*2.1.2. Particle size in the suspension*

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 reports showing a reduction in the amount of codeposited particles as their size decreases [5, 6]. In aqueous ionic solution, the particles are easily bonded to each other due to the compres‐ sion of the diffusion double layer around them, originated by the high ionic strength. Although the stirring of the electrolytic bath may decrease the particle aggregation and favors their codeposition, this effect is more pronounced for particles of micrometric size, as the shear forces on the agglomerates generated by the stirring process decrease with the particle size [5].

The influence of SiC particle size (1 μm and 45–55 nm) in the microstructure and morphology of the nickel MMC coatings was evaluated [46]. The results showed that, whereas the pure nickel coating presented a pyramidal morphology, the addition of SiC nano- or microparticles produced a coating with a rough nodular surface. This new morphology was related to the presence of nickel grains surrounding the SiC particles, forming globular aggregates. It has been argued that the introduction of the ceramic particles disturbed the formation of the metallic matrix and decreased the grain size of the coating [46]. However, no differences were observed concerning the size of the particles.

The effect of microparticle concentration of SiC (5 and 0.3 μm) in the volumetric percentage of metal deposited on the nickel matrix was evaluated [26], and it was observed that there was an increase in the volumetric percentage of deposited SiC particles as their concentration in the bath increased, independent of the particle size. However, the greatest volumetric percentage of codeposited SiC particles was obtained using 5 μm particles, whereas the lowest percent‐ age was deposited using 0.3 μm particles. It was pointed out that a direct comparison of the volumetric percentage of codeposited particles with different sizes may lead to a misunder‐ standing because of the differences in the particle density on the composite coating [26].

The effect of SiC particle size (1.2, 8.0, 14.0, and 20.0 μm) on nickel composite coatings was investigated by Kim and Yoo [48], and it was verified that an increase in the size of the particle (until 14 μm) added to the electrolytic bath induced an increase in the content of the particles in coatings. This fact was explained by the adsorption of Ni2+ ions on the particles, resulting in a strong Coulomb force and an increase of the SiC particles codeposited. Above 14 μm, however, the particles presented a sedimentation tendency due to their weight, which decreased the suspension stability and the content of SiC particles codeposited in the metallic matrix [48].
