Effect of indentation unloading conditions on phase transformation induced events in silicon
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More than 2500 indentations were made on a silicon wafer surface using a range of different unloading rates and maximum applied loads. The unloading curves were examined for characteristic events (pop-out, kink pop-out, elbow followed by pop-out, and elbow) that were assigned to different phase transitions within the affected material based on Raman microspectroscopy analysis of residual imprints. The effect of unloading rate and maximum applied load on the average contact pressure at the beginning of the event was found. A permissible range for each event to occur was established.
I. INTRODUCTION
The response of single-crystal silicon to loading and unloading during indentation has been widely researched during the past decade.1 It has been established that discontinuities in its load–displacement curve under various loading conditions appear, referred to as pop-in and popout events.1–3 The pop-in event in silicon has been seen for spherical indentations2,4 but has never been reported for indentations created by sharp tools. The popout event has been seen for both spherical and sharp indentation, however.2,4,5 Furthermore, the influence of a size effect for this phenomena within the microscale has been shown to not exist.6 These events have also been seen to occur multiple times in unloading curves during cyclic loading conditions.7 It was suggested that the pop-in event for spherical indenters is the result of a phase change from cubicdiamond Si-I to metallic -tin Si-II,2,8 which is known to occur between 11.3 and 12 GPa under hydrostatic loading in high-pressure cells.9,10 The pop-in discontinuity is reasoned to occur because there is a sudden volume change when this critical pressure is reached, as the volume decrease from Si-I to Si-II is about 22%. This transition would be at an extremely low load for sharp indenters because the contact pressure quickly exceeds this critical value, and the transformation to Si-II occurs at shallow depths, starting from the very beginning stages of the indentation if the indenter is perfectly sharp. However, the pop-in discontinuity has also been found for Fe–3 wt.% Si and other materials and has been shown to be caused by yielding by dislocation nucleation.11 For a)
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J. Mater. Res., Vol. 18, No. 5, May 2003 Downloaded: 29 Nov 2014
sharp indenters, these pop-in effects have routinely been found at loads below about 2 mN. For the case of a pop-out event, it has been suggested that the pressurized Si-II phase suddenly transforms upon unloading into the rhombohedral r8 (Si-XII) phase. This is based on transmission electron microscopy (TEM) and Raman analysis of the residual imprints.5,8,12,13 It is known that the Si-II to Si-III phase change is accompanied by a 9% increase in volume. Upon further decompression, Si-XII transforms gradually into the body-centered-cubic bc8 (Si-III) phase according to high-pressure cell data.14 It was experimentally found that the resulting Si-III and Si-XII formed
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