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Abstract
This thesis presents a study of the design and fabrication of nanolayered aluminium/palladium (Al/Pd) thin films and their characterization to determine the mechanical properties and microstructure, and to attempt to elucidate the mechanism(s) responsible for enhancement of these properties. Nanolayered Al/Pd thin films and monolayer Al and Pd thin films on silicon [Si(111)] wafer substrates were deposited by DC magnetron sputtering and investigated using X-ray diffraction (XRD), Focussed ion beam (FIB) milling, Scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and nanoindentation. Two different sets of nanolayered Al/Pd samples were investigated: for Set-I, Al layer thickness = Pd layer thickness = 1-40 nm; and for Set-II, Al layer thickness = 25-29 nm while Pd layer thickness = 2-10 nm. All films had a total thickness of approximately 1 micron. XRD and XTEM results indicated polycrystalline face-centred cubic Al and Pd layers with epitaxial Al(111)/Pd(111) relationship and the formation of AlPd compound at the Al-Pd interfaces. Also, XTEM micrographs indicated sharp but not flat Al-Pd interfaces.
Nanoindentation results showed that, for both sets of samples, the hardness of nanolayered Al/Pd films was higher than the hardness of both constituent monolayer films. For Set-I samples, enhancement of hardness up to 350% and modulus were observed for decreasing bilayer-thickness to 2 nm compared to the hardness of pure Al film. For Set-II samples, hardness enhancement of ~200% was observed compared to the hardness of pure Al film when Pd content was just 6.5% (vol./vol.). The validity of different strengthening mechanism models for Al/Pd multilayers is also discussed. Uniaxial compression testing of submicron-sized pillars milled in nanolayered Al/Pd thin films using FIB followed by SEM and XTEM analyses showed that the deformation behaviour of such pillars differs depending on bilayer-thickness, changing from dislocation driven plasticity at large bilayer-thickness to shear due to grain rotation via grain boundary sliding at small bilayer-thickness. The transition occurs at about bilayer-thickness = 20 nm where a mixture of the two mechanisms is apparent. The compositional stability of nanolayered Al/Pd structures in the XTEM sample, at room temperature, with respect to different layer thicknesses and time is also discussed.