Identification of Pressure-Induced Phase Transformations Using Nanoindentation
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Identification of Pressure-Induced Phase Transformations Using Nanoindentation Vladislav Domnich1, Yury Gogotsi2 and Michael Trenary3 1 Department of Mechanical Engineering, University of Illinois at Chicago, 842 West Taylor Street, Chicago, IL 60607, USA. 2 Department of Materials Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA. 3 Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA. ABSTRACT A combination of depth-sensing indentation and Raman microspectroscopy has been used for the identification of pressure-induced phase transformations in silicon, germanium, boron carbide and partially stabilized zirconia single crystals. Phase transformations during nanoindentation may be revealed through deviations in the shape of the load-displacement curves from that of a perfect elastoplastic material. Such deviations are often more readily identified if the nanoindentation data are presented as average contact pressure vs. contact depth curves, allowing assessment of the corresponding transformation pressures. INTRODUCTION It is known that the indentation of materials with diamond indenters creates high stresses (hydrostatic and deviatoric) under the indenter that can cause phase transformations [1-4]. It has also been shown that the hardness correlates with metallization pressures for a number of semiconductors [5]. Unfortunately, the successful use of indentation technique to study phase transformations in materials is inhibited by difficulties associated with monitoring the process. Indirect monitoring of phase transformations during indentation through conductivity measurements [1-3, 6] was used in many of the indentation experiments. However, the conductivity measurements can only be used to detect metallic-nonmetallic transitions in materials and the obtained data are significantly affected by a particular experimental setup. Direct studies of the indentations have been conducted using transmission electron microscopy (TEM) [2, 7-9]. However, only amorphous material was repeatedly observed in the residual impressions. Thus, although numerous indirect indications of the structural changes in materials (mostly Si and Ge) during indentation have been observed, direct evidence of the phase transformations was still lacking. Recent advances in the use of Raman microspectroscopy to study indentation-induced phase transformations in semiconductors and ceramics [10-13] opened new horizons and substantially revived research activity in this area. For example, the most recent, carefully performed TEM experiments [14, 15] indeed revealed metastable high-pressure phases in the residual impressions on Si, confirming the Raman spectroscopy results. Much effort has also been put lately in understanding of the deformation behavior of Si and other semiconductors by combining the depth-sensing indentation data with the new experimental findings of Raman microspectroscopy [16, 17] and TEM [15]. In the present study, we examine the cyclic nanoindent
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