Ag transport in CrN–Ag nanocomposite coatings

Abstract

2-μm-thick CrN–Ag composite coatings containing 22at.% Ag were deposited on Si(001) by reactive co-sputtering at Ts=300, 400, and 500°C. Subsequent vacuum annealing at Ta=425, 525, and 625°C causes Ag transport to the surface. Auger electron spectroscopy and plan-view microscopy are used to quantify the Ag transport to the surface, which increases strongly with increasing ΔT=Ta−Ts. Compositional depth profiles and cross-sectional microscopy show that annealing causes a negligible Ag gradient through the composite layer, suggesting that the Ag transport is detachment-limited as opposed to diffusion-limited. Statistical analyses of Ag aggregate size-distributions within the matrix show that large aggregates (≥50nm) are unaffected by annealing, while the Ag in a large fraction of small aggregates (<50nm) moves to the surface, leaving behind 10–50nm wide voids in the annealed composite. This indicates that the Ag from the smaller grains, with a higher chemical potential and thus a higher detachment rate, is transferred to the large grains on the surface which are 200–1000nm wide.