A new insight on reversible deformation and incipient plasticity during nanoindentation test in MgO

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.