Mechanical and tribological characterization of electrolytically elaborated (Ni and Ni-Co) deposits

Abstract

The objective of this thesis was the electrodeposition and the characterization of the eco-friendly and the cost-effective Ni Co alloy coatings intended to replace the eco-harmful hard chromium coatings in the tribological applications. In the first part, an attempt was done to prove the performances of Ni-Co alloy nanocrystals as anti-wear coatings through their electrodeposition on StW24 mild steel, and then comparing their tribological behavior with those of the uncoated StW24 steel substrate and a coating used commonly against wear, i.e. the electrodeposited nanocrystalline Ni coating. The results of the mechanical and tribological analysis have shown that the nanocrystalline Ni-Co coatings have better anti-wear performances compared to the uncoated substrate and the nanocrystalline pure Ni coating. However, it has been noticed that the Ni-rich nanocrystalline Ni-Co coating presents a higher friction coefficient than the uncoated substrate, while the Co-rich Ni-Co coating exhibits a lower hardness than the pure Ni coating, which means that the Co content in the coating has a great influence on its properties. For this purpose, we aimed in the second part of this thesis to study the effect of two essential parameters of electrodeposition, namely the Co2+ ions concentration in the bath and the bath temperature on the Co content in the coatings. It was observed that the Co content in the coatings varied proportionally to the variation of the Co2+ ions concentration in the bath, resulting in significant changes in their morphology, structure and consequently significant changes in their mechanical and tribological properties. The hardness of the coatings decreased with the increase of their Co content, but in spite of this, the wear resistance increased, which is attributed to the lubricating effect of Co and to the progressive variation of the crystalline structure from the α (fcc) phase to the ɛ (hcp) phase that has an excellent friction-reduction behavior. The bath temperature also showed its influence on the composition and properties of the elaborated coatings. It has significantly affected their Co content, their crystallite size as well as their porosity rate, resulting in a change in their hardness, their friction coefficient, as well as their wear resistance. It has been concluded also that the coatings deposited at room temperature (25°C) have the best anti-wear performances thanks to their low porosity rate, their high hardness and thanks to their low friction coefficient compared to the other coatings. The ability of Ni-Co alloy nanocrystalline coatings to maintain its anti-wear performances on different metal substrates (StW24 steel, copper and brass) and with different thicknesses (43.2, 79.8 and 110.6 µm) was also examined. The results showed only a slight effect of the latter parameters on the properties of the coatings. In addition, these coatings have proven to have a high adhesion capacity on all the studied substrates and with the different thicknesses, which would offer the possibility of their use in many industrial applications