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TEM Investigation of Nanoindentation Induced Deformation Structures in W/NbN Nanolaminates

M.F. Savage, D.E. Kramer, A. Lin and T. Foecke, S.A. Barnett

Understanding the deformation mechanisms in nanolaminated composites is an important step towards developing constitutive laws that describe their mechanical behavior. Several mechanisms have been proposed, however experimental corroboration is difficult due to the small length scales associated with these materials and limited means of mechanical testing. The nanoindentation test is an attractive choice as a mechanical probe due to the ease of sample preparation and available analysis procedures. The measure of a material's resistance to plastic deformation, the hardness, is often related to yield strength through Tabor's relationship, in which hardness is three times the yield stress, or according to Johnson's spherical cavity model. However, these models assume that the material is a homogeneous isotropic solid. Their applicability to nanolaminated composites is unknown. In this study, the deformation mechanisms in single crystal tungsten and epitaxial W/NbN superlattices with bilayer spacings from 5 nm to 30 nm are investigated by means of a novel indentation technique. Cross-section transmission electron microscopy is then used to study the deformation mechanisms, and compare the size and shape of the plastic zones underneath the contact. An example of such a cross-sectional micrograph is shown below.  Differences within the range of bilayer spacings and between the superlattices and tungsten single crystal will be discussed.