K. Wasmer

Plastic deformation of gallium arsenide micropillars under uniaxial compression at room temperature

The authors have experimentally investigated the compressive strength of GaAs pillars with a diameter of 1μm by uniaxial compression tests. The tests were performed at room temperature and, contrary to macroscopic tests, the micropillars were found to exhibit ductile plasticity comparable to that of metal single crystal micropillars. The yield stress was 1.8±0.4GPa and, for one pillar that was more closely examined, a total deformation of 24% was observed. In the diffraction patterns from transmission electron microscopy studies of this pillar, a high density of twins was observed.

Studying grain boundary regions in polycrystalline materials using spherical nano-indentation and orientation imaging microscopy

In this article, we report on the application of our spherical nanoindentation data analysis protocols to study the mechanical response of grain boundary regions in as-cast and 30% deformed polycrystalline Fe–3%Si steel. In particular, we demonstrate that it is possible to investigate the role of grain boundaries in the mechanical deformation of polycrystalline samples by systematically studying the changes in the indentation stress–strain curves as a function of the distance from the grain boundary. Such datasets, when combined with the local crystal lattice orientation information obtained using orientation imaging microscopy, open new avenues for characterizing the mechanical behavior of grain boundaries based on their misorientation angle, dislocation density content near the boundary, and their propensity for dislocation source/sink behavior.

In-situ SEM indentation studies of the deformation mechanisms in TiN, CrN and TiN/CrN.

In this study, the microstructure and the deformation mechanisms of TiN, CrN and multilayer TiN/CrN thin films on silicon substrates were investigated. Cross-sectional lamellas of nanoindents were prepared by focused ion beam milling to observe by transmission electron microscopy the microstructure of the as-deposited and deformed materials. TiN film exhibits nanocrystalline columns, whereas CrN shows large grains. The TiN/CrN multilayer presents microstructural features typical for both materials. A film hardness of 16.9GPa for CrN, 15.8GPa for TiN and 16.6GPa for TiN/CrN was found by the nanoindentation. Reduced modulus recorded for TiN and CrN reference coatings were 221.54 and 171.1GPa, respectively, and 218.6GPa for the multilayer coating. The deformation mechanisms were observed via in-situ scanning electron microscope nanoindentation. The TiN thin film showed short radial cracks, whereas CrN deformed through pile-up and densification of the material. For TiN/CrN multilayer pile-up and cracks were found. Transmission electron microscopy observations indicated that TiN deforms through grain boundary sliding and CrN via densification and material flow. The deformation mechanism observed in TiN/CrN multilayer was found to be a mixture of both modes.

Plastic deformation modes of gallium arsenide in nanoindentation and nanoscratching

The mechanical deformation by nanoindentation and scratching of gallium arsenide has been investigated using cross-sectional transmission electron microscopy. Twinning was found to be the main deformation process occurring during indentation while only slip bands and perfect dislocations are observed during scratching. This behavior is explained, in the authors’ experiments, with the strain rate in scratching being hundred times greater than in indentation. Hence, the low indentation velocity allows twins to be nucleated and propagated from surface inhomogeneities, whereas in scratching, the deformation occurs first in front of the indenter and the scratching speed allows only perfect α dislocation to propagate.

In situ compression tests on micron : sized silicon pillars by Raman microscopy-Stress measurements and deformation analysis

Mechanical properties of silicon are of high interest to the microelectromechanical systems community as it is the most frequently used structural material. Compression tests on 8 μm diameter silicon pillars were performed under a micro-Raman setup. The uniaxial stress in the micropillars was derived from a load cell mounted on a microindenter and from the Raman peak shift. Stress measurements from the load cell and from the micro-Raman spectrum are in excellent agreement. The average compressive failure strength measured in the middle of the micropillars is 5.1 GPa. Transmission electron microscopy investigation of compressed micropillars showed cracks at the pillar surface or in the core. A correlation between crack formation and dislocation activity was observed. The authors strongly believe that the combination of nanoindentation and micro-Raman spectroscopy allowed detection of cracks prior to failure of the micropillar, which also allowed an estimation of the in-plane stress in the vicinity of the crack tip.

Prediction of Failure in Lubricated Surfaces Using Acoustic Time–Frequency Features and Random Forest Algorithm

Scuffing is one of the most problematic failure mechanisms in lubricated mechanical components. It is a sudden and almost not predictable failure that often leads to extensive cost in terms of damages and/or delay in production lines. This study presents a promising solution that can prevent scuffing for the machinery industry in the future. To achieve this goal, a signal processing approach by means of an acoustic emission is introduced for the prediction of scuffing. An acoustic dataset was collected from metallic surfaces reciprocating under a constant load (typical conditions for semi journal bearings). The coefficient of friction values were measured during the entire experiments and were referred to as the ground truth of the momentary surface state. Based on the friction behavior, three friction regimes were defined that are running-in, steady-state, and scuffing. The present approach is based on tracking the changes in acoustic emission by means of three sets of wavelet-derived features. Those features include: 1) energy, 2) entropy, and 3) statistical information about the content of acoustic emission and the response of each feature to the different friction regimes was individually investigated. The applicability of machine learning classification and regression was studied for scuffing prediction. Both approaches were applied separately but can be unified together to increase the prediction time interval of surface failure. For classification, an extra friction regime was introduced designating as pre-scuffing and is defined as a time span of 3 min before the real surface failure. Random forest classifier was used to differentiate the features from the different friction regime. The best performance in classification of features from pre-scuffing regime reached a confidence level as high as 84%. In regression approach, the extracted features sequences were used together with random forest regressor. Our strategy allowed predicting scuffing up to 5 min preceding its real occurrence.

In-Situ Scanning Electron Microscope Indentation of Gallium Arsenide

Indentation is a commonly used method to evaluate the fracture toughness of brittle materials through crack length measurements. Many models for toughness estimations are available and rely on the assumption that surface traces of the cracks are either of the Half Penny type, Lawn and Evans, [1], Lawn, et al., [2], or of the radial type, Niihara, et al., [3], Niihara, [4]. However, it has been shown that, especially at low loads, the crack morphology depends strongly on material and thus this dependence affects calculated toughness values, Cook and Pharr, [5].

Dry-epitaxial lift-off for high efficiency solar cells

There is a need for lightweight and flexible solar cells and arrays with efficiency > 30% and specific power > 1000 W/kg for commercial and military terrestrial and aerial (UAV) applications and for space missions, that are cost-effective. Inverted metamorphic triple-junction (IMM3J) solar cells achieve the highest efficiencies, but cost >

Scratching and Brittle Fracture of Semiconductor In-Situ Scanning Electron Microscope

250/W. This high cost is split among the substrate (40%), epitaxial growth (30%) and front side processing including metallization (30%). It is desired to reduce the cost to

In Situ Quality Monitoring in AM Using Acoustic Emission: A Reinforcement Learning Approach

50/W. This is especially important for space missions exceeding 10 kW, for short-term missions to low earth orbit, and for picosatellites such as cubesats. Epitaxial lift-off (ELO) is used to transfer the epi-layer to a flexible substrate and reuse the Ge or GaAs wafer to grow another epi-layer. A dry epitaxial lift-off (DELO) process is presented by driving a controlled crack at the epi/wafer interface and transferring wafer scale to a polyimide substrate permanent carrier of the fragile solar cell, thus eliminating the need for detaching the substrate. The crack is driven purely by the thermal stresses due to the mismatch in CTE between GaAs and polyimide without the necessity for any external mechanical force or tool to aid crack propagation. This process yields single atomic plane cleavage which reduces the need for post lift-off polishing, and allows multiple reuses of the substrate up to 10 times. This allows the use of automated roll-to-roll processing. This combined with cheaper metallization using aerosol jet 3D printing and additive manufacturing techniques allows reducing the cost down to