Emission of Dislocation Loops at Zero Indentation Load in Adhesive Nanocontacts
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Nanoindentation has recently gained widespread acceptance as a technique for exploring the mechanics of incipient plasticity phenomena. During the advance of the indenter into the bulk material, the onset of plasticity is often characterized by discrete pop-in events associated with dislocation emission.[1] In a traditional Hertzian contact analysis, when adhesive effects are ignored, dislocation emission is understood to occur when the stresses reach a certain critical value following sufficient penetration of the indenter.[1] However, it has also been recognized that the contact mechanics of indentation and the evolution of the nanocontact zone can be strongly modified by the presence of adhesive interactions between the indenter and the bulk material. This phenomenon became vividly apparent during nanoindentation experiments wherein tips of Ni were advanced into Au surfaces and the modification of the mechanics of contact in this mutually adhesive system was studied.[2] More recently, during in-situ nanoindentation of Al in the transmission electron microscope (TEM),[3] it was found that discrete dislocation emission events occurred at negligible indenter loads much before the usual perfusion of dislocations associated with the pop-in events at the larger loads. The analysis presented M. RAVI SHANKAR, Assistant Professor, is with the Department of Industrial Engineering, University of Pittsburgh, Pittsburgh, PA 15261. R. MEENAKSHI SUNDARAM, Graduate Student, is with the Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, United Kingdom. Contact e-mail: shankarr@engr. pitt.edu Manuscript submitted September 5, 2007. Article published online August 23, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
in the article in Reference 3 justified this dislocation nucleation by calculating shear stresses from indenter loads (~1.5 lN) that were barely above the force noise floor and of the same order as the resolution of the indentation device (~0.5 lN). Here, we present an alternative paradigm wherein we demonstrate that emission of dislocations is energetically feasible, even at zero indentation loads, by drawing from the much more reliable observation of adhesive interaction between the indenter and the bulk. More broadly, it is noteworthy that such emission of dislocations at vanishing indentation loads offers a peculiar nanomechanical system wherein to explore the energy balance. Here, the release of the interfacial surface energy during the formation of the contact is transformed into crystal lattice defects that are then stored within the volume of the bulk material—a curious consequence of the novel mechanics associated with adhesive interactions. Prior to analyzing this energy conversion, it is useful to qualitatively recapitulate the salient mechanics of this interaction. When an indenter tip that is inherently adhesive to a bulk substrate approaches a flat surface, as illustrated in Figure 1, a finite contact region is known to form even prior to the application of any indentation load.[4] It is al
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