Investigation of the deformation behavior in nanoindented metal/nitride multilayers by coupling FIB-TEM and AFM observat
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Investigation of the deformation behavior in nanoindented metal/nitride multilayers by coupling FIB-TEM and AFM observations G. Abadias, C. Tromas, Y.Y. Tse, A. Michel Laboratoire de Métallurgie Physique, UMR 6630, Université de Poitiers, SP2MI, Téléport 2, 86962 Chasseneuil-Futuroscope, FRANCE
ABSTRACT Epitaxial TiN/Cu bilayers and multilayers with periods Λ between 5 and 50 nm have been grown by ultrahigh vacuum ion beam sputtering on Si and MgO(001) substrates at room temperature. The deformation modes induced by a Berkovich nanoindent have been imaged using Focused Ion Beam – Transmission Electron Microscopy (FIB-TEM) and Atomic Force Microscopy (AFM). The observations suggest that the mechanical response of the multilayers is essentially governed by an extensive plastic flow inside the Cu layers, which is confined by a bending of the more rigid TiN layers. This specific deformation behavior, with no contribution of the interfaces as a barrier for dislocation motion could explain the absence of significant hardness enhancement in this system. INTRODUCTION Transition-metal nitride thin films (such as TiN, ZrN, or VN) are a technologically important class of materials due to their mechanical properties as well as chemical stability. Combining different nitrides or coupling nitrides with metals in the form of nanocomposites or superlattices offers new opportunities to design superhard (hardness > 40 GPa) coatings [1-4]. Enhanced hardness with respect to the monolithic constituent materials has been reported in many multilayered structures, such as Mo/NbN [5,6], W/ZrN [4,5] or TiN/NbN [3,7,8], when the bilayer period (Λ) decreases down to the nanoscale. These experimental findings have been mainly explained by invoking Koehler’s original model [9]. This theory, revisited later by Chu and Barnett [10], is based on dislocation hindrance at sharp interfaces between nanometer thin epitaxial layers of materials with a large difference in elastic shear moduli. Other analyses refer to deformation processes based on confined plasticity, where dislocations bow within individual layers, but this behavior concerns essentially the case of metal/metal multilayers [11-13]. However, the mechanical deformation modes in nitride nanolayers during indentation remain poorly understood, especially regarding the role of dislocation motion. Only few studies [14-17] have reported direct experimental observations of dislocation activity in these films to validate theoretical mechanisms. This is mainly due to the difficulty in preparing samples with appropriate imaging conditions in the transmission electron microscope (TEM), due to small grain sizes and high defect densities in the as-deposited films. Recently, it has been shown that the Focused Ion Beam (FIB) is a well suited technique for the preparation of TEM cross-section specimens from samples that have been locally deformed by nanoindentation [18]. Additionally, hardening is not always observed in nanoscale multilayers [15,19,20], suggesting that the deformation acro
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