Effects of Additives on the Electrodeposition of Nano-Grained Ni-Mo Alloys

Abstract

Nano-grained (ng) materials have attracted much interest in the past few decades, as these materials are expected to have superior properties to their coarse-grained counterparts. Nevertheless, owing to the large fraction of grain boundary, the grain boundary energy becomes so high with the grain size down to a few nanometers that it is challenging to synthesize ng metals with extremely fine grain size. One possible way to do this is to prepare ng alloys instead of pure ng metals by choosing appropriate alloy systems with a tendency of segregation, as the segregation of solute atoms at grain boundaries will reduce grain boundary energy and thus stabilize the nanostructure. Electrodeposition has been found very promising in synthesis of ng metals, where additives are usually introduced to reduce grain size by moving the onset deposit potential negatively and consequent increasing the nucleation rate. In the electrodeposition of ng alloys, however, it is to be explored the effect of additives to the composition and the grain size of the synthesized ng alloy. It is the purpose of the present work to explore the effect of additives (saccharin and 1, 4-butynediol) in the synthesis of ng Ni-Mo alloy film in aqueous citrate-ammonia electrolyte with different concentration of sodium molybdate (Na2MoO4·H2O) ranging from 0.5 to 7 g/L. The chemical composition of ng deposits were determined by Energy-dispersive X-ray spectroscopy (EDS). The microstructure of the synthesized ng Ni-Mo films were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) with the grain size determined by TEM dark field images. Voltammetry was employed to evaluate the effect of additives and also sodium molybdate to the deposition of Ni-Mo. XRD results demonstrate that the as-deposited Ni-Mo films are all single fcc solid solutions. It is found Mo content in the deposits increases with the concentration of sodium molybdate in the electrolyte irrespective of the presence of additives. In addition, additives do not induce significant difference in the composition of the alloy deposits at a certain concentration of molybdate. The average grain size of the deposits decreases with the increase of sodium molybdate concentration from 44 nm to 5 nm corresponding to 0.5 g/L and 7g/L Na2MoO4·H2O respectively in plain solution, but the grain size reduces to 16 nm and 4 nm respectively in additives-added electrolyte corresponding to the same Na2MoO4·H2O concentration range. It is evident that saccharin and 1, 4-butynediol pronouncedly reduce the grain size of ng Ni-Mo when the Na2MoO4·H2O concentration is low but this reduction gradually gets small with the increase of molybdate concentration. Steady state polarization curves with the addition of additives and molybdate indicate an interaction between the saccharin, 1, 4-butynediol and the sodium molybdate, which correlates well with the composition and the characterized microstructure of ng Ni-Mo films.