Gaurav Mohanty

Elevated temperature, strain rate jump microcompression of nanocrystalline nickel

Nanocrystalline and ultrafine-grained materials show enhanced strain rate sensitivity (SRS) in comparison to their coarse grained counterparts. Majority of SRS measurements on nanocrystalline thin films reported in literature have focused on nanoindentation-based approaches. In this paper, micropillar strain rate jump tests were demonstrated on an electrodeposited nanocrystalline nickel film from 25 to 100 °C. SRS exponent, m, and activation volume, V, values were determined as a function of temperature. The measured values were found to be in good agreement with previously reported literature on bulk and nanoindentation measurements. Apparent activation energy for deformation was found to be about 100 kJ/mol, which is close to that for grain boundary diffusion in nickel. Grain boundary sliding was observed in the deformed pillars from scanning electron microscopy images.

Ultrafast elemental mapping of materials combinatorial libraries and high-throughput screening samples via pulsed glow discharge optical emission spectroscopy

Combinatorial and high-throughput techniques have become well-established in materials science to aid in the accelerated discovery of novel functional materials. Thin film composition spreads are frequently used for this purpose by performing localized measurements and exploring different properties throughout the phase diagram. Nevertheless, the thin film composition map has to be determined to understand the behavior of the compound or alloy of interest. There are several established techniques that can give elemental composition maps of large areas but the total mapping time can range from several hours to tens of hours or more. Thus, faster elemental mapping techniques are needed. In this study, the applicability of GDOES elemental mapping towards combinatorial and high-throughput screening samples is explored by characterizing a CuNi thin film composition spread. Qualitative analysis images show that Cu and Ni composition gradients can be obtained in a matter of seconds. In addition, the use of reference materials allowed quantitative elemental analysis maps to be extracted from the emission intensity images. The images of emission intensities and curve fitting parameters are discussed. Finally, the GDOES results are compared to profiles obtained via EDX. The GDOES figures-of-merit relevant to elemental mapping are contrasted against selected alternative techniques from the literature to put the potential impact into perspective.

Image denoising techniques applied to glow discharge optical emission spectroscopy elemental mapping

Glow discharge optical emission spectroscopy (GDOES) is becoming a mature technique for depth profiling analysis. The advantages it affords, including fast, quantitative, and multi-elemental analysis, as well as allowing very high depth-resolution, have attracted the attention of the thin film community. Recently, the use of GDOES under pulsed-mode operation and coupled to hyper-spectral imaging techniques has been proposed to perform surface elemental mapping. Several manuscripts have reported on the underlying mechanisms in GDOES pertaining to the spatial resolution, while other manuscripts have reported on elemental mapping applications, for example, regarding separated proteins or thin film combinatorial libraries. Only a couple of studies have reported image processing techniques applied to GDOES elemental mapping and none having to do with image denoising purposes. Herein, image denoising techniques are compared in several scenarios: (a) mapping of homogeneous samples; (b) mapping of heterogeneous samples in two dimensions; (c) mapping of heterogeneous samples in three dimensions. Denoising techniques compared include averaging, median filtering, principal component analysis (PCA), and local pixel grouping-PCA. The peak signal-to-noise ratio is used to show the efficiency of noise removal, while the full-with-half-maximum of emission from sharp features is used to demonstrate the resolution degradation effects of each denoising technique. In general, it is observed that PCA outperforms other techniques albeit with a higher cost of image processing time. Also, it becomes evident that having multiple image slices (a 3rd dimension) affords more efficient noise removal while minimizing losses in spatial resolution.

Microscale resolution fracture toughness profiling at the zirconia-porcelain interface in dental prostheses

The high failure rate of the Yttria Partially Stabilized Zirconia (YPSZ)-porcelain interface in dental prostheses is influenced by the micro-scale mechanical property variation in this region. To improve the understanding of this behavior, micro-scale fracture toughness profiling by nanoindentation micropillar splitting is reported for the first time. Sixty 5 μm diameter micropillars were machined within the first 100 μm of the interface. Berkovich nanoindentation provided estimates of the bulk fracture toughness of YPSZ and porcelain that matched the literature values closely. However, the large included tip angle prevented precise alignment of indenter with the pillar center. Cube corner indentation was performed on the remainder of the pillars and calibration between nanoindentation using different tip shapes was used to determine the associated conversion factors. YPSZ micropillars failed by gradual crack propagation and bulk values persisted to within 15 μm from the interface, beyond which scatter increased and a 10% increase in fracture toughness was observed that may be associated with grain size variation at this location. Micropillars straddling the interface displayed preferential fracture within porcelain parallel to the interface at a location where nano-voiding has previously been observed and reported. Pure porcelain micropillars exhibited highly brittle failure and a large reduction of fracture toughness (by up to ~90%) within the first 50 μm of the interface. These new insights constitute a major advance in understanding the structure-property relationship of this important bi-material interface at the micro-scale, and will improve micromechanical modelling needed to optimize current manufacturing routes and reduce failure.