A study of the mechanics of microindentation using finite elements
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In this paper the finite element method is used to explore the mechanics of the microindentation process. In the simulations discussed, aluminum and silicon are investigated both in their bulk forms and in thin film-substrate combinations. Among the quantities readily computed using this approach and given in this paper are hardness (computed using actual contact area), contact stiffness, effective composite modulus, and surface profile under load. Importantly, this investigation builds on previous work by providing a more critical examination of the amount of pileup (or sink-in) around the indenter in the fully loaded configuration, as well as the variation of the actual contact area during indenter withdrawal. A key conclusion of this study is that finite element simulations do not support the widely used assumption of constancy of area during unloading (for either bulk materials or thin film systems). Furthermore, the amount of pileup or sink-in can be appreciable. The implication of these findings is that for many situations the commonly used straight-line extrapolation of a plastic depth may render an estimate for the contact area that is quite distinct from the actual area. This assertion is demonstrated herein through comparison of hardnesses calculated using actual contact area with values calculated using the straight-line extrapolation of plastic depth.
I. INTRODUCTION In recent years, the mechanical behavior of very thin films on substrates has been a topic of increasing interest, in great part due to applications in magnetic data storage and retrieval, integrated circuit design, the design of ultra smooth reflective coatings, and countless other situations. The determination of mechanical properties (hardness, modulus, etc.) of the film in situ is in general quite difficult for structures of the type mentioned, largely because of the extremely thin films involved (often having thicknesses as small as a few nm). In response to this situation, a variety of experimental techniques have been proposed for thin film property determination. Among the most important of these is the microindentation test. As is the case with more traditional hardness testing techniques, the microindentation test is capable of measuring the amount of force required to push a nearrigid indenter a given distance into a bulk material or film system. In fact, modern experimental setups perform this function very accurately by providing continuous traces of indenter load versus indentation depth.1'2 These data can be converted to a measure of average pressure (hardness) induced under the indenter tip, provided the area of contact between indenter and material is known. To do this requires some assumptions on the relation between the contact area and the actual measurable physical parameters, since the contact area itself (during the actual indent) is not in general available to the experimentalist. Additionally, the modulus of the thin 618 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 3, Mar 1992 Downloaded: 27 May 2014
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