Study of the microstructure, morphology, micro- and nanoindentation hardness, and corrosion behaviour of Ni-SiO ₂ composites electrodeposited from a deep eutectic solvent

Composite coatings containing Ni and small amounts of SiO 2 nanoparticles (NPs) were electrodeposited from an environmentally friendly deep eutectic solvent (DES) containing 0, 15, and 30 g/L of SiO 2 . The effects of the presence of SiO 2 in the Ni coatings were investigated in relation to their microstructure, as well as their mechanical and corrosion properties. The average crystallite size of Ni decreased by approximately 9% with the incorporation of 30 g/L SiO 2 . At this SiO 2 concentration, the Si content reached 0.56 wt.% in the bulk and 4.1 wt.% at the surface of the coating based on the results obtained by EDX and X-ray photoelectron spectroscopy, respectively. Microstructure studies of all coatings identified a granular growth with a broad Ni cluster size distribution. With the addition of 30 g/L of SiO 2 , the surface roughness decreased, and the normality of the surface texture improved. Maximum micro- and nanoindentation hardness values of 4.9 GPa 0.0981 N (500 Hv 0.0981 N ) and 6.7 GPa 20 mN , respectively, were achieved with the addition of 30 g/L of silica nanopowder, representing increases of 18% and 8% compared to pure Ni. Electrochemical results showed that the incorporation of 15 and 30 g/L SiO 2 , along with reduced surface defects, significantly improved polarization behaviour. This enhancement is attributed to improved passive film formation and the role of SiO 2 as a corrosion barrier. Corrosion current density decreased by factors of 11 and 7, respectively, compared to unreinforced Ni.