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M.V. Swain Biomaterials Science Research Unit, Department of Mechanical and Mechatronic Engineering and Faculty of Dentistry, The University of Sydney, Eveleigh, NSW 1430, Australia

P. Munroe Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia (Received 27 December 2000; accepted 5 March 2001)

The mechanical deformation of crystalline silicon induced by micro-indentation has been studied. Indentations were made using a variety of loading conditions. The effects on the final deformation microstructure of the load–unload rates and both spherical and pointed (Berkovich) indenters were investigated at maximum loads of up to 250 mN. The mechanically deformed regions were then examined using cross-sectional transmission electron microscopy (XTEM), Raman spectroscopy, and atomic force microscopy. High-pressure phases (Si-XII and Si-III) and amorphous silicon have been identified in the deformation microstructure of both pointed and spherical indentations. Amorphous Si was observed using XTEM in indentations made by the partial load–unload method, which involves a fast pressure release on final unloading. Loading to the same maximum load using the continuous load cycle, with an approximately four times slower final unloading rate, produced a mixture of Si-XII and Si-III. Slip was observed for all loading conditions, regardless of whether the maximum load exceeded that required to induce “pop-in” and occurs on the {111} planes. Phase transformed material was found in the region directly under the indenter which corresponds to the region of greatest hydrostatic pressure for spherical indentation. Slip is thought to be nucleated from the region of high shear stress under the indenter.

I. INTRODUCTION

The response of crystalline silicon to indentation has been a topic of interest for many years, and it is now well established that this material undergoes a series of hysteretic phase changes when subjected to high pressures.1–5 Diamond anvil techniques have been used to study these high-pressure phases under hydrostatic loading conditions. Diamond cubic Si-I, transforms to the metallic ␤–Sn phase, Si-II, involving a 22% increase in density at a pressure of 11.3–12.5 GPa.6 At higher pressures, Si undergoes further transformations, forming a primitive hexagonal phase at approximately 16 GPa. Work by Gilman,7 indicating shear stress can lower transformation thresholds, is especially significant for indentation loading where high shear stresses can be present.8 Upon pressure release, rhombohedral (r8) Si-XII, bodycentered-cubic (bc8) Si-III, and the amorphous phase have all been reported to form.9,10 a)

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J. Mater. Res., Vol. 16, No. 5, May 2001

A number of authors have reported discontinuities in both the loading and unloading portions of the force– displacement curves of silicon.11–13 The loading discontinuity (“pop-in”) is reported to be caused by the Si-I to Si-II transformation. The discontinuity on unloading (“pop-out”), has been attributed to the tr