Thermal stability of self-supported metallic multilayered thin films

摘要

The morphological stability and strength retention following elevated temperature exposure or thermal cycling will be crucial in exploiting the extremely high strengths of nanolayered materials in advanced engineering applications. The effects of elevated temperature (≤ 800 °C) vacuum annealing on the morphological stability and mechanical properties of sputter deposited Cu-Nb multilayers with 75 nm bilayer period are reported here. Even after 800 °C/1 hour anneal, the continuity of the layered structure is maintained and the bilayer periods are unchanged. The in-plane grain sizes in both Cu and Nb coarsened but were anchored at grooved boundaries preventing further growth. For a constant bilayer period, the effect of increasing the in-plane grain size on the multilayer hardness is found to be insignificant. After annealing, the layers are observed to be offset by shear along a vertical plane at the triple point junctions that have equilibrium groove angles aligned in a zig-zag pattern. A new mechanism is proposed for the evolution of this "anchored" structure that is resistant to further morphological instability.