Introduction

MULTIPLE-LAYER ALLOY PLATING is an emerging technology for engineering desirable properties into thin surface layers through the use of carefully controlled deposit microstructures. As implied by the name, multiple-layer alloy electrodeposition involves the formation of an inhomogeneous alloy consisting of lamellae of different composition, as shown schematically in Fig. 1 for a binary alloy composed of species A and B. Each lamella of species A (or species B) in the film has a nearly uniform thickness XA (or XB). The modulation wavelength (X = XA + XB) characterizes the imposed compositional microstructure and typically takes a value anywhere from angstroms to microns in thickness. Multiple-layer thin films with spatially periodic compositional microstructures of the type shown in Fig. 1 are sometimes referred to in the literature as composition-modulated alloys (CMAs) or as superlattice alloys. A wide variety of binary and ternary alloy systems have been electroplated as multiple-layer films, including Ni/Cu, Ag/Pd, Cu/Ni-Fe, Cu/Ag, Cu/Co, Cu/Pb, Cu/Zn, Ni-P/Ni-Co-P, and Ni/Ni-P, to name a few. In many cases these alloys can be electroplated from a single electrolyte bath using either current or potential pulsing schemes. A common feature to many single-bath electroplating strategies is the use of hydrodynamic modulation that is synchronized in some manner with the pulsed plating. Multiple-layer alloys are often found to exhibit unusual (and sometimes highly desirable) mechanical, magnetic, electrical, and chemical properties, especially when the modulation wavelength X is of the order of nanometers.

Fig. 1 Schematic representation of a multiple-layer alloy consisting of alternating lamellae of species A and species B. The thicknesses of the A and B layers are given by ÀA and ÀB, respectively. The modulation wavelength that characterizes the multiple-layer superlattice structure is À= ÀA + ÀB. Multiple-layer alloys often exhibit a spatially periodic compositional wave throughout the film, rather than the discrete interface depicted between each lamella.

Fig. 1 Schematic representation of a multiple-layer alloy consisting of alternating lamellae of species A and species B. The thicknesses of the A and B layers are given by ÀA and ÀB, respectively. The modulation wavelength that characterizes the multiple-layer superlattice structure is À= ÀA + ÀB. Multiple-layer alloys often exhibit a spatially periodic compositional wave throughout the film, rather than the discrete interface depicted between each lamella.

In short, multiple-layer alloy plating combines the best attributes of electroplating--high throughput, low cost, and simple equipment—with an extra degree of freedom to engineer surface film properties. The potential impact of multiple-layer plating on the performance and economics of engineered surface layers appears to be large, although most commercial applications of the technology are still being developed. This article is focused mainly on the science and engineering of multiple-layer metallic alloys with nanometer-scale modulation wavelengths, because these are the materials that have gained the most attention for surface engineering. Throughout this chapter a solidus, or virgule (/) is used to denote the two materials that are spatially modulated to form a superlattice structure, whereas a dash between elements indicates that the species is an alloy. Using this nomenclature, Fig. 1 shows an A/B alloy. If species A happens to be copper and species B is a Ni-Fe alloy, then the figure denotes a Cu/Ni-Fe multiple-layer alloy.

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