*2.1.3. Current density*

**2.1. Parameters affecting the codeposition of particles in metallic matrix**

their suspension, agglomeration, and precipitation [27,28].

which could lead to uneven distribution of particles in the coatings.

The particle concentration in the electrolyte may affect the deposition process, changing the ratio metal/particle in the coating and its grain size and causing variations on the coating properties. The incorporation rate per volume of the particles in the deposit is an increasing function of the concentration of particles in the electrolyte suspension [23,41] and is a param‐ eter often used to control the amount of particles in the coating [27,28]. However, as shown in several studies, it is evident that the amount of particles in the deposit does not grow infinitely but reaches a limit value [5,27,42], which depends on the deposition conditions. The concen‐ tration of particles in the electrolyte can also result in problems relating to the homogeneity of

Composite coatings of Zn-SiO2 were produced in the presence of N,N-dimethyldodecylamine and there was a direct trend in the increase of the amount of incorporated particles up to 100 g L-1 of silica particles in the solution [6]. Beyond this concentration, however, the increasing incorporation response is oscillating. This behavior was related to the fact that any concentra‐ tion beyond 100 g L-1 of particles might be sufficiently high to induce localized agglomeration,

The TiO2 particle concentration (5.0, 10.0, and 15.0 g L-1) in the electrolytic bath also influenced the content of these particles in a zinc matrix composite coating [43]. Although the increase of particle concentration in the bath elevated the content of particles in the coatings, concentra‐ tions higher than 10.0 g L-1 caused a decrease in the codeposited particle content in the metallic matrix. This effect was explained by the agglomeration of particles in the coating due to their

The dependence of TiO2 nanoparticles (1.0, 1.6, 10.0, and 16.0 g L-1) added to different concen‐ trations of Zn(II) electrolyte (0.5, 0.3, and 0.1 mol L-1) on the grain size of the Zn matrix was also investigated [44]. It was observed that the grain size of the metal matrix decreases with the increase of the nanoparticles added to the bath, which was related to changes in the nucleation and growth processes of zinc crystals due to the presence of these particles. Similar results were obtained for nickel deposition in the presence of SiC nanoparticles [44] and for copper codeposition with SiO2 nanoparticles [45]. These results are in agreement with the literature, which relates this grain refining effect to the nanoparticle abilities of providing more nucleation sites and, consequently, decreasing the velocity of the crystal growth process [45].

Several works report the influence of the particle size in suspension on the homogeneity of the coating, its microstructure and morphology, and on the particle incorporation in the metal matrix [37,46,47]. The number of particles incorporated per area is associated with the selectivity related to the particle size in suspension, as observed for the NiP–SiC coatings, where the amount of SiC particles in the coating increased due to the decreased size of the SiC particles in the electrolyte [27,28]. However, it is not a consensus in the literature, as there are

*2.1.1. Particle concentration in solution*

152 Electrodeposition of Composite Materials

poor wettability [43].

*2.1.2. Particle size in the suspension*

Besides the particle concentration of the suspension, the applied current density is certainly the parameter most studied by several authors [37,38,47,49] and there are evidences concerning its effect on the particle incorporation. The literature [5] reports that the particle incorporation in the coating is a function of the current density and that this effect can be divided in four current density regions. Initially, there is a region where the particle incorporation increases rapidly reaching a maximum followed by the second region where a marked decrease in the process occurs. Then, a third region takes place, where the process is fairly constant, followed by another fall in the particle content in the current density region where the metal reduction is limited by mass transfer. It seems that an optimum range of current density is necessary to favor the codeposition of the particles. This range, however, depends on the studied particle and on other deposition parameters, such as bath composition or stirring.

For example, the greatest amount of α-Al2O3 nanoparticles was incorporated to a copper matrix at low current density values (between 1 and 2 A dm-2), whereas, outside this range, the content of particles in the coating significantly decreased [11]. This dependence between the content of nanoparticles in the coating and the current density is explained by the mechanism by which the particles are captured. The increase in the particle content in the coatings occurs in the region where the reduction of the metallic ions is under charge transfer control and where the reduction of adsorbed cations on alumina is the determining step of the deposition rate. As the reduction of metal ions occurs under diffusion control at current densities higher than 2 A dm-2 (the hydrodynamic conditions are not mentioned), the codeposition of alumina particles gradually decreases with increasing current density [11].

It was also observed that increasing current density caused a decrease in the initial rate of incorporation of SiO2 particles in the zinc matrix; however, at current densities approximately 30 A dm-2, an increase in SiO2 deposition rate was noted, mainly for the particles with the highest size (2 μm) [6]. Moreover, the regions in which the particle incorporation increased or decreased markedly with current density were sensitive to particle size. The authors suggested that the deposition process was controlled by mass transfer until the maximum value of current density, where the codeposition process was favored. Otherwise, the process was controlled by the particle adsorption on the substrate, and a further increase in current density resulted in the rapid deposition of the metallic matrix and less particles were included in the coating [6]. Similar results were obtained for composite coatings of Ni-Co alloy matrix containing SiC particles with 50 nm of diameter [37].

The literature reports that this relationship between the current density and the amount of incorporated particles also influences the microstructure of the produced MMC coatings [47,49]. For example, the Ni/Al2O3 coatings produced at current density values of 10 A dm-2 from an acid sulfamate-based bath (pH 4.3) presented a more refined microstructure than the coatings produced at lower current density values [50]. It is an important feature, as the microstructure of the coatings can be consequently related to their corrosion resistance.

In addition, the mode of applied current [continuous (DC) or pulsed current (PC)] may also influence the microstructure and corrosion properties of the MMC coatings. Composite coatings of copper matrix and β-SiC particles were produced under DC and PC conditions [51]. The coatings containing β-SiC particles deposited under DC conditions showed grains more refined than those observed from the copper coating. However, this coating presented voids between the incorporated particles and the metal matrix, which influenced the anticorrosive properties of the composite coating, as evidenced by the corrosion tests performed in 0.5 mol L-1 Na2SO4 solution (pH 2). These coatings showed less resistance to both uniform and localized corrosion compared to the pure metal coating. On the contrary, the coatings produced using PC presented a more compact microstructure and, as a consequence, showed elevated resistance to uniform corrosion, similar to pure copper coating and greater than the coatings produced under DC [51].

Another example is the study that evaluated the influence of DC, PC, and pulsed reverse current (PRC) on the content of particles incorporated to a nickel metallic matrix [36]. The particles used were Al2O3 (150 nm), SiC (30–60 nm), and ZrO2 (200 nm), and for all systems studied, the particle content in the composite coatings increased when the coatings were produced by PC and PRC; the amount of particles obtained using DC was always lower. The periodic switching of current (between positive values and zero for PC and between positive and negative values for PRC) permitted the discharging of the double layer, allowing a higher access of the particles (with adsorbed ions on their surfaces) toward the cathode. The ions adsorbed on the particles were subsequently reduced on the cathode surface causing a capture of the reinforcement material during coating growth.
