Combining hardness indentation tests and micro-Raman spectroscopy it is shown that metallic Si-II is produced near the interface of a diamond indenter and silicon to a depth of about 0.5 μm, where the highest stresses (hydrostatic and deviatoric) exist. At fast unloading rates Si-II transforms to the amorphous state, whereas a mixture of the r8 high pressure polymorph Si-XII and the bc8 phase Si-III forms upon a slow load release. The region of Si-III+Si-XII is surrounded by the wurtzite structured Si-IV, where the stresses during the indentation had not been high enough to cause the transition to the metallic state. Thus, because of shear deformation a direct transformation to Si-IV takes place. Outside the phase-transformed regions the classical aspects of indentation-induced deformation by dislocation glide, twinning and crack formation are observed. Annealing of the high pressure phases leads to the formation of Si-IV at moderate temperatures and to the reversal to the original diamond structure (Si-I) at temperatures above 500 °C. Using the laser beam of the Raman spectrometer to anneal the samples the phase transitions could be monitored directly. The formation of silicon polymorphs other than amorphous or metallic structures during hardness indentation is, to the best of our knowledge, reported here for the first time. The results compare well with the polymorphism in Si that is known from diamond anvil cell experiments.
During hardness indentation, materials are subjected to highly localized stresses. These stresses not only cause crack formation and plastic deformation by dislocation gliding, but a complete change of the crystal structure and formation of amorphous phases or high-pressure polymorphs can occur in the zone of maximum contact stresses. Such contact-induced phase transformations were observed in hard and brittle materials including semiconductors (Si, Ge, GaAs and InSb) and common ceramic materials such as SiC and SiO 2 (a-quartz and silica glass). A prime tool for their investigation is the Raman microspectroscopy of hardness indentations.
In Si and Ge, there is an initial transformation to metallic high-pressure phases upon hardness indentation and a subsequent formation of crystalline, nanocrystalline, or amorphous phases depending on the conditions of the hardness test, in particular the unloading rate. A phase transformation occurs also in InSb, whereas the results for GaAs do not give sufficient evidence for phase transformations. Indentationinduced amorphization has been observed in SiC and quartz. Even diamond has been shown to undergo amorphization and phase transformation under nonhydrostatic stress conditions imposed by indentation tests.Plate 1. Light micrograph of (a) a Vickers impression in diamond, (b) typical Raman spectrum from the central part of the impression and Raman intensity images showing (c) distribution of diamond and (d) graphite. Colour change from dark blue to yellow indicates the increase of the Raman band intensity from zero to the maximum. (Excitation wavelength: 632.8nm)
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