3D FIB and AFM mapping of Nanoindentation Zones
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3D FIB and AFM mapping of nanoindentation zones T.J. Steer, G. Möbus, O. Kraft*, T. Wagner* and B.J. Inkson Department of Materials, Oxford University, Parks Road, Oxford, OX1 3PH, UK. *Max-Planck-Institut für Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany. ABSTRACT A novel technique has been developed to examine site-specific, subsurface microstructures in three dimensions. A 3D data set is collected by successive cross-sectional slicing using a gallium focused ion beam (FIB) and imaging using ion-induced secondary electrons, enabling a 3D microstructure map to be generated using computer-based reconstruction techniques. In the first instance, this 3D FIB mapping technique has been applied to copper-based epitaxial metal multilayer coatings which have been deformed by nanoindentation. It is possible to produce 3D profiles of the deformed subsurface interfaces. These individual interface maps allow analysis of the deformation in terms of both the thickness of individual layers and that of the entire film. Material flow, which is seen as pile-up and residual indent zones around the indent, can thus be precisely characterised. The site at which the sectioning is to be carried out can be chosen with high spatial resolution; consequently, nanoscale mechanical properties can be linked directly with an area’s microstructure. In an attempt to examine the errors involved in this 3D mapping method the 3D FIB map of the surface of a residual indent has been compared to an atomic force microscopy (AFM) scan of the same region. The sources and significance of the errors are discussed with reference to ways in which they might be reduced. INTRODUCTION The technique of nanoindentation is widely used to investigate the mechanical properties of nanostructured materials [1]. The evolution of the microstructure around the nanoindentation site and the related mechanical response to the deformation are of great interest. FIB has been used to cut and view 2D cross-sections from underneath Knoop microindentations in multilayers [2,3]. In addition to direct FIB analysis of the area underneath indent sites [4], site-specific TEM samples have been produced using FIB [5,6]. We have previously carried out 3D investigations into surface and sub-surface structure evolution under nanoindentations using FIB [7]. This was achieved by taking and aligning a sequential set of cross-sectional 2D images from an indent site. In this paper we have applied the method of 3D FIB analysis to nanoindentation deformation (Berkovich tip) of a Cu-Ti bilayer grown on an Al2O3 substrate. The surface profile of a residual indent as given by this method is compared to that obtained by AFM in order to examine the errors present in the subsurface FIB maps. EXPERIMENTAL METHOD A Cu-Ti bilayer was grown on the (0001) basal plane of an Al2O3 single crystal substrate using UHV molecular beam epitaxy (DCA Instruments). The growth was monitored by in-situ RHEED Q3.7.1
investigations. The Ti and Cu layers grew- with the following epitaxial orientation relations
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