Preferred orientations in nickel electro-deposits: I. The mechanism of development of textures in nickel electro-deposits

Abstract

This paper presents a mechanism for (a) the preferential formation of electro-deposit facets of a particular type and (b) the alignment of these facets normal to the substrate, and then shows that the result of these two growth processes is the development of a preferred orientation in thick electro-deposits. The discussion is confined to the growth of nickel electro-deposits from unadulterated baths. Facets are formed because of differences in growth velocities. Both crystallographic and electrochemical factors produce these differences. The crystallographic factors are considered, in terms of the Bravais law of crystal growth. It is suggested that different lattice planes have different free growth metallic overpotentials. The electrochemical disturbance to free growth, stems from the hydrogen evolution reaction. Beeck's experimental studies on hydrogen adsorption on nickel are used to argue that the intermediately-produced hydrogen atoms are adsorbed more easily on lattice planes which during free growth have faster growth velocities. If hydrogen atom adsorption increases metallic overpotential, then facets of different types are formed with different surface coverages. The {hkl} facets so formed are aligned normal to the substrate because the deposits tend to a lopt an outward mode of growth which requires that the slowest-growing {hkl} facets (and therefore the zone-axis <uvw> of the {hkl} Wilman facets) stand perpendicular to the substrate surface. All deposit-crystals being subject to similar growth influences, they get identically oriented, i.e., a preferred orientation develops with the texture-axis <uvw> being the zone-axis of the Wilman facets. The random orientation stage-which precedes the texture stage-exercises a substrate influence and therefore permits only a gradual development of texture. Two important consequences follow from the theory proposed in this paper : (1) texture changes are induced by changes in surface coverage with hydrogen atoms, and by factors such as temperature which affect this coverage; (2) deposits having different textures should have different hydrogen contents. The experimental data of Yang supports the latter conclusion.