Discrete dislocation simulation of nanoindentation

A methodology to describe nanoindentation by means of discrete dislocations is presented. A collocation method is used to calculate the arising contact stresses at each indentation step, which permits to realize an arbitrary shape of the indenter. Distributed dislocation sources are allowed to emit dislocations on predefined slip planes, when the critical value of the local shear stress for the emission is reached. After each indentation step, the newly emitted dislocations are brought to their equilibrium positions under the influence of the stresses induced by the contact stresses and the dislocations. As an application of our model, the plastic behavior of two materials with different densities of dislocation sources will be studied in detail.This work was financially supported by the FWF (Fonds zur Förderung der wissenschaftlichen Forschung) Project P13908-N07.     

Surface effects on Young’s modulus and hardness of fused silica by nanoindentation study

The surface Young’s modulus (E) and hardness (H) of fused silica samples have been studied by nanoindentation. Two factors strongly affect the results of E and H. One factor is the polishing quality of the fused silica surface. Poor polishing quality produces much smaller E and H than the literature values for bulk fused silica. The second factor is surface flatness. Even for a well-polished silica surface, an “arch bridge effect” may hinder the measurements of the true values of E and H. A correction procedure is proposed to eliminate this effect, and the corrected results show substantial improvements.     

Surface crystalline phases and nanoindentation hardness of explanted zirconia femoral heads

One new and nine explanted zirconia femoral heads were studied using glancing angle X-ray diffraction, scanning electron microscopy, and nanoindentation hardness techniques. All starting zirconia implants consisted only of tetragonal zirconia polycrystals (TZP). For comparison, one explanted alumina femoral head was also studied. Evidence for a surface tetragonal-to-monoclinic zirconia phase transformation was observed in some implants, the extent of which was varied for different in-service conditions. A strong correlation was found between increasing transformation to the monoclinic phase and decreasing surface hardness. Microscopic investigations of some of the explanted femoral heads revealed ultra high molecular weight polyethylene and metallic transfer wear debris.     

Errors in determining hardness by means of impact-type instruments

Conclusions The comparative evaluation of errors for impact-type instruments leads to the conclusion that the Nikolaev instrument (± 11.5%) has the smallest error in determining the HB hardness of various ferrous metals. After certain improvements this error was reduced to 10%. The Poldi instrument has an error of ±7%, providing the reference sample is made of the same material as the one whose hardness it is required to determine.     

Influence of sintering temperatures on hardness and Young''s modulus of tricalcium phosphate bioceramic by nanoindentation technique

Nanoindentation experiments on tricalcium phosphate (TCP) bioceramic sintered at different temperatures were performed with a Berkovich indenter for determining hardness and elastic modulus from load and displacement data. The hardness and Young's modulus increased with the increase of sintering temperature up to 1300 °C, but the Young's modulus decreased with the further increase of sintering temperatures at 1400 and 1500 °C. X-ray diffraction (XRD) results showed that the transformation β→-TCP happened when the sintering temperature reached around 1400 °C, which contributed to the decreases of modulus at 1400 and 1500 °C. Scanning electron microscopy (SEM) results showed that the sintering effect was improved with the increase in sintering temperature.     

Thermal conductivity and nanoindentation hardness of as-prepared and oxidized porous silicon layers

Porous silicon (PS) offers promising possibilities to be applied as thermal insulating material in thermal effect microsystems for its thermal conductivity (TC) is up to two orders smaller than that of bulk silicon. In order to find a compromise between efficient thermal isolation and good mechanical stability of PS, thermal oxidation of PS is commonly used to tune the mechanical and thermal properties of PS. Both TC and the hardness of as-prepared and oxidized PS have been thoroughly investigated. TC and the hardness of as-prepared and oxidized PS were measured using micro-Raman scattering and nanoindentation, respectively. Experimental results revealed that TC and the hardness of as-prepared PS, exhibiting a strong dependence on the preparing conditions, decrease with increasing porosities. After oxidization at different temperatures, TC of oxidized PS decreases with increasing oxidation temperatures, whereas the hardness increases a lot. PS with a moderate porosity of 73.4% oxidized at 600 °C has a compromise between low TC [2.100 W/(m K)] and high hardness (∼1.160 GPa). So this process finalizes this kind of oxidized PS to be used as a suitable thermal insulation substrate in thermal effect microsystems.     

Molecular dynamics simulation of incipient plasticity of nickel substrates of different surface orientations during nanoindentation

The present study aims to elucidate the anisotropic characteristics in material responses for crystallographic nickel substrates with (001), (011) and (111) surface orientations during nanoindentation. Molecular dynamic simulation is applied to compensate for the experimental limitation of nanoindentation, particularly for pure nickel substrates. Defect nucleation and evolution in Ni single crystal of these three crystal orientations was examined. Hardness and Young’s modulus are also extracted in different orientations. The Young’s modulus of (111) crystallographic orientation is the largest, while that of (001) surface is the smallest. The sensitivity of the yield point for face centred cubic crystals depends on the crystallographic orientation. The (001) crystallographic orientation reaches the yield point first, while the (111) crystallographic orientation is the most difficult in which to achieve yield. Using a visualisation method of centrosymmetry parameter, the homogeneous nucleation and early evolution of dislocations were investigated, deepening understanding of incipient plasticity at the atomic scale. The present results suggest that defect nucleation and evolution are the root of curve jitter. The indentation depth of the elastic–plastic transition point varies in the different crystallographic orientation models, and appears latest in the (111) model. The strain energy of the substrate exerted by the tip is stored by the formation of homogeneous nucleation and is dissipated by the dislocation slide in the {111} glide plane. The three nickel substrates with different crystallographic orientations exhibit different forms of dislocation propagation.     

Modeling of isothermal recovery and recrystallization kinetics by means of hardness measurements

Recovery and recrystallization, which may happen during annealing of materials, are very important technological processes through which many properties of the material can be changed. Hence, reliable and efficient modeling of the softening process is vital. Hardness can be used as a simple and cheap measure to model the softening processes. In the present study, the isothermal recovery and recrystallization kinetics of a commercial purity aluminum alloy AA1050 is experimentally modeled using hardness test at four temperatures from 285 °C to 400 °C after cold rolling to various reductions. In order to achieve more accurate analysis, hardness data points are increased and a statistical method is used. The isothermal recovery and recrystallization kinetics are described by a logarithmic and the Johnson‐Mehl‐Avrami‐Kolmagorov relationships, respectively. It is found that hardness measurements can be performed as a parameter to quantify recovery and recrystallization kinetics. The constants of models and the activation energies for both processes are calculated in the different conditions. These constant parameters are dependent to the temperature. Also, the activation energies of recovery and recrystallization rise as these transformations proceed.     

Identification of stiffness tensor components of wood cell walls by means of nanoindentation

A new procedure to characterize the full set of elastic constants of wood cell walls was developed. For the first time, not only the longitudinal modulus, but also the transverse- and the shear modulus were determined in one experimental setup at micron scale. For this purpose, nanoindentation experiments were performed at variable angles between the indentation direction and the direction of cellulose microfibrils in wood cell walls. Using an approach based on anisotropic indentation theory a relationship between the indentation moduli obtained experimentally and the elastic material constants of the cell wall was derived. Using an error minimization procedure, the values of the elastic material constants were finally calculated. As typically observed for natural materials, our experimental results are characterized by high variability. Particularly the elastic modulus in longitudinal cell direction is highly sensitive to small changes in the local orientation of cellulose microfibrils. Nonetheless, reasonable estimates of 26.3 GPa for the longitudinal elastic modulus of the secondary wood cell wall S2, 4.5 GPa for the transverse modulus, and – for the first time – a value of 4.8 GPa for the shear modulus of wood cell wall material were obtained.     

Interaction of lattice dislocations with a grain boundary during nanoindentation simulation

Interaction of dislocations with a Σ = 5 (210) [001] grain boundary was investigated using molecular dynamics simulation with EAM potentials. The results showed that the dislocation transmitted across the grain boundary during nanoindentation and left a step in the boundary plane. Burgers vector analysis suggested that a partial dislocation in grain I merged into the grain boundary and it was dissociated into another partial dislocation in grain II and a grain boundary dislocation, introducing a step in the grain boundary. Simulation also indicated that, after the transmission, the leading partial dislocation in the grain across the boundary was not followed by the trailing partials, expanding the width of the stacking fault. The results suggested that the creation of the step that accompanied grain boundary motion and expansion of the stacking fault caused resistance to nanoindentation.     

Effect of surface step on nanoindentation of thin films by multiscale analysis

Nanoindentation simulations on flat and stepped surfaces are respectively investigated using the quasicontinuum method based on the embedded-atom method potential. Effect of surface step considering indenter size and step height is studied. Results show that the critical load for the first dislocation emission will be decreased with the increase of step height. However, the effect of surface step will be weakened if the indenter size continues to increase. Initial atomistic structures after dislocation nucleation and emission are discussed systematically. The initial dislocations are not quite identically nucleated under the stepped surface. Stress distribution analysis reveals that the shear stress in the slip planes close to the step is much larger than the shear stress in the slip planes far from the step for nanoindentation on the stepped surface. The multiscale simulation results are consistent with experimental results and analytic solutions. The conclusions about step effect considering indenter size and step height are helpful for understanding the microscopic mechanism of nanoindentation tests on thin films with surface step.     

Laboratory tests of a motorized snow surface hardness gage

A motor-driven, spherical probe controlled by a handheld calculator and a microprocessor developed to measure the force and probe travel distance needed to break individual snow surface elements was tested for consistency and reliability. Consistency tests were run on automatic pencil leads; reliability tests on ice rods. Both proved satisfactory. Tests on two man-made snow surfaces showed the probe capable of measuring the strength of individual snow surface elements. Brittle failure occurred in such elements at loads varying from 0.200 to 174.631 grams (median 11.560 grams) for 118 samples. Probe travel distance varied from 0.003 to 0.099 mm (median 0.017 mm). The load needed to cause the first break was less than 1 gram for 21 (18%) of the samples.     

Determining emissivity and true surface temperature by means of a pyrometer and an attachment

We examine a method of measuring the true surface temperature by means of a pyrometer with an adiabatic attachment designed to eliminate the introduction of a correction factor for emissivity. We propose a method of determining the emissivity by means of two pyrometers with cold and adiabatic attachments. We present the formulas and the curve of the functions needed to design the attachments and to evaluate the errors of the method.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 16, No. 4, pp. 723–730, April, 1969.