*2.1.7. Electrical nature of the particles*

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

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

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

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

by the presence of the surfactant in the bath [6].

158 Electrodeposition of Composite Materials

CTAB chain was grafted on their surfaces.

their incorporation in coatings [7].

in the zinc matrix [44].

The electrical nature of the particles used to produce the MMC coatings has drawn the attention of very few researchers. However, it is important to have information about how the codeposition of inert, semiconductor, or conductive particles influence the overall process [11]. Apparently, conductive particles are readily incorporated into the metallic coatings, although dendritic, uneven, and very rough deposits are obtained. On the contrary, the codeposition of insulating particles is very slow, producing relatively homogeneous and smooth deposits [17]. This fact can be used for the production of composite coatings with different applications: the smooth ones could be applied for mechanical and/or anticorro‐ sion uses, whereas those with very high specific surface area and using conductive parti‐ cles with some catalytic properties entrapped in the coatings could act as catalysts in some chemical or electrochemical processes [60].

The codeposition of inert (α-Al2O3), semiconductive (SiC, MoS2), and conductive (graphite) particles in a copper metallic matrix was studied and it was found that the distribution of inert particles in the coating was uniform and an acceptable surface quality (no roughness) was obtained even at high current density values [11]. When semiconductive and, especially, conductive particles were incorporated to the copper matrix, however, spongy and irregular coatings were produced, showing high surface roughness mainly at high values of current density. Therefore, the increase in the conductivity of the particles changed the specific surface area and increased the roughness of the coating.

These results confirm that the codeposition process as well as the morphology and micro‐ structure of the coatings are affected by the electrical nature of the particles. The presence of more conductive particles in suspension, such as graphite for example, may act as a suspension electrode in the process, as shown by Iwakura [60]. Graphite particles in contact with the cathode were polarized and the deposition of copper could have occurred onto the particles as well as onto the cathode. Such codeposition mechanism could explain the unsuitable deposit structures and the coppering of graphite particles, as observed by Stankovic and Gojo [11]. However, to the best of our knowledge, this mechanism has not been validated yet.

#### *2.1.8. Previous stirring time of the particles in the solution*

The effects of the period of time the particles are maintained suspended in the solution before the electrochemical codeposition process occurs (the previous stirring time) on the character‐ istics of the codeposited coatings are not usually investigated in the literature. However, it is possible to verify that composite coatings are produced after previous stirring time values ranging from 30 min to 24 h, without any explanation about this choice even if the same kind of particle was used [18,26,38,51]. The suspension is generally stirred to enhance the dispersion of the particles and promote their codeposition [11,18,26,34,38,51,61,66]. Therefore, the time used to keep the nanoparticles suspended must be directly related to the codeposition process.

An introductory study concerning the effect of the previous stirring time was performed to produce copper MMC coatings reinforced with micrometric γ-Al2O3 onto steel substrate (AISI 1020) [61]. The coatings were produced by chronoamperometry, using a pyrophosphate-based bath, under constant stirring speed (800 rpm) and using different values of previous stirring time (ranging from 1 to 5 h). Figures 1 to 3 present the surface morphology of the coatings obtained under these conditions.

**Figure 1.** Surface morphology of Cu/γ-Al2O3 coatings produced at -1.20 VSSE from pyrophosphate bath previously stir‐ red for 1 h at 800 rpm [61].

**Figure 2.** Surface morphology of Cu/γ-Al2O3 coatings produced at -1.20 VSSE from pyrophosphate bath previously stir‐ red for 3 h at 800 rpm [61].

**Figure 3.** Surface morphology of Cu/γ-Al2O3 coatings produced at -1.20 VSSE from pyrophosphate bath previously stir‐ red for 5 h at 800 rpm [61].

It is possible to observe that the previous stirring time influenced the dispersion of the γ-Al2O3 particles in the copper matrix. The particle agglomerates in the coating produced after 1 h of previous stirring (Figure 1) seem to decrease when the previous stirring time increased to 3 h (Figure 2). After 5 h of previous stirring, the particles seem to be homogeneously dispersed in the copper matrix (Figure 3). These results indicated that this parameter may have a fundamental role to produce coatings presenting dispersed second-phase particles and consequently enhanced mechanical and anticorrosive resistances.

Additionally, the coatings produced after 1, 3, and 5 h of stirring (Figures 1–3) were chemically analyzed, producing 9.74 wt% Al2O3, 16.7 wt% Al2O3, and 9.98 wt% Al2O3, respectively [61]. There is an increase in the Al2O3 content from the experiment conducted after 1 to 3 h of stirring and a small decrease when the previous stirring time increases to 5 h. This result indicates that it must be an optimum time to stir the suspension before electrodeposition be performed to produce coatings with completely dispersed particles and presenting high amount of partic‐ ulate material incorporated into the metallic matrix. These initial results suggest that the previous stirring time must be carefully studied to produce high-quality composite coatings.
