Indentation-Induced Damage Mechanisms in Germanium
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Indentation-Induced Damage Mechanisms in Germanium David J. Oliver1, Jodie E. Bradby1, Jim S. Williams1, Michael V. Swain2, Damien McGrouther3, and Paul Munroe3 1 Electronic Materials Engineering, Australian National University, Canberra, ACT 0200, Canberrra, 0200, Australia 2 Biomaterials Science Research Unit, University of Sydney, Sydney, Australia 3 Electron Microscope Unit, University of New South Wales, Sydney, 2052, Australia
ABSTRACT The response of crystalline Ge to indentation has been studied over a range of maximum loads. At a certain load, an unusual ‘giant pop-in’ event occurs, in which a discontinuous extension of >1 µm is observed in the force-displacement curve. In such cases, load release curves show a pronounced ‘elbowing’ response, leading to increased depth recovery. TEM and Raman microspectroscopy revealed the presence of amorphous material in the residual impression. To examine cracking, a sequence of cross-sections was milled through the indent and images taken using an automated method (the ‘slice-and-view’ method). Using 3-D reconstruction software, the data was segmented and reconstructed into a 3-dimensional representation of the cracks around the indent. Applying this technique to indents featuring a giant pop-in, it was deduced that the inelastic elbowing observed was a bending response of material detached by lateral cracking. The giant pop-in is attributable to material removal, caused by lateral cracks formed during loading.
INTRODUCTION Crystalline germanium (Ge) exhibits a wide range of responses to mechanical deformation with a sharp diamond indenter, including dislocation plasticity,1 twinning,1,2 phase transformations3 and at higher loads cracking.4,5 The present study has been prompted by the observation of novel features in the indentation force-displacement (P-h) curve of Ge. A sudden displacement excursion, or ‘pop-in’, of unusually large magnitude (1 µm or greater) was observed above a threshold load. This giant pop-in event was accompanied by a discontinuous change in slope, or ‘elbow’, on unloading. To understand the causes of these force-displacement features, the microstructural changes in the indent were examined using several FIB techniques, detailed below. The results indicate a material removal event is responsible for the giant pop-in, whilst the elbowing is due to the opening of lateral cracks during unloading. EXPERIMENTAL DETAILS Undoped crystalline Ge(100) was indented with a UMIS-2000 indenter (CSIRO), using a spheroconical diamond tip (R ≈ 4.3 µm). Some indents were created in a single load-unload cycle; other indents were created in a multiple loading fashion, in which the load-unload cycle was
repeated 5 times. Maximum loads ranged up to 400 mN. Loading rates of 1.0 – 1.5 mN.s-1 and unloading rates of 1.4 – 1.9 mN.s-1 were used. Raman spectra were collected from the centers of indents using a Raman microscope (Dilor Super LabRam), using the HeNe laser line of 632.8 nm, with a spot size of 1 µm2. The A low laser power (
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