Vladislav Domnich

Effect of phase transformations on the shape of the unloading curve in the nanoindentation of silicon

Silicon wafers subject to depth-sensing indentation tests have been studied using Raman microspectroscopy. We report a strong correlation between the shape of the load-displacement curve and the phase transformations occurring within a nanoindentation. The results of Raman microanalysis of nanoindentations in silicon suggest that sudden volume change in the unloading part of the load-displacement curve (“pop-out” or “kink-back” effect) corresponds to the formation of Si–XII and Si–III phases, whereas the gradual slope change of the unloading curve (“elbow”) is due to the amorphization of silicon on pressure release. The transformation pressures obtained in nanoindentation tests are in agreement with the results of high pressure cell experiments.

Cyclic Nanoindentation and Raman Microspectroscopy Study of Phase Transformations in Semiconductors

This paper supplies new interpretation of nanoindentation data for silicon, germanium, and gallium arsenide based on Raman microanalysis of indentations. For the first time, Raman microspectroscopy analysis of semiconductors within nanoindentations is reported. The given analysis of the load-displacement curves shows that depth-sensing indentation can be used as a tool for identification of pressure-induced phase transformations. Volume change upon reverse phase transformation of metallic phases results either in a pop-out (or a kink-back) or in a slope change (elbow) of the unloading part of the load-displacement curve. Broad and asymmetric hysteresis loops of changing width, as well as changing slope of the elastic part of the loading curve in cyclic indentation can be used for confirmation of a phase transformation during indentation. Metallization pressure can be determined as average contact pressure (Meyers hardness) for the yield point on the loading part of the load-displacement curve. The pressure of the reverse transformation of the metallic phase can be measured from pop-out or elbow on the unloading part of the diagram. For materials with phase transformations less pronounced than in Si, replotting of the loaddisplacement curves as average contact pressure versus relative indentation depth is required to determine the transformation pressures and/or improve the accuracy of data interpretation.

Effect of indentation unloading conditions on phase transformation induced events in silicon

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.

Nanoindentation and Raman spectroscopy studies of boron carbide single crystals

The measurements of hardness and elastic modulus have been conducted on the (0001) and (1011) faces of B4.3C single crystals using nanoindentation. The results are in good agreement with the corresponding values obtained using a conventional microhardness technique on polycrystalline ceramics. Raman microspectroscopy analysis of the nanoindentations shows the appearance of several bands which suggest dramatic structural changes in the indented material. Localized contact loading may lead to damage in boron carbide resulting in disorder or a pressure-induced solid state phase transformation in the region under the indenter, although the exact mechanism responsible for the observed Raman spectra could not be identified at this time. This may explain why little variation in mechanical properties was observed with respect to the crystallographic orientation.

High-resolution transmission electron microscopy study of metastable silicon phases produced by nanoindentation

Plan-view transmission electron microscopy (TEM) and Raman microspectroscopy were used to identify metastable silicon phases in nanoindentation. A mixture of metastable Si-III and Si-XII phases was observed by both selected area diffraction in TEM and Raman analysis. High resolution TEM observations provided detailed structural information about the metastable phases of silicon and the interfaces between different silicon structures. A mechanism of dislocation-induced lattice rotation that leads to a phase transition and distortion-induced amorphization in nanoindentation is proposed.

Examining pressure-induced phase transformations in silicon by spherical indentation and Raman spectroscopy: A statistical study

Unloading rate and maximum load have been previously shown to affect the response of silicon to sharp indentation, but no such study exists for spherical indentation. In this work, a statistical analysis of over 1900 indentations made with a 13.5-μm radius spherical indenter on a single-crystal silicon wafer over a range of loads (25–700 mN) and loading/unloading rates (1–30 mN/s) is presented. The location of “pop-in” and “pop-out” events, most likely due to pressure-induced phase transformations, is noted, as well as pressures at which they occur. Multiple occurrences of pop-in and pop-out events are reported. Raman micro-spectroscopy shows a higher intensity of metastable silicon phases at some depth under the surface of the residual impression, where the highest shear stresses are present. A stability range for Si-II is demonstrated and compared with previous results for Berkovich indentation.

Thermal stability of metastable silicon phases produced by nanoindentation

Raman spectroscopy and transmission electron microscopy are used to investigate the temperature effects on the stability of metastable silicon phases (Si–III and Si–XII) produced by nanoindentation. It is found that the thickness of the specimen beneath the residual imprint plays an important role in the phase transformation sequence during heating up to 200 °C. Amorphization is preferred in nanoindents located in thin and loosely constrained areas; formation of Si–IV from Si–III/Si–XII is observed in the residual imprints located in the areas with an intermediate thickness; and the formation of an unidentified “Si–XIII” structure, which precedes the formation of Si–IV, is observed in nanoindents constrained by the bulk wafer. The phase transformation sequence in the indented samples under annealing is established as follows: Si–XII→Si–III→Si–XIII (thick sample only)→a-Si or Si–IV→nanocrystalline Si–I→Si–I.

GaN nanoindentation: A micro-Raman spectroscopy study of local strain fields

We have investigated strain fields around GaN nanoindentations. Stress relaxation around the edges of the nanoindentation was evident in atomic force microscopy images. More detailed information on the strain fields was obtained from Raman scattering, which has been used to analyze the shape of the strain field around the indentation. We find that the Berkovich tip giving a triangular imprint on the sample generates a strain field, which represents a hexagonal pattern. Negative values of the strain indicate that the residual stress is compressive. Strain is larger in the center of the indentation than outside. Analysis of the ratio of the frequency shift of the E2 and A1(LO) modes suggests that the residual strains are close to biaxial state outside the indentation contact zone, and mostly hydrostatic within the indentation center.

High Pressure Surface Science and Engineering

Introduction. High Pressure Surface Science and Engineering - A New Area of Research (Y Gogotsi, Drexel University, USA) Chapter 1 Phase Transitions Induced by Mechanical Compression (JJ Gilman, University of California at Los Angeles, USA) Chapter 2 Simulation of Pressure-Induced Phase Transformations 2,1 Contact Mechanics Models Accounting for Phase Transformations (BA Galanov and V. Kindrachuk, National Academy of Sciences, Ukraine) 2,2 Molecular Dynamics Simulation of Phase Transformations in Monocrystalline Silicon (L Zhang and WCD Cheong, University of Sydney, Australia) 2,3 High-Pressure Phases of Group IV and III-V Semiconductors (G Ackland, University of Edinburgh, UK) Chapter 3 Continuum Mechanical Fundamentals of Mechanochemistry (VI Levitas, Texas Tech University, USA) Chapter 4 Experimental Techniques in High Pressure Surface Science 4,1 Depth-Sensing Nanoindentation (A Fischer-Cripps, CSIRO, Australia) 4,2 Nanomechanical Characterization of Ceramic Materials (B Bhushan, Ohio State University, USA X Li, University of South Carolina, USA) 4,3 Raman Microspectroscopy (V Domnich and Y Gogotsi, Drexel University, USA) 4,4 Transmission Electron Microscopy (D Ge and Y Gogotsi, Drexel University, USA) Chapter 5 Experimental Studies of Phase Transformations Induced by Contact Loading 5,1 Indentation Induced Phase Transformations in Semiconductors (V Domnich, D Ge and Y Gogotsi, Drexel University, USA) 5,2 Indentation Induced Phase Transformations in Ceramics (V Domnich and Y Gogotsi, Drexel University, USA) 5,3 Zirconia Ceramics: Phase Transformations and Micro Raman Spectroscopy (M Dorn and KG Nickel, University of Tubingen, Germany) 5,4 Phase Transformations Under Dynamic Loading (T Juliano, V Domnich and Y Gogotsi, Drexel University, USA) Chapter 6 Ductile Regime Machining of Semiconductors and Ceramics (J Patten and H Cherukuri, University of North Carolina at Charlotte, USA J Yan, Kitami Institute of Technology, Japan)

Phase transformations in silicon under dry and lubricated sliding.

Sliding friction and wear mechanisms of silicon/silicon nitride test pairs were investigated under conditions of both dry and lubricated sliding. High-resolution surface topography mapping and electron microscopy studies revealed that microfracture was the predominant wear mechanism under dry and grease-lubricated sliding conditions. Raman spectroscopy suggested that in certain areas of the sliding contact, silicon underwent phase transformation and reached a metallic state because of high contact pressures. The extent of phase transformation was greater during the very early stages of the run-in period than during steady-state sliding regimes. The use of grease and oil as lubricants led to a substantial reduction in friction and greatly diminished wear due to microfracture. Furthermore, almost all areas on Si surfaces subjected to lubricated sliding contact underwent pressure-induced phase transformation. Both amorphous material and crystalline Si phases were identified by Raman spectroscopy. The experimental observations suggested that the wear process in lubricated sliding contacts was mainly dominated by the formation of a ductile metallic Si phase and subsequent removal of the transformed layers. The results of this study demonstrate that pressure-induced phase transformation must be taken into account when considering possible wear mechanisms of silicon in contact with other hard materials, inasmuch as it contributes notably to the wear of silicon under lubricated sliding. Presented at the 56th Annual Meeting in Orlando, Florida May 20–24, 2001