Dissolution modes of Fe/Cu and Cu/Fe deposits

Using a kinetic model that includes bulk and surface driving forces, we stu dy the different modes occurring during the kinetics of dissolution of thin (1 or 2 ML) and thick (10 ML) deposits of Cu on Fe and Fe on Cu. For a thi n deposit, due to the lower surface energy of Cu, the dissolution kinetics is slower for the Cu on Fe case than for Fe on Cu. In the first case the Cu remains at the surface up to the total dissolution. For the inverse deposi t a surfactant effect takes place where the deposit is buried by one or two planes of the substrate element. Before the total dissolution and dependin g on the temperature and the deposit thickness the kinetics can slow down d ue to the appearance of quasistationary configurations that corresponds to equilibrium solutions of an equivalent finite system having the same instan taneous quantity of matter. For a thick deposit of Cu on Fe the deposit als o remains at the surface and the dissolution rakes place following a layer by layer dissolution mode, which corresponds to the successive dissolution of each precipitate plane, starting from the plane at the interface between the deposit and the substrate. The shape of this interface corresponds to the equilibrium interface between two semi-infinite phases having the bulk solubility limit concentrations. For the inverse deposit, first a surfactan t effect occurs, leading to a copper bilayer floating on the surface. Then, two layer by layer dissolution modes take place, which correspond to the d issolution either from the bottom or from the top of the precipitate. These layer by layer dissolution modes are linked to the large miscibility gap o f the phase diagram. We use a "local equilibrium" concept that allows us to compare all the configurations obtained during the kinetics of dissolution to concentration profiles of "stable" or "metastable" solutions of related equilibrium situations. [S0163-1829(98)04147-2].