Microstructure and mechanical properties of disperse Ni-Co alloys electrodeposited on Cu substrates

Abstract

The nickel and cobalt disperse alloy deposits of five different compositions were obtained by electrodeposition on Cu substrates in the galvanostatic regime from an ammonium sulfate-chloride solution. The effect of cathodic current density and Ni2+/Co2+ ions concentration ratio in the electrolyte on the composition, microstructure, morphology and mechanical properties of Ni-Co alloys were investigated. Conditions for formation of nanostrustured disperse deposits and surface roughness was determined by 3D SEM reconstruction of the specimen surface. It was established that formation of the disperse deposits with highly developed structures is favored from bath with equal Ni2+/Co2+ ions concentration ratio in the electrolyte. Cathodic polarization diagrams determined for all investigated alloys have shown shift of the cathodic potential for alloy deposition to the more negative values with increasing Ni2+/Co2+ ions concentration ratio in the electrolyte. An increase of in the cobalt content in the alloy was observed with decreasing the current density and increasing of the Co2+ ions concentration ratio in the bath. X-ray analyses of nanocrystalline Ni-Co deposits show formation of a single phase face-centered cubic (FCC), a single phase hexagonal-close packed (HCP) and mixture of FCC solid solutions and HCP phase depending on the current density applied and electrolyte composition. The increase of HCP phase content in the nanocrystalline deposits appears as a result of both, the increase in Co2+ ions concentration in the bath and decrease of deposition current density. The mechanical properties of nanocrystalline deposits have shown increase of the hardness with increasing Ni content in the alloy. The cross-section of the samples electrodeposited on Cu substrates from electrolytes with equal ion metal concentration at lower current density values revealed the beginning of a dendrite structure formation. © 2009 Elsevier B.V. All rights reserved.