Effects of elevated temperature annealing on the structure and hardness of copper/niobium nanolayered films

We investigated the effects of elevated temperature vacuum annealing on the morphological stability and hardness of self-supported, textured, polycrystalline Cu-Nb nanolayered films with individual layer thickness varying from 15 to 75 nm. Films with layer thickness greater than approximately 35 nm are found to resist layer pinch-off and spheroidization even after long annealing times at 700 degrees C, while films with layer thickness similar to 15 nm exhibit layer pinch-off and evolve into an equiaxed grain microstructure. Nanoindentation measurements indicate almost no change in hardness after annealing for films that retain the layered morphology, in spite of the increase of in-plane grain dimensions. Significant decreases in hardness are noted for films that develop a coarsened equiaxed grain microstructure after annealing. The mechanism that leads to the development of a thermally stable nanolayered structure is analyzed. Also, the relative effects of in-plane grain size and layer thickness on the multilayer hardness are discussed.