Mechanical properties of nanostructured amorphous metal multilayer thin films

The hardness of amorphous metal multilayered films was investigated by nanoindenation. Bilayer material systems of amorphous CuNb, FeB, and FeTi were produced by dc sputtering on <112(-)0> sapphire substrates to a total thickness of 1 mum. The bilayer periods (Lambda) ranged from 2 to 50 nm. The films' noncrystallinity was verified by x-ray diffraction (XRD) and electron diffraction. The layer structure was verified by transmission electron microscopy and grazing angle XRD. The hardness and elastic modulus properties of the films, measured by nanoindentation, were shown to be statistically equivalent to the rule mixtures predictions. The hardness behavior is in contrast with the behavior of crystalline multilayered films, which generally display significant enhancements as the bilayer period is decreased below 10 nm. The lack of a significant hardness variation in the amorphous films strongly suggests that dislocation-mediated mechanisms do not govern inhomogeneous flow in amorphous metals.