Initial Contact Behavior of Nanograined Ni-25at.%Al Film During Nanoindentation
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Initial Contact Behavior of Nanograined Ni-25at.%Al Film During Nanoindentation Han. Li and Alfonso H. W. Ngan Department of Mechanical Engineering, the University of Hong Kong Pokfulam Road, Hong Kong, P. R. China
ABSTRACT Cyclic indentation was performed on standard fused quartz, single crystal Ni3Al (111) and nanocrystalline Ni-25at. %Al alloy thin film with average grain size of a few nanometers. For the thin film sample, it is found the scattering of the effective Young’s modulus at small depths goes far beyond the expectation from effects due to surface roughness alone. Three representative deformation mechanisms during initial contact stage were identified to be responsible for the scattering with the assistance of immediate pre and post indentation atomic force microscopy imaging. Furthermore, repeated loading was found to stiffen the thin film sample, but not the bulk ones. I. INTRODUCTION The successful design and overall reliability of novel micro and nanoscale devices rely on the accurate knowledge of the mechanical behavior and thorough understanding of the underlying operative deformation mechanisms of the materials from which they are built. Depth-sensing indentation has been widely used to address this issue due to its capability to study highly localized deformation in samples with small volumes or in the form of thin films on substrates [1-3]. The current study investigates the indentation deformation of a nanocrystalline alloy thin film during the initial contact stage, where the contact size is comparable with the grain size. With immediate pre and post indentation atomic force microscopy (AFM), topography changes are correlated with the indentation curves. The aim is to identify deformation mechanisms responsible for the observed scatter of the mechanical properties in nanocrystalline materials. II. EXPERIMENTAL PROCEDURES The experimental material is a nanocrystalline Ni-25at.%Al alloy thin film with average grain size of about 5-10 nm. The thin film was prepared by dc-magnetron sputtering on nickel substrates. The film thickness was about 3 µm and further details of this film can be found in our earlier work [4-7]. The film deposited without in situ substrate heating is nanocrystalline with a face-centered-cubic lattice structure, and the grain size can be controlled by suitable post-deposition heat treatment [4,7]. The film used in this work was in the as-deposited state.
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The roughness of the film sample has been determined by AFM to be less than 20 nm over a scanning range of 5 µm. Two bulk samples, namely, a fused quartz standard specimen and a (111) oriented Ni3Al single crystal, were also used as control. The roughness values of these control samples were 1 nm and 2 nm respectively over a scanning range of 5 µm. Indentation experiments were carried out at ambient temperature using a Hysitron TriboScope® nanoindenter mounted on a Thermomicroscopes® scanning probe microscope. To preclude any possible substrate influence on the indentation response, the maximum indentation
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