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Fig. 1: During hardness indentation, there is a transformation to either amorphous or crystalline high-pressure phases at locations of maximum stresses.

225 Mat. Res. Soc. Symp. Proc. Vol. 499 01998 Materials Research Society

Especially in semiconductors, there is a transformation to metallic high-pressure phases during hardness indentation, followed by further transformations to metastable phases during pressure release [2-7]. Indentation-induced phase transformations and amorphization effects can also be observed in ceramic materials and minerals. The possibility of phase transformations under contact loading has been known for years [2]. It was also shown that extremely high pressures could be achieved using a diamond anvil and a diamond indenter [8]. However, the use of this technique in high-pressure research was hindered by the difficulties with the characterization of transformation products in the small (several microns) area of contact. By combining hardness indentation testing and micro-Raman spectroscopy, we developed a simple technique for examining contact-induced phase transformations. In the present paper, we show some examples for contact-induced phase transformations in various materials in order to demonstrate the usefulness of this new and simple investigation technique. EXPERIMENT As a model case of mechanical contact, we chose hardness indentation devices, which are commonly used in materials science laboratories. Numerous indentations were produced on the polished sample surfaces using Vickers (tetragonal diamond pyramid) and Rockwell (spherical diamond tip of 0.2 mm radius) type indenters (Fig. 2a). The samples were then examined with a micro-Raman spectrometer (LabRam, Dilor, France) using the 632.8 nm excitation line of a HeNe laser and the 514.5 nm line of an Ar÷ laser (Fig. 2b). The main advantage of the experimental technique is the simplicity of both sample preparation and measurement, because only a small area of a sample surface is needed to produce the indentations. For micro-Raman investigations no further sample preparation is required. During the Raman observations, the laser intensities were kept low to avoid further transformations or chemical reactions in the samples. Despite of this, we could easily conduct Raman spectral analyses of regions as small as 1 pm in diameter. The time needed to record a Raman spectrum ranged from 1 to 60 seconds.

Spectrometer

Vickers

Rockwell

(a)

(b)

Hardness impression

Fig. 2: Sketch of the experimental procedure. A number of hardness indentations were produced (a), which then were examined with a microRaman spectrometer (b).

226

RESULTS Silicon and Germanium From DAC experiments, Si and Ge are known to undergo a transition to the metallic P3-Sn structure upon compression to 10-13 GPa and 8-10 GPa respectively. This is nearly the same value as the Vickers hardness of these materials [4,5]. However, the metallic phases are not stable at ambient pressure and further metastable phases form during pressure release. From measurements of th