Observation of largely enhanced hardness in nanomultilayers of the Ag–Nb system with positive enthalpy of formation

Abstract

Ag ∕ Nb nanomultilayers with different modulation wavelengths Λ were prepared on silicon wafers by electron beam evaporation. Nanoindenter measurements show that with decreasing Λ of the multilayers, the nanohardness increases up to ∼80% for Λ=4nm, whereas the modulus is almost unchanged. This unusual behavior originates from a unique microstructure where amorphous Ag–Nb alloys form at the interfaces and grain boundaries of silver nanoparticles, as observed by cross-section high resolution transmission electron microscopy. The amorphous phases favor hardness enhancement by preventing dislocation emission and movement, whereas they have a negative contribution to the modulus because of their free volume.