cu–Ni–TiO2 composite coatings. By increasing the current density from 0.33 to 1.33 Adm–2 for the Cu–Ni–αAl2O3 and Cu–Ni–TiO2 depositions, an increase in the Ni percent‐ age in the coating and hardness was observed versus the pure Cu–Ni coating, but the per‐ centage of the ceramic particles in the coating decreased. Increasing the pH of the plating solutions from 2.5 to 4.05 lead to an escalation in the Ni percentage in the film, loading percentage of the ceramic particles, and hardness. The hardness increased as the Al2O3 and TiO2 increased in the plating solution from 170 to 248 kgf mm–2 for the addition of αAl2O3 particles with no effect to the Ni concentration in the coating, whereas the addition of TiO2 particles produced an increase from 170 to 231 kgf mm–2 and helped to increase the Ni percentage in the coating. The optimal coating parameters were determined to be 1.33 A dm– <sup>2</sup> for the current density, at pH 4.05, and incorporating 20 g dm–3 of the ceramic particles into

Hashemi et al. [38] studied the electrodeposition of Cu–Ni–W incorporated with SiC nano‐ particles. They investigated the effects of different concentrations of SiC ranging from 0 to 25 g/L, stir rates ranging from 100 to 600 rpm, and the change in current density from 10 to 50 mAcm–2. They optimized the plating conditions to obtain the best wear protection and hardness for the coatings. With the change in concentration from 0 to 25 g/L SiC in the plating solution, 15 g/L was found to have the highest incorporation of SiC into the coating. Also, the 15 g/L incorporation of SiC into the Cu–Ni matrix increased hardness and had the lowest weight loss factor and friction factor from the wear results. It was hypothesized that any amount over 15 g/L caused agglomeration of the SiC particles, which produced observable voids on the surface of the coating in the SEM pictures. A 400 rpm stir rate and the 20 mAcm–

 current density proved to be the optimal conditions for the best wear resistance and hardness. The SEM of the coating incorporated with 15 g/L of SiC, stirred at 400 rpm, and electrodepos‐

Wan et al. [39] developed a continuous three step deposition process to produce Cu, Cu–Fe, and Cu–Ni reinforced carbon fiber (6–8 μm in diameter) composites. Cu–C, Cu–Fe–C, and Cu– Ni–C composites display similar strength vs. temperature graphs to each other, but the decreasing trend seen in the tensile strength is controlled by a different mechanism. A decrease in the tensile strength for the Cu–C composite was due to interfacial bonding at the carbon fiber interface. Interfacial debonding was found to be absent for Cu–Ni–C and Cu–Fe–C composites because its larger interfacial bonding strength is due to a diffusion reaction and a chemical reaction at the interface of the fiber–metal matrix. The maximum tensile strength value was obtained with the addition of Cu–Ni onto the carbon fiber interface, but the optimal tensile strength value was not found to be proportional to the interfacial bonding strength, whereas the highest bonding strength was found for the Cu–Fe–C composite [39].

Our group studied electrodeposited 70–30 Cu–Ni coatings incorporated with a platelet-clay known as montmorillonite (MMT) from a citrate bath. Layered silicates have several advan‐ tageous properties to be utilized for composites, such as a high surface area, chemical inertness, resistance to extreme temperatures, and resistance to pH changes. The inclusion of layered silicates, such as MMT, has shown an increase in hardness, adhesion, and corrosion resistance in Ni and Ni–Mo coatings [56–59]. MMT has a 2:1 layered structure, with two layers of the silicon tetrahedral sandwiching one layer of an aluminum octahedral. Seen in Figure 8, MMT is a hydrous aluminum silicate with the formula (Na,Ca)(Al, Mg)6(Si4O10)3-(OH)6⋅nH2O and measures 1 nm in height and 1–2 microns in width [12]. The Al3+ and Si4+ locations can be replaced by lower valent cations, causing the montmorillonite structure to have an excess of electrons. The negative charge is compensated through loosely held cations from the associated water. Within aqueous solutions, MMT can be completely delaminated and incorporated into other materials, forming continuous, crack-free films, which is a necessary requirement of corrosion resistant coatings [12]. For this work, a solution of MMT was stirred vigorously for 24–48 h to exfoliate the layered silicates. Seen in Table 2, the zeta potential of the plating solution with the dispersed MMT was also evaluated to check the stability of the particles in solution. When dealing with the electrocodeposition of a MMC, understanding the particle stability in the colloidal plating solution is vital because the properties of composites increase significantly with the preferential inclusion of individual particles [2].The exfoliated MMT is stable in solution and easily dispersed into aqueous solutions, while the non-exfoliated MMT precipitates. For stable nanoparticle suspensions, the ideal zeta potential would be a value greater than ±25 mV. The Cu–Ni–MMT plating solutions were around –19 to –20 mV. The adsorption of the Ni and Cu at the surface of the MMT platelet moves the zeta potential toward a more positive value and helps to increase the particle size, which slightly decreases the electrostatic stabilization of the dispersion compared to pure MMT solution. However, the plating solutions still had enough stability for deposition purposes to stay suspended in solution throughout the deposition cycle. Once exfoliated and freely dispersed throughout the electrolytic bath, the MMT platelets can slowly settle down onto the surface of the electrode and be incorporated into the forming alloy coating structure as shown in Figure 1.

**Figure 8.** The structure and thickness of montmorillonite [12].


**Table 2.** Zeta potentials for pure MMT and 70–30 Cu–Ni–MMT plating solutions [60].

A copper–nickel alloy (70–30 ratio) was electrochemically deposited from a citrate bath and compared to a composite coating incorporated with 0.05–0.2% MMT to study the effects of the mechanical properties.The adhesion shear strength (Figure 9) and the hardness (Figure 10) for the MMC coatings were investigated. The adhesion shear strength (measured by resistance to knife movement) (Figure 9) was evaluated with MMT amounts in solution ranging from 0 to 0.2% and all of the nanocomposite coatings surpass the strength of the pure Cu–Ni matrix. The shear adhesion tests were measured using the XYZTEC instrument paired with a 2-mm wide knife. The knife was placed at 5 μm above the substrate-coating interface and moved 2 mm horizontally at a velocity 150 (μm/s) through the coatings. With the addition of MMT into the Cu–Ni coating, a greater resistance to the knife movement was observed. The nearly 300% increase from pure Cu–Ni to Cu–Ni–0.05% MMT displays the value that the platelets can have at low loading values.As the MMT in solution increases to 0.1–0.2%, the mechanical resistance of the coating begins to decrease. This indicates that the increased amount of platelets incorporated into the coating leads to more substrate–platelet contact which would reduce the effective area of the matrix–substrate contact and leads to a reduction in the adhesion strength of the coatings. The microhardness test (Figure 10) revealed an increase of about 25% for the Cu–Ni coatings incorporated with MMT versus the pure Cu–Ni coating [60].

**Figure 9.** The adhesion shear strength of different coating layers with varying amounts of MMT in the electroplating solution [60].

**Figure 8.** The structure and thickness of montmorillonite [12].

94 Electrodeposition of Composite Materials

**Solutions Zeta Potential (mV)**

0.05% MMT –38.8 0.1% MMT –39.2 0.2% MMT –38.7 **Cu–Ni–0.05% MMT** –20.2 **Cu–Ni–0.1% MMT** –19.5 **Cu–Ni–0.2% MMT** –19.2

A copper–nickel alloy (70–30 ratio) was electrochemically deposited from a citrate bath and compared to a composite coating incorporated with 0.05–0.2% MMT to study the effects of the mechanical properties.The adhesion shear strength (Figure 9) and the hardness (Figure 10) for the MMC coatings were investigated. The adhesion shear strength (measured by resistance to knife movement) (Figure 9) was evaluated with MMT amounts in solution ranging from 0 to 0.2% and all of the nanocomposite coatings surpass the strength of the pure Cu–Ni matrix. The shear adhesion tests were measured using the XYZTEC instrument paired with a 2-mm wide knife. The knife was placed at 5 μm above the substrate-coating interface and moved 2 mm horizontally at a velocity 150 (μm/s) through the coatings. With the addition of MMT into the Cu–Ni coating, a greater resistance to the knife movement was observed. The nearly 300% increase from pure Cu–Ni to Cu–Ni–0.05% MMT displays the value that the platelets can have at low loading values.As the MMT in solution increases to 0.1–0.2%, the mechanical resistance of the coating begins to decrease. This indicates that the increased amount of platelets incorporated into the coating leads to more substrate–platelet contact which would reduce the effective area of the matrix–substrate contact and leads to a reduction in the adhesion strength of the coatings. The microhardness test (Figure 10) revealed an increase of about 25% for the

**Table 2.** Zeta potentials for pure MMT and 70–30 Cu–Ni–MMT plating solutions [60].

Cu–Ni coatings incorporated with MMT versus the pure Cu–Ni coating [60].

**Figure 10.** Vickers microhardness for the 70–30 Cu–Ni Coatings incorporated with MMT [60].

#### **4. Cu–Ni MMCs in Electrochemistry and Improved Corrosion Protection**

Understanding the relationship between the electrochemical deposition parameters and the resulting corrosion properties is of great importance for creating optimal coatings that will endure harsh environments. MMC coatings that reduce the rate of corrosion at a lower cost have been extensively studied. Exposing films to an unfavorable environment accelerates the degradation of the coating, which can lead to many different types of corrosion phenomena [61, 62]. The rate of corrosion can be slowed using five universal approaches which include the choice of materials, chemical inhibitors, altercations in the environment, cathodic protec‐ tion, or coatings [63]. Al particles and MMT were incorporated into the Cu–Ni matrix to understand how magnetic particles affect the metal matrix and MMT to observe how platelets affect the corrosion resistance of the Cu–Ni coating [40, 60].

Cui et al. [40] successfully codeposited Cu–Ni–Al MMCs and noted that high amounts of Al (~3 μm) particles, 29 vol. %, could be deposited into the Cu–Ni coating matrix. They investi‐ gated whether the conductive Al particles would behave similar to inert particle codeposition according to the Guglielmi's model. Adding the conductive aluminum particles into the Cu– Ni matrix caused the polarization curve to shift to more negative potentials, which was credited to the non-active surface of the aluminum metal particles during the deposition of Cu–Ni following a similar codeposition path for inert particles defined by Guglielmi. The parameters used in Guglielmi's model for the codeposition of non-conductive inert particles can also model the deposition of charged particles presented in this research [40].

**Figure 11.** Tafel plots for the 70–30 Cu–Ni coatings with and without MMT after being submerged in a 3.5 % NaCl solution for two weeks [60].

In addition to the mechanical properties, our group studied the corrosion behaviors of the 70– 30 Cu–Ni–MMT coatings by the use of Tafel polarization and electrochemical impedance spectroscopy (EIS). The corrosion of 70–30 Cu–Ni and 70–30 Cu–Ni–MMT composite coatings were evaluated using potentiodynamic polarization, as seen in Figure 11. The corrosion parameters were measured after immersing the 70–30 Cu–Ni coatings for two weeks in 3.5% NaCl solution at 25°C. The *E*corr and *I*corr correlation for the Cu–Ni–MMT coatings can be seen in Figure 12. Figure 12 shows *E*corr shifting to more positive potentials and the *I*corr shifting to lower current values leading to the Cu–Ni–0.2% MMT having the best corrosion properties. Nyquist plots (Figure 13) of pure Cu–Ni and Cu–Ni–0.2% MMT composite coatings after 14 days immersion in 3.5 % NaCl at 25°C showed the diameter of the depressed uncompleted semicircles is larger in case of Cu–Ni–MMT compared to pure Cu–Ni, which displays increased stability of the passive film in the case of Cu–Ni–MMT compared to that of pure Cu–Ni coating. The equivalent circuit parameters of the fitting procedure showed an increase in *Rp* from 2.87 kΩcm<sup>2</sup> (pure Cu–Ni) to 13.77 kΩcm<sup>2</sup> (Cu–Ni–0.2% MMT), as seen in Table 3. The calculated parameters show higher resistance for the inner layer in case of Cu–Ni–MMT composite coating compared to that of pure Cu–Ni; this resistance increases as the MMT content in the metallic matrix increases, which is consistent with the data obtained from potentiodynamic polarization, and also confirms that embedding of layered silicate particles into the Cu–Ni metallic matrix increases its corrosion resistance [60].

**Figure 12.** *Ecorr* and *icorr* of 70–30 Cu–Ni coatings incorporated with MMT [60].

degradation of the coating, which can lead to many different types of corrosion phenomena [61, 62]. The rate of corrosion can be slowed using five universal approaches which include the choice of materials, chemical inhibitors, altercations in the environment, cathodic protec‐ tion, or coatings [63]. Al particles and MMT were incorporated into the Cu–Ni matrix to understand how magnetic particles affect the metal matrix and MMT to observe how platelets

Cui et al. [40] successfully codeposited Cu–Ni–Al MMCs and noted that high amounts of Al (~3 μm) particles, 29 vol. %, could be deposited into the Cu–Ni coating matrix. They investi‐ gated whether the conductive Al particles would behave similar to inert particle codeposition according to the Guglielmi's model. Adding the conductive aluminum particles into the Cu– Ni matrix caused the polarization curve to shift to more negative potentials, which was credited to the non-active surface of the aluminum metal particles during the deposition of Cu–Ni following a similar codeposition path for inert particles defined by Guglielmi. The parameters used in Guglielmi's model for the codeposition of non-conductive inert particles

**Figure 11.** Tafel plots for the 70–30 Cu–Ni coatings with and without MMT after being submerged in a 3.5 % NaCl

In addition to the mechanical properties, our group studied the corrosion behaviors of the 70– 30 Cu–Ni–MMT coatings by the use of Tafel polarization and electrochemical impedance spectroscopy (EIS). The corrosion of 70–30 Cu–Ni and 70–30 Cu–Ni–MMT composite coatings were evaluated using potentiodynamic polarization, as seen in Figure 11. The corrosion parameters were measured after immersing the 70–30 Cu–Ni coatings for two weeks in 3.5% NaCl solution at 25°C. The *E*corr and *I*corr correlation for the Cu–Ni–MMT coatings can be seen in Figure 12. Figure 12 shows *E*corr shifting to more positive potentials and the *I*corr shifting to lower current values leading to the Cu–Ni–0.2% MMT having the best corrosion properties. Nyquist plots (Figure 13) of pure Cu–Ni and Cu–Ni–0.2% MMT composite coatings after 14 days immersion in 3.5 % NaCl at 25°C showed the diameter of the depressed uncompleted

can also model the deposition of charged particles presented in this research [40].

affect the corrosion resistance of the Cu–Ni coating [40, 60].

96 Electrodeposition of Composite Materials

solution for two weeks [60].

**Figure 13.** Nyquist impedance plots of pure Cu–Ni (A) and Cu–Ni–0.2% MMT (B) after being submerged in a 3.5% NaCl solution for two weeks [60].


**Table 3.** The equivalent circuit parameters of Cu–Ni and Cu–Ni–MMT composite coatings after two weeks submersion in a 3.5% NaCl solution [60].
