New evidence for a pressure-induced phase transformation during the indentation of silicon
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New evidence for a pressure-induced phase transformation during the indentation of silicon G.M. Pharr Department of Materials Science, Rice University, P.O. Box 1892, Houston, Texas 77251
W. C. Oliver Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
D. S. Harding Department of Materials Science, Rice University, P.O. Box 1892, Houston, Texas 77251 (Received 29 October 1990; accepted 6 February 1991)
Scanning electron micrographs of indents in (111) silicon reveal that a thin layer of material immediately adjacent to the indenter is plastically extruded. The fact that the material can be deformed in this way indicates that it has metallic-like mechanical properties. This is presented as new evidence that a pressure-induced phase transformation to the metallic state occurs during the indentation of silicon.
Recent studies have shown that the hardness of silicon is very close to the pressure needed to transform it from its normal, diamond cubic structure (4-fold coordinated) to the denser, /5—tin structure (6-fold coordinated). 1 " 5 Various values for the transformation pressure have been reported, with careful studies by Hu et al. narrowing in on 11.3-12.5 GPa as the range in which the transformation occurs under hydrostatic conditions.6 Gupta and Ruoff have shown that for nonhydrostatic conditions, the transformation pressure may be reduced to as low as 8 GPa.7 The transformation is accompanied by a large, reversible drop in electrical resistivity resulting from the fact that the /3-tin form of silicon has metallic electrical properties. The resistivity drop is frequently used in high pressure diamond anvil studies as an indicator that the transformation has taken place.7 What is interesting about the above values for the transformation pressure is that they are very close to the hardness of silicon, which is generally reported in the range 11-12 GPa. 3 " 5 ' 8 The fact that the hardness and the transformation pressure are so similar has led several investigators to suggest that a pressure-induced phase transformation takes place during the indentation of silicon, and several studies have been undertaken to confirm it.1'3"5 The primary supporting evidence is a large, reversible drop in electrical resistivity in a thin layer surrounding the indent. Such a drop has been reported by Gridneva et al.,1 who have estimated the J. Mater. Res., Vol. 6, No. 6, Jun 1991
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thickness of the layer to be about 0.05 /xm for Vickers indents made at 2 N (200 g), and by Clarke et al? Other evidence for the transformation is found in an unusually large hysteresis in indentation load-displacement curves. 45 The argument is that since the transformation is itself sluggish and hysteretic, that portion of the indentation displacement resulting from the transformation must also be hysteretic. It is worth noting that the displacement associated with the transformation may be quite large, since the transformation is accompanied by a 21% volume r
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