*2.1.6. Composition of the electrolyte bath*

The production of MMC coatings by electrodeposition is highly influenced by the composition of the electrolytic bath, as the presence of complexants, surfactants, or dispersant agents may affect the metallic ion deposition process, the suspension stability, and the particle incorpora‐ tion in the coating. Moreover, the acidity of the bath (pH) as well as the concentration of the baths components may also influence the particle dispersion and the codeposition of the species. Therefore, bath composition is one of the most studied topics in the production of MMC coatings by this technique, and the properties of the produced coatings obtained from different baths may certainly vary.

Another study [47] showed that there is a maximum value of particles of CeO2 (between 15 and 20 nm) incorporated to nickel matrix when the stirring speed was varied (100, 250, 350, 450, and 550 rpm). The highest quantity of particles in the matrix was obtained at 450 rpm; above this value of stirring speed, the content of CeO2 decreases. This phenomenon was explained by the collision of particles on the cathode under high values of stirring speed. Moreover, the increase of stirring speed also enhanced the turbulence in the flux suspension, which could have removed the weakly adsorbed particles from the cathode surface [47]. Similar results were obtained by other research groups [15,49,54] for different nickel composite

The solution temperature affects several physical properties of the suspension (such as the viscosity and the sedimentation rate) and influences the reduction kinetics of the free and adsorbed cations as well as the possible adsorption of particles on the cathode [27]. Although there are few works in the literature studying the effects of this parameter in the electrochem‐ ical codeposition process, these works show the relationship between the solution temperature and the increase of codeposited particles in metallic matrix or the morphology of the coating.

The temperature of 50°C was considered the most beneficial one for the incorporation of alumina particles in Co-Ni matrix from an acidic Ni(II)/Co(II) sulfamate bath instead of 60°C [12]. For the codeposition of alumina particles in Cr matrix from a sulfate bath containing a rare earth element (not mentioned), the best temperature range for particle incorporation was from 30°C to 40°C. When the temperature was below 30°C, the composite coating was rough, whereas, when it was above 40°C, the composite coating decreased; beyond 55°C, there was

Research works carried out for Zn-SiC composite coatings [56] showed that the increase of solution temperature (33–45ºC) caused a significant reduction of particle content in the coating. The authors considered that the increase in the solution temperature favored the electroactive species reduction, while it did not increase the codeposition of the SiC particles. Kim and Yoo

It is important to mention that the overall result concerning the temperature variation on the codeposition process for producing MMC coatings is difficult to predict, as the parameter most affected by the solution temperature is not generally identified in the usual used deposition

The production of MMC coatings by electrodeposition is highly influenced by the composition of the electrolytic bath, as the presence of complexants, surfactants, or dispersant agents may affect the metallic ion deposition process, the suspension stability, and the particle incorpora‐ tion in the coating. Moreover, the acidity of the bath (pH) as well as the concentration of the baths components may also influence the particle dispersion and the codeposition of the

Thus, this parameter is still scarcely explored and understood.

[48] verified the same behavior for Ni-SiC composite coatings.

coatings.

*2.1.5. Solution temperature*

156 Electrodeposition of Composite Materials

no Al2O3 codeposited with Cr [55].

*2.1.6. Composition of the electrolyte bath*

mechanisms [12,27].

The electrolyte composition and the solution pH directly influence the ζ potential measure‐ ments, which is the main variable related to the stability of solid particle dispersions in aqueous solution [57]. Charged surface particles form more stable suspensions because the mutual repulsion between the particles increases, decreasing their agglomeration [27,45,58]. There‐ fore, the increase in the particle charge (in modulus) will result in a higher (also in modulus) ζ potential and in a more stable suspension.

Surfactants agents are generally used to promote a better dispersion of suspensions because they reduce the surface energy of the particles, deeply influencing the codeposition process. The surfactants or surface-active agents are characterized by having two distinct regions on the same molecule: a hydrophilic polar region and a hydrophobic nonpolar region. These compounds have activity at the surface interfaces between two phases, such as water–air and oil–water, and the solid–liquid interface [46]. For experimental evaluation, these additives apparently act in two ways: by modifying the properties of the particle surface and stabilizing the suspension and/or by affecting the reduction of metal ions during electrodeposition [4].

Nanoparticles of Al2O3 suspended in a 0.001 mol L-1 KCl solution presented ζ potential values almost constant and positive for a pH range between 2 and 6. However, at higher pH values, the ζ potential shifted to less positive values until it reached the isoelectric point (IEP), where the particles had no charge and then precipitated (pH 9.2) [59]. On the contrary, in the presence of nickel sulfamate bath, the ζ potential of these particles was positive in the entire pH range between 2 and 12 probably due to the adsorption of nickel cations on the alumina particles; in the case of pyrophosphate electrolytes, the ζ potential remained negative, which was related to the adsorbed pyrophosphate anion. Both in the presence of sulfamate and the pyrophos‐ phate baths, it was not possible to determine the IEP in the pH range studied. Thus, the authors concluded that alumina particles take on negative charges in alkaline baths, whereas, in acidic electrolytes, they assume positive charges.

The sign of the charges on the particle surface will also influence the deposition process, although both positive and negative charges are considered to improve the incorporation of the particles. For example, the codeposition of α-Al2O3 particles in a Co-Ni matrix from a Ni(II)/ Co(II) acid sulfamate bath was found to be enhanced by the presence of Co(II) ions adsorbed on the particle surface, which charged them positively [12]. In opposition, the presence of negatively charged alumina particles (due to the presence of citrate or pyrophosphate anions adsorbed on them) also increased the codeposition of Al2O3 particles on Cu matrix [10]. In this last case, the negatively charged particles were codeposited in the metallic matrix in higher amounts than the positively charged ones. A possible explanation is that the negatively charged particles would be attracted by the double layer of the substrate that was charged with excessive positive charge [probably Cu(II) ions] under the conditions of the electrodeposition experiment [10]. Although these arguments do not imply that the electrodeposition is com‐ pletely governed by electrostatic forces, the proposed mechanism helps streamline the experimental results for the present system [10,59].

However, each system can produce unique responses to the addition of surfactants or changes in the bath composition. The addition of N,N-dimethyldodecylamine in Zn(II) baths contain‐ ing SiO2 particles (20 nm) has not caused a significant increase in the incorporation rate of these particles, as similar amounts (approximately 5 wt%) were obtained in both the presence and absence of the amine. Nonetheless, for particles with size of 2 μm, there was an increase in the incorporation rate with the addition of the surfactant and the SiO2 content on the coating was 14 wt%. Therefore, there was a joint effect of the particle size and the dispersion effect caused by the presence of the surfactant in the bath [6].

The effects of adding cetyltrimethylammonium bromide (CTAB), a cationic surfactant, at concentrations of 10-5 to 10-3 mol L-1 on the codeposition process of SiO2 particles in a copper matrix was also investigated [45]. It was found that the ζ potential of the suspension in the absence of the additive was negative in the whole pH range studied (3–9). It was assumed that, at high pH values, there were hydroxyl groups (negative ions) on the surface of silica particles.

The introduction of small concentrations of CTAB in the CuSO4 solution induced changes in the ζ potential of the nanoparticles, which assumed positive values for all CTAB concentration ranges. A reasonable explanation for this behavior would be the easy adsorption of the cationic surfactant on the surface of silica nanoparticles because of the positive charge in the polar part of CTAB molecules. These molecules reacted with the hydroxyl groups on the silica surface, decreasing the surface energy of the nanoparticles, improving the state of dispersion of SiO2 particles [45]. In addition, the steric effect between the SiO2 nanoparticles became higher as the CTAB chain was grafted on their surfaces.

However, it is very difficult to correlate the results obtained in the ζ potential measurement, which is generally carried out at low electrolyte ion concentrations (low ionic strength), with the behavior of a particle in an electrolyte used for electrodeposition (i.e., high ionic strength). Therefore, it is not easy to verify the dispersion influence of the surfactants in the electro‐ chemical codeposition process [10,48]. Moreover, the addition of surfactants should be carefully used, because an excessive increase in their concentration may create a large repul‐ sion force between the surfactant layer next to the cathode and the surface of the particle. At high concentrations, the surfactants form micelles in solution and the uniform dispersion of previously formed nanoparticles is interrupted, promoting their agglomeration and reducing their incorporation in coatings [7].

The concentration of the metallic ion in the electrolyte also affects the properties of the MMC coatings produced by electrodeposition. The grain size of Zn metallic matrix was substantially independent of the concentration of TiO2 particles for concentrated ionic solutions. However, for more dilute solutions (0.1 mol L-1), it was observed that the grain size decreased with the increase of the nanoparticle amount. This was explained mainly by changes in the growth and nucleation of zinc crystals due to the concentration of the metallic ions and the presence of semiconductor particles. For dilute solutions, the evolution of hydrogen and the presence of particles should promote a detrimental effect on the crystal growth, leading to smaller grains in the zinc matrix [44].
