Effect of the individual layer thickness on the transformation of Cu/W nano-multilayers into nanocomposites

Thermal treatment of nano-multilayers (NMLs) constituted of alternating immiscible metals, like W and Cu, can evolve in a nanocomposite (NC) with tailored mechanical, electrical and/or thermal properties. The NML-to-NC transformation can result in unique material properties of the resulting NC, which can hardly be achieved by conventional composite fabrication. In this paper, we systematically study the role of the individual Cu and W layer thicknesses in Cu/W NMLs on the diffusion, internal stress evolution and resulting NC microstructure upon heating by X-ray diffraction, high-resolution scanning electron microcopy and Auger electron spectroscopy. In the as-deposited state, a strong compressive stress state of Cu and W is observed, which is governed by a dominating interface stress contribution of the Cu{111}/W{110} interfaces of 11.25 +/- 0.56 J/m(2). Isothermal annealing of the as-deposited NMLs with different Cu and W layer thicknesses yields to the formation of nets of Cu protrusions on the NML surface, acting as a stress relaxation mechanism, which becomes thermally activated at 400 degrees C. The kinetics of Cu surface outflow are governed by the initial Cu/W stress state and the individual layer thickness ratio. During annealing at 700-800 degrees C, the Cu surface particles diffuse back into the volume of the near stress-free NML (dissolution), which defines the onset of accelerated degradation of the nanolaminated structure by thermal grooving, eventually forming a NC. The thickness ratio of Cu and W layers is shown to strongly affect the onset of the NML-to-NC transformation and the final NC microstructure.