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

Ag/Nb nanomultilayers with different modulation wavelengths Lambda were prepared on silicon wafers by electron beam evaporation. Nanoindenter measurements show that with decreasing Lambda of the multilayers, the nanohardness increases up to similar to 80% for Lambda=4 nm, 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. (c) 2007 American Institute of Physics.