Numerical derivative analysis of load-displacement curves in depth-sensing indentation

Materials Research Society Symposium Proceedings, 791: pp. 191-202. Retrieved September 19, 2006 from http://nano.materials.drexel.edu/Papers/NumericalDerivativeAnal.pdf.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 A1sLOd modes suggests that the residual strains are close to biaxial state outside the indentation contact zone, and mostly hydrostatic within the indentation center

Locating Si atoms in Si-Doped Boron Carbide: a Route to Understand Amorphization Mitigation Mechanism

The well-documented formation of amorphous bands in boron carbide (B4C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure. To mitigate amorphization, Si-doping was suggested by an earlier computational work, which was further substantiated by an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing atom/s in B_(12) icosahedra or the C-B-C chain. Dense single phase Si-doped boron carbide was produced through a conventional scalable route. A powder mixture of SiB_6, B_4C, and amorphous boron was reactively sintered, yielding a dense single phase Si-doped boron carbide material. A combined analysis of Rietveld refinement on XRD pattern coupled with electron density difference Fourier maps and DFT simulations were performed in order to investigate the location of Si atoms in the boron carbide lattice. Si atoms occupy an interstitial position, between the icosahedra and the chain. These Si atoms are bonded to the chain end C atoms, which result in a kinked chain. Additionally, these Si atoms are also bonded to the neighboring equatorial B atom of the icosahedra, which is already bonded to the C atom of the chain, forming a bridge like structure. Owing to this bonding, Si is anticipated to stabilize the icosahedra through electron donation, which is expected to help in mitigating stress-induced amorphization. Possible supercell structures are suggested along with the most plausible structure for Si-doped boron carbide

Phase transformations in silicon induced by contact loading.

Phase transformations in silicon induced by contact loading

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

Journal of Materials Research, 19(10): pp. 3099-3108. Retrieved September 19, 2006 from http://nano.materials.drexel.edu/Papers/SphericalSiliconJMR.pdf. DOI: http://dx.doi.org/10.1557/JMR.2004.0403Unloading 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-mum 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