XAFS characterization of electrodeposited Ni-W coatings

JOSE MARTIN RAMALLO LOPEZ; M.P. QUIROGA ARGAÑARAZ; MARTÍN DANIEL MIZRAHI; F. G. REQUEJO; S.B. RIBOTTA

Campinas

Workshop; 23ª Reunión Anual de Usuarios del LNLS; 2013

LNLS

Alloys of tungsten and molybdenum with iron group metals have attracted considerable attention due to their specific magnetic, electrical, mechanical, thermal and corrosion resistance properties. Among them, nickel and tungsten (Ni-W) alloys are particularly interesting because they exhibit good corrosion resistance in different aqueous media, high hardness and ductility, and anisotropic magnetic properties. Therefore, they have been studied as candidates to protect structural metals such as steels and copper, for magnetic recording systems, and for MEMS and NEMS applications. In this work a detailed characterization by X-ray absorption spectroscopy of electrodeposited Ni-W coatings prepared by pulsating electrodeposition on steel and copper substrates is presented. X-ray Diffraction, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) results are also presented. These results are discussed in terms of the good hardness and ductility that the coatings present. Different Ni-W coatings were prepared with different pulse programs in order to determine the effect of the preparation conditions on the final structures that were formed. Both XANES and EXAFS experiments at both Ni-K and W-L3 edges were studied at the XAFS2 beamline of the Laboratorio Nacional de Luz Sincrotron (LNLS), Campinas, Brazil. Experiments were performed in fluorescence mode at room temperature. The results indicate that two main phases are found in the bulk structure: a W-rich amorphous phase and Ni-rich crystalline phases. The crystalline phases consist of crystalline domains of 7-8 nm in size of Ni(W) (fcc) solid solution (12% W content). The amorphous phase exhibits a less compact Ni-W structure and a higher W content. We have found that in both phases the W is under a metallic state without evidence of significant amounts of tungsten carbide, tungstates or citrate-tungsten complexes. The good hardness and ductility of the coatings can be related to the structure consisting of a W-rich Ni-W matrix composite reinforced by the 7-8 nm crystalline domains of the Ni(W) solid solution.