Nanocomposite coatings produced by electrodeposition from additive-free bath: the potential of the ultrasonic vibrations

The main objectives of this Ph.D. research work are the development of enhanced nickel matrix nanocomposite coatings and the optimization of the codeposition parameters. Two different nanopowder, i.e. silicon carbide and alumina, were added to a Watts type galvanic bath in order to produce the nanocomposites coatings and ultrasonic vibrations have been considered as an alternative to pitting control agents in order to produce pore-free layers. The powders and the stability of their suspensions have been studied by DLS and ζ-potential measurements. After the study of the relationship between process parameters and embedded ceramic particle amount, the optimized deposition condition have been evaluated and used for the production of the sample for the final properties characterization and to test the use of the ultrasounds. Unique, functional properties of composite coatings are derived not only from the presence of the particles dispersed in the bulk of the metallic matrix but also on the matrix microstructural changes induced by the interaction between particles and electrocrystallization. Therefore the microstructure of all type of coatings have been analyze by SEM on the top-view and on the cross-section, the agglomeration of the powder have been observed by LOM in case of Ni/Al2O3 and by TEM in case of Ni/SiC. It has been demonstrated that the codeposition of the SiC particles induces an important microstructural refinement while the Al2O3 powder is strongly agglomerated and only under ultrasonic vibrations can be dispersed and change the field oriented columnar structure into un-oriented fine grains. Ultrasounds revealed to have positive effect not only in avoiding the porosity but also dispersing the ceramic powder and increasing the codeposition rate. This allowed to produce protective and very refined coatings. All these interactions between ultrasounds, nanopowder and electrocrystallization lead to improved mechanical properties and the enhancement is proportional both to powder content and dispersion degree. Moreover a well dispersed powder induce, an improvement also in the corrosion protection leading to the formation of a more stable and resistant passive oxide. Concluding, Ni/Al2O3 nanopowder codeposition leads to hardening effect, but does not affect the corrosion resistance because the particles agglomeration is not completely avoided even if deposited under ultrasonic vibrations. The SiC particles, on the contrary, can be better dispersed thus leading to improved both mechanical and protective properties.