In situ observation of the spatial distribution of crystalline phases during pressure-induced transformations of indente
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Indentation-induced phase transformation processes were studied by in situ Raman imaging of the deformed contact region of silicon thin films, using a Raman spectroscopy-enhanced instrumented indentation technique (IIT). In situ Raman imaging was used to study the generation and evolution of the phase transformation of silicon while performing an IIT experiment analyzed to determine the average contact pressure and indentation strain. This is, to our knowledge, the first sequence of Raman images documenting the evolution of the strain fields and changes in the phase distributions of a material while conducting an indentation experiment. The reported in situ experiments provide insights into the transformation processes in silicon during indentation, confirming, and providing the experimental evidence for, some of the previous assumptions made on this subject. The developed Raman spectroscopy-enhanced IIT has shown its potential in advancing the understanding of deformation mechanisms and will provide a very useful tool in validating and refining contact models and related simulation studies.
I. INTRODUCTION
Silicon (Si) in its pristine diamond cubic (dc) phase can be transformed to different crystallographic structures through the application of mechanical stress.1 Given its importance to electronic devices and microelectromechanical systems, and given that modifications of the Si crystallographic structure are connected to its performancerelated properties, Si phase transformations have been intensively studied. At hydrostatic pressures of 10–16 GPa, Si undergoes a nonmetallic–metallic transition, as the dc structure of Si-I transforms to the denser body centered tetragonal (bct) b-tin structure of the Si-II phase.2–4 Further compression leads to a sequence of transitions to other crystallographic phases.1 On release of the hydrostatic pressure at ambient temperatures, the high-pressure phases do not recover to the dc structure but rather to other metastable phases in secondary phase transition processes. Slow decompression from the b-tin phase leads to the rhombohedral r8 (Si-XII) phase at about 10 GPa, which subsequently transforms to the body-centered cubic bc8 (Si-III) phase at about 2 GPa.5 The possibility of generating large hydrostatic (and deviatoric) stresses when loading a sharp indenter onto sample surfaces, led researchers to suggest that a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.316 390
J. Mater. Res., Vol. 30, No. 3, Feb 14, 2015
http://journals.cambridge.org
Downloaded: 08 Sep 2015
indentation experiments could be used to study the phase transformation of materials,6 especially in the case of Si, for which the hardness and the pressure needed to initiate the transformation to the b-tin phase are very similar.7 Initial evidence that indeed phase transformation processes took place during indentation experiments in Si was a large change in the electrical resistivity in a thin layer surrounding the indent.7–9 With the introduction of instrumented
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