**1. Introduction**

Metal matrix composite (MMC) coatings engineered using an electrodeposition method are examined in this chapter. The electrodeposition of a composite involves the electrolysis of plating baths where nano- to micro- sized particles are dispersed and various quantities of the particles become imbedded within the plated metal matrix, providing special properties to the coating (Figure 1) [1]. The process of particle incorporation into metal coatings can be simpli‐ fied into four steps: (1) particles dispersed in solution form a surface charge; (2) from the bulk solution, there is mass transport of the particles to the surface of the electrode typically through convection; (3) there is interaction between the particle and the electrode; (4) the particles become trapped within the growing metallic film [2]. The earliest example of electrodeposited

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composites dates back to the 1920s where Cu–graphite coatings were developed for automo‐ tive bearings [3, 4]. Enhanced corrosion, tribological, and mechanical properties were the main research focus of the automotive and aerospace industries in the 1970s–1990s, leading to significant technological advancements. In the early 2000s, some of the focus started to shift to electrical components and electronic devices [5–10]. As the accessibility of nanoparticles continues to rise, the interest in reduced cost and low-temperature electrodeposited MMCs continues to escalate [2].

**Figure 1.** Schematic displaying the growth of MMCs by electrodeposition.

Coatings can be produced by using several different approaches, and electrodeposition remains a prominent technique to produce novel materials for science and engineering applications. Electrodeposition also offers low cost, convenience, ability to work at low temperatures, and the ease of application to complex geometries [11, 12]. Other advantages for using electrodeposition include the ability to quickly scale to an industrial setting, uniform coating of large samples, and accurate control of the coating thickness [2]. Other methods for producing MMCs include commercial pressing [13], laser cladding [14], hot pressing [15], plasma transferred-arc surfacing [16], stir casting [17], diffusion bonding [18], powder metallurgy [19], and chemical vapor deposition [20]. Each of these methods has different advantages and disadvantages. However, some of the drawbacks include production at high temperatures or under vacuum, difficulty in controlling the thickness, and cost.

MMCs coatings combine the advantageous properties of each individual material together which is not possible with the non-composite metal films [21, 22]. An extensive amount of work has gone into examining copper-based MMCs [23–27] and nickel-based MMCs [28–32] since individual metals can exhibit a limited range of properties. An important route to improving the properties of individual metals is the deposition of alloys such as Zn–Ni, Ni– Mo, and Cu–Ni [33–35]. Successful incorporation of particles into the metal matrix by electro‐ codeposition relies on many different parameters, including the composition of the electrolyte, pH, current density, and properties of the particles [36]. Incorporating particles such as TiO2, SiC, Al2O3, carbon fibers, Ni, Al, and Cr into the Cu–Ni matrix of different coatings enhances the electrical, mechanical, and corrosion properties of the coatings [5–9, 21, 37–40].

Copper alloys, such as Cu–Ni, have been studied because of their good electrical and thermal properties, machinability, and resistance to corrosion [41–44]. Copper is relatively soft and needs to be alloyed with another metal, such as nickel, to increase the hardness of the material [43, 45]. Also, with the addition of Ni into the Cu matrix during electrodeposition, it is possible to grow films with minimal strain due to both Cu and Ni having face-centered cubic crystal structures and similar lattice parameters [46]. The electroplating of copper alloy films has an instrumental role in many different industry-related applications. For instance, electroplating has found a niche in microelectromechanical systems (MEMS) because the process can deposit different alloys onto depressed and oddly shaped geometric substrates [5, 6, 10, 45]. Cu–Ni coatings have been evaluated for use as inert anodes in the fabrication of aluminum because of their enhanced electrical and thermal conductivity [8, 9]. In marine environments, copper alloys are used to defend against biofouling of materials by inhibiting microbial-induced corrosion (MIC) [47–50].

composites dates back to the 1920s where Cu–graphite coatings were developed for automo‐ tive bearings [3, 4]. Enhanced corrosion, tribological, and mechanical properties were the main research focus of the automotive and aerospace industries in the 1970s–1990s, leading to significant technological advancements. In the early 2000s, some of the focus started to shift to electrical components and electronic devices [5–10]. As the accessibility of nanoparticles continues to rise, the interest in reduced cost and low-temperature electrodeposited MMCs

Coatings can be produced by using several different approaches, and electrodeposition remains a prominent technique to produce novel materials for science and engineering applications. Electrodeposition also offers low cost, convenience, ability to work at low temperatures, and the ease of application to complex geometries [11, 12]. Other advantages for using electrodeposition include the ability to quickly scale to an industrial setting, uniform coating of large samples, and accurate control of the coating thickness [2]. Other methods for producing MMCs include commercial pressing [13], laser cladding [14], hot pressing [15], plasma transferred-arc surfacing [16], stir casting [17], diffusion bonding [18], powder metallurgy [19], and chemical vapor deposition [20]. Each of these methods has different advantages and disadvantages. However, some of the drawbacks include production at high

MMCs coatings combine the advantageous properties of each individual material together which is not possible with the non-composite metal films [21, 22]. An extensive amount of work has gone into examining copper-based MMCs [23–27] and nickel-based MMCs [28–32] since individual metals can exhibit a limited range of properties. An important route to improving the properties of individual metals is the deposition of alloys such as Zn–Ni, Ni– Mo, and Cu–Ni [33–35]. Successful incorporation of particles into the metal matrix by electro‐ codeposition relies on many different parameters, including the composition of the electrolyte, pH, current density, and properties of the particles [36]. Incorporating particles such as TiO2, SiC, Al2O3, carbon fibers, Ni, Al, and Cr into the Cu–Ni matrix of different coatings enhances

temperatures or under vacuum, difficulty in controlling the thickness, and cost.

the electrical, mechanical, and corrosion properties of the coatings [5–9, 21, 37–40].

Copper alloys, such as Cu–Ni, have been studied because of their good electrical and thermal properties, machinability, and resistance to corrosion [41–44]. Copper is relatively soft and

continues to escalate [2].

84 Electrodeposition of Composite Materials

**Figure 1.** Schematic displaying the growth of MMCs by electrodeposition.

Understanding the incorporation of particles into these alloys requires mathematical model‐ ing.Earlymodelsdatingbacktothe1960s fromWilliamsandMartin[51]proposedthatparticles were transported to the cathode surface via a convection transport mechanism facilitated by stirring the plating bath. Brandes and Goldthorpe [52] hypothesized that entrapment by mechanical means was not the only factor at play and decided that an electrostatic force must be aiding the inclusion of the particles into the metal matrix. In 1972, Guglielmi [53] became the first to propose a mathematical model that explained the inclusion of particles into the metal matrix. The model followed a simple two part approach: (a) the particles slowly move toward the surface of the cathode and adsorb very loosely and (b) then the particles become securely adsorbed by shedding their ionic cloud. The derived model equations are [40, 53]:

$$\frac{\mathbb{C}\left(1-\alpha\right)}{\alpha} = \frac{\mathsf{W}\vec{i}\_{\circ}}{nFd\upsilon\_{\circ}}e^{(A-B)\eta}\left(\frac{1}{k} + \mathsf{C}\right) \tag{1}$$

$$\dot{\mathbf{u}} = \left(\mathbf{1} - \mathcal{B}\right) \dot{\mathbf{i}}\_o e^{A\eta} \tag{2}$$

where *C* is the vol.% of particles in the electrolyte solution and *α* is the vol.% of the particle incorporated in the composite film. *W* corresponds to the atomic weight of the metal in the coating, F is Faraday's constant, *d* is the density of the metallic coating, and *n* is the valence of the metallic coating. *η* relates to the overpotential, *i* is the current density, and *io* is the current density from the metallic coating. The υ term is a constant from the deposition of the particle, A is a constant from the deposition of the metal, and B is a constant from the particle inclusion. The ϑ relates to the coverage of the surface by the particle that is incorporated into the metal coating and *k* is the adsorption coefficient. Although the model has some holes, such as not taking into account the mass transport of the metal ions or the particles and the nature of the particle or the shape, it is still one of the most widely used models to date. In 1987, Celis et al. [54] hypothesized a nnew five part model, which is built on the idea of Guglielmi's two part model. The drawback of this model is that factors need to be created that are specific to each individual system. As late as 2002, Bercot's group [55] proposed an addition to Guglielmi's original model that included a polynomial for the purpose of correcting for different effects presented by adsorption and flow.

A survey of the literature for the incorporation of particles into Cu–Ni coatings is shown in Table 1. The composite coatings described in Table 1 are for electrodeposited processing only. Other Cu–Ni composites have been made using different techniques, but fall outside the scope of this chapter.

This chapter will cover the electrodeposition of Cu–Ni alloys onto steels and other substrates to improve corrosion resistance and mechanical properties. The influence of the deposition parameters will be covered as well as the electrodeposition mechanism. The resulting me‐ chanical and corrosion properties will also be discussed in the chapter.



**Table 1.** A summary of the work produced on electrodeposited Cu–Ni composite coatings.
