Nanoindentation: From forces to energies
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Nanoindentation: From forces to energies Sergi Garcia-Manyes, Pau Gorostiza and Fausto Sanz Research Center for Bioelectronics & Nanobioscience and Department of Physical Chemistry, University of Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain ABSTRACT We have performed nanoindentations using the tip of an AFM cantilever. Experiments have been made on a freshly etched hydrogen terminated Silicon (111) surface, yielding penetrations of less than 1 nm so that the onset of the plastic region(or yield threshold) determination have been carefully investigated. Furthermore, other nanomechanical properties such as hardness have been also investigated. The energy involved in such indentation processes where a nanometric deformation of the surface takes place, has been for the first time calculated by calibration against a non deformable surface, and a correlation with the atomic structure of the indented single crystal is inferred. INTRODUCTION Indentors have been widely used to measure the mechanical properties of materials. Besides, the improvements in developing more sensitive techniques have reduced the applied loads from the millinewton scale and penetrations in the micrometer scale [1] to the micronewton scale and penetrations of a few nanometers [2]. At this scale, the nanomechanical response often differs from the macroscopic one, and, in some cases, approaches theoretical limits. One of these examples is the doubtful meaning of hardness when penetration depths in the order of the nanometer are involved [3]. Thus, the nanometer depth scale is appropriate to accurately investigate the elastic behavior before plastic deformation, as well as the onset of this plastic deformation, the so-called yield threshold, followed by the irreversibly induced cavity formation on the studied surface. Recently [4, 5], we have proposed a simple model based on springs that takes into account the in-plane interactions on the indented surface during the elastic deformation, giving rise to amazingly good results for ionic compounds (NaCl, KBr and KCl), semiconductors (GaSe, InSe, GaS) as well as HOPG. Molecular dynamics simulations on ionic compounds [5] also gives support to the model conclusions[4]. We have now gone a step further by changing the studied magnitude from the applied force to the energy, that has more to do with ‘chemical bond sense’. Energy is the conservative magnitude that really explains the exchanging process in the perturbed surface. The conversion of the y-axis in the indentation curve from forces to energies is not straightforward, due to the fact that penetration is obtained by comparing the cantilever deflection and the total z-piezo displacement. In this novel results we have overcome these constrictions and we have been able to separate the total energy into the energy that is used to deflect the tip and the energy that the surface accumulates. The latter is actually the energy used either to break locally the atomic bonds or eventually to propagate dislocations. In this short presentation we have
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