Yvonne Gerbig

Instrumented indentation is a technique used for measuring the mechanical response of materials to deformation. The accuracy of these measurements relies on force and displacement values recorded during the indentation process. This paper presents a miniature fused silica parallelogram flexure to serve as a reference object for calibrating the applied indentation force based on Hooke’s law, F = kz, where k is the stiffness of the reference object and z is the displacement of the flexure under force F. This study discusses two independent experimental approaches to determine the stiffness of the flexure. In both cases, the stiffness was calculated from the measured flexure displacement while known forces are applied. In the first method, the displacement is measured using a built-in fiber-based interferometer while reference masses of 100 mg and 200 mg are placed on the flexure. A stiffness of k = ( 80.3 ± 0.79 ) k N m − 1 is determined with this method. The second method uses a commercial indenter to apply a cyclic force of 100 mN magnitude on the flexure while recording the displacement of the indenter probe. Experiments using a 5 μm and a 50 μm conospherical tip estimate the stiffness to be k = ( 80.74 ± 0.79 ) k N m − 1 , and ( 80.55 ± 0.33 ) k N m − 1 respectively. Disclaimer: Certain commercial equipment, instruments are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

The strain field surrounding a spherical indentation in silicon is mapped in two dimensions (2-D) using electron backscatter diffraction (EBSD) cross-correlation and confocal Raman spectroscopy techniques. The 200 mN indentation created a 4 μm diameter residual contact impression in the silicon (001) surface. Maps about 50 μm × 50 μm area with 128 pixels × 128 pixels were generated in several hours, extending, by comparison, assessment of the accuracy of both techniques to mapping multiaxial strain states in 2-D. EBSD measurements showed a residual strain field dominated by in-surface normal and shear strains, with alternating tensile and compressive lobes extending about three to four indentation diameters from the contact and exhibiting two-fold symmetry. Raman measurements showed a residual Raman shift field, dominated by positive shifts, also extending about three to four indentation diameters from the contact but exhibiting four-fold symmetry. The 2-D EBSD results, in combination with a mechanical-spectroscopic analysis, were used to successfully predict the 2-D Raman shift map in scale, symmetry, and shift magnitude. Both techniques should be useful in enhancing the reliability of microelectromechanical systems (MEMS) through identification of the 2-D strain fields in MEMS devices.

Indentation-induced plastic deformation of amorphous silicon (a-Si) thin films was studied by in situ Raman imaging of the deformed contact region of an indented sample, employing a Raman spectroscopy-enhanced instrumented indentation technique. Quantitative analyses of the generated in situ Raman maps provide unique insight into the phase behavior of as-implanted a-Si. In particular, the occurrence and evolving spatial distribution of changes in the a-Si structure caused by processes, such as polyamorphization and crystallization, induced by indentation loading were measured. The experimental results are linked with previously published papers on the plastic deformation of a-Si under hydrostatic compression and shear deformation to establish a sequence for the development of deformation of a-Si under indentation loading. The sequence involves three distinct deformation mechanisms of a-Si: (1) reversible deformation, (2) increase in coordination defects (onset of plastic deformation), and (3) phase transformation. Estimated conditions for the occurrence of these mechanisms are given with respect to relevant intrinsic and extrinsic parameters, such as indentation stress, volumetric strain, and bond angle distribution (a measure for the structural order of the amorphous network). The induced volumetric strains are accommodated solely by reversible deformation of the tetrahedral network when exposed to small indentation stresses. At greater indentation stresses, the increased volumetric strains in the tetrahedral network lead to the formation of predominately fivefold coordination defects, which seems to mark the onset of irreversible or plastic deformation of the a-Si thin film. Further increase in the indentation stress appears to initiate the formation of sixfold coordinated atomic arrangements. These sixfold coordinated arrangements may maintain their amorphous tetrahedral structure with a high density of coordination defects or nucleate as a new crystalline β-tin phase within the a-Si network.