Spherical Load Indentation in Submicron NiTiCu Shape Memory Thin Films
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Spherical Load Indentation in Submicron NiTiCu Shape Memory Thin Films R. Hassdorf 1, J. Feydt1, S. Thienhaus1, L. Buforn2, N. Conté2, O. Pykhteev1, M. Kružík1,3, N. Botkin1, and M. Moske1 1 Center of Advanced European Studies and Research (caesar), D-53175 Bonn, Germany 2 CSM Instruments SA, CH-2034 Peseux, Switzerland 3 Institute of Information Theory and Automation, Academy of Sciences, CZ-182 08 Prague, Czech Republic ABSTRACT Nanoindentation with spherical tipped indenters provides a powerful technique to explore surface and thin film mechanical properties through the application of Hertzian contact mechanics. The full range of mechanical response can be obtained from elastic, through the yield point, to permanent deformation. In this study spherical indentation has been used for probing MBE-grown NiTiCu alloy thin films into superelasticity or stress-induced martensitic transformation. By this way, obstacles typically occurring related to the fabrication of freestanding films (film thickness < 1 µm) are avoided. The indentation measurements were performed starting from the parent austenite state. Notably, for loads as small as 0.5 mN, deformation appears to be completely reversible. As loading is increased (up to 5 mN) the indent becomes irreversible following local plastic deformation within the tip-specimen contact area. Using finite-element simulations the indentation data were converted into a stress-strain diagram aimed at simulating uniaxial tension load. Therefrom, the superelastic strain is estimated to be around 3%.
INTRODUCTION As microstructured thin film devices progressively come into focus for system control options in form of microactuators and sensors, it is increasingly demanding to locally test their mechanical properties. One class of materials with high prospects are shape memory and superelastic materials which can be vapor-deposited and microstructured by lithographical processes. High technological importance has already evolved in superelastic and shape memory medical devices and thermal actuators [1,2]. However, the characterization of nonlinear superelastic behavior, especially with shrinking size scale, now has become more and more demanding. In this respect, we present a study demonstrating the use of nanoindentation with spherical tipped indenters tested on MBE-grown submicron NiTiCu alloy thin films. By applying Hertzian contact mechanics, the full range of mechanical response can be obtained from elastic, through the yield point, to permanent deformation [3]. To date, just a few reports are known dealing with superelastic behavior of indented NiTi foils or sputter-deposited thin films [4–6]. In these studies, however, mostly pyramidal indenters have been employed which result in inaccuracies due to the development of pile-up during indentation and the occurrence of plastic strains even at small indentation depths. In an earlier study [7,8] we established that using MBE deposition the microstructure of NiTiCu alloy films is very different from conventionally sputter-depos
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