Characterisation of nanolayered aluminium/palladium thin films using nanoindentation P. Dayal a, ⁎, N. Savvides b , M. Hoffman a a School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia b CSIRO Materials Science and Engineering, Lindfield, NSW 2070, Australia abstract article info Article history: Received 30 September 2008 Received in revised form 12 December 2008 Accepted 28 January 2009 Available online 8 February 2009 Keywords: Nanoindentation Al/Pd multilayer Hardness Elastic modulus Transmission electron microscopy Structure, hardness, and elastic modulus of nanolayered aluminium/palladium thin films, with individual layer thickness varying from 1 nm to 40 nm, were investigated using transmission electron microscopy (TEM) and nanoindentation. TEM micrographs indicated a sharp but not flat Al–Pd interface. With just 6.5% (v/v) Pd a hardness enhancement of ~200% was observed for nanolayered Al/Pd compared to the hardness of pure Al film. A maximum hardness enhancement of up to 350% was observed for nanolayered Al/Pd samples compared to the hardness of pure Al film when bilayer thickness was 2 nm and Pd was 50% (v/v). Modulus enhancement was also observed for the nanolayered thin films. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Currently there is world-wide interest in enhancing the mechan- ical properties of aluminium-based light metals by tailoring the microstructure at the nanometer scale. Over the past few years, it has been shown that aluminium-based nanolayered materials exhibit enhanced mechanical properties compared to those of pure alumi- nium [1–6]. However, the exact mechanism of strengthening at the smallest length scale is not well understood for nanolayered materials, including those based on aluminium [7,8]. Better knowledge of the strengthening mechanisms is expected to assist in designing future advanced aluminium-based systems. Koehler [9] proposed a nanolayered structure of two different materials, A and B, in an attempt to design a strong solid, provided A and B follow certain criteria. In the present paper, we report a unique nanolayered system, aluminium (Al) and palladium (Pd), chosen on the basis of Koehler's theory. For selection of A and B Koehler suggested that: (a) The lattice parameters of A and B, at the operating tempera- ture, should be nearly equal to favour epitaxial growth of multilayers A/B without having large strains present at the interfaces caused by lattice mismatch. Both Al and Pd possess a face-centered cubic (fcc) crystal structure with lattice parameters of 4.0496 Å [10] and 3.8818 Å [11], respectively, at room temperature. (b) The elastic constants of A and B should differ by as much as possible. Because of the large elastic mismatch between the layers, a large external stress will be required to drive dislocations across the layers which will lead to the hardness enhancement of the multilayer structures. For Al, the elastic constants are: c 11 =114.0 GPa, c 12 = 65.3 GPa, c 44 = 28.5 GPa [12], whereas for Pd, the elastic constants are: c 11 =227.1 GPa, c 12 =176.04 GPa, c 44 = 71.73 GPa [11]. (c) The thicknesses of the A and B layers should be of the order of 100 atomic layers or less to prevent dislocations pile up within the layers. Dislocations pile up may produce the necessary stress concentrations to move dislocations across the layers and hence decrease the hardness of the multilayer structures. This suggests that Al and Pd layers should be less than approxi- mately 40 nm. This criterion was maintained for all the nanolayered Al/Pd samples reported in the present work. This paper describes the fabrication of Al/Pd nanolayered thin films by DC magnetron sputtering and their characterisation by nanoinden- tation and transmission electron microscopy (TEM). We report the effect of layer thickness on the hardness and elastic modulus of Al/Pd nanolayered thin films with different Al/Pd thickness ratios. Al/Pd nanolayered thin films have not been studied before for their mechanical properties. However, authors are aware of solid-state amorphization study on Al/Pd multilayer thin films where individual layer thickness was less than 1 nm [13]. The study by Kingetsu et al. showed that the Al/Pd multilayers form amorphous structures when individual layer thickness is kept below 1 nm. Therefore, in the present work, individual layer thickness was not kept less than 1 nm to avoid the formation of amorphous structure. Here it should be Thin Solid Films 517 (2009) 3698–3703 ⁎ Corresponding author. E-mail address: pranesh@unsw.edu.au (P. Dayal). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.174 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf