Mechanical properties of nanostructured amorphous metal multilayer thin films

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A.B. Mann Department of Ceramic & Materials Engineering, Rutgers University, Piscataway, New Jersey 08854

H. Kungb) Division of Materials Sciences and Technology, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

C.L. Chien Department of Physics & Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218

T.P. Weihs and R.C. Cammaratac) Department of Materials Science & Engineering, The Johns Hopkins University, Baltimore, Maryland 21218 (Received 30 December 2003; accepted 22 March 2004)

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 ␮m. The bilayer periods (⌳) 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.


Nanostructured crystalline composite materials can exhibit mechanical properties that differ significantly from those of their components in bulk form.1 For example, significant hardness enhancements (50–400% over the rule of mixtures) have been observed in crys-


Present address: Advanced Products Research and Development Lab, Motorola Inc., Tempe, AZ 85684. Address all correspondence to this author. email: [email protected] b) Present address: Division of Materials Sciences and Engineering (SC-13), U.S. Department of Energy, Germantown, MD 20874. c) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// DOI: 10.1557/JMR.2004.0248 1840

J. Mater. Res., Vol. 19, No. 6, Jun 2004 Downloaded: 14 Mar 2015

talline multilayer thin films, especially in films having bilayer periods below 10 nm.2–6 This phenomenon can be explained by three major mechanisms of dislocation entrapment associated with the high density of interfaces.7–11 If there is significant lattice matching between the layers of differing equilibrium lattice spacing, the resulting coherency stress can lead to a Peach–Koehler force on dislocations, impeding their motion. Near an interface where a change in elastic modulus occurs, dislocations can also experience an image force that causes a net repulsion from the layers with a higher elastic mod