A new insight on reversible deformation and incipient plasticity during nanoindentation test in MgO
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In this study, nucleation of dislocations in magnesium oxide (MgO) during nanoindentation with a spherical indenter is investigated. For flat and defect-free surfaces prepared by chemo/mechanical polishing, reversible load–displacement curves have been obtained for load as high as 300 mN, whereas on a cleaved MgO surface, pop-in and plastic deformation occur at 10 mN with the same indenter. Furthermore, these reversible curves deviate from the Hertz contact theory. Indented areas have then been characterized by atomic force microscopy and nanoetching. In some cases, few slip lines are observed for reversible indentation tests. However, the slip lines position indicate that the nucleation process of the corresponding dislocations is different from that involved during a pop-in phenomenon.
I. INTRODUCTION
During the past two decades, nanoindentation has become a common technique for local determination of hardness and Young’s modulus.1 Combined with atomic force microscopy (AFM)2–4 or transmission electron microscopy (TEM),5,6 nanoindentation is also a powerful technique to study the elementary deformation mechanisms at the nanometer scale. During a nanoindentation test, penetration depth is monitored as the load is applied to the indenter (in the so-called force controlled mode). When the applied load is sufficient to generate plastic deformation, the typical load–displacement curve, or P–h curve (with P the applied load and h the penetration depth), exhibits some hysteresis. As a consequence, a residual imprint is produced on the surface. For lower loads, the deformation is elastic: the indentation curve is reversible and no residual imprint is left on the surface. The transition from reversible to elastoplastic deformation often occurs suddenly, leading to a displacement burst on the loading curve. This yielding, which is also called pop-in, is commonly associated with the homogeneous nucleation of dislocations.5,7,8 This hypothesis is justified by the small volume of material solicited by nanoindentation, which can be considered as free of pre-existing dislocations. This phenomenon has aroused considerable interest and attention in the past few years. For Page et al.,5 the pop-in was clearly associated to the first nucleation of dislocation. They used TEM to investigate the residual microstructure around indents in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0113 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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sapphire. For nanoindentation tests stopped before popin no dislocations were observed, whereas for nanoindentation curves showing a pop-in, a high density of dislocations was observed. Similarly, Gerberich et al.7 using AFM also reported that plastic yielding corresponds to the appearance of a pop-in in Fe–3%wtSi single crystals. It was also suggested that pop-in is a time-dependent phenomenon9,10: when the load is held at a value lower than that for spontaneous pop-in, a holding time is ne
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