Ion exchange at a metal-ceramic interface

We present the results for an interfacial reaction between magnesium aluminate spinet and aluminum metal at 640 degreesC which. for the first time, show the occurence of ion exchange at a metal-ceramic interface. The exchange reaction occurs by the diffusion of magnesium and aluminum ions across the interface in opposite directions. The enrichment of the spinel in aluminum cations, and the metal in magnesium is confirmed by the appearance of phases of aluminum oxide and magnesium-aluminum intermetallics in the X-ray diffraction spectra. Electron spectroscopic images of spatial elemental distribution provide further evidence for the reaction. A drop in the thermal and electronic conductivities are explained by enhanced scattering of the electrons by solid solution Mg in the aluminum metal. The experiments were carried out on composites consisting of an aluminum matrix with a fine dispersion of spinet particles, whose size was varied while the volume fraction was held constant. Measurable ion exchange reaction was seen only in the composites containing the smaller (one micrometer size) particles. The influence of surface to volume ratio of the particles on the reaction rate confirms that the reaction was interface controlled. The possibility of a nucleation and growth process for the conversion of spinel into alumina and solid solution magnesium was ruled out by quantitative thermodynamic analysis. (C) 2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc