Roman Nowak

Effect of deformation temperature on Hall-Petch relationship registered for polycrystalline magnesium

Abstract Temperature dependence of the frictional stress derived from the Hall–Petch relation for pure magnesium polycrystals was compared with the critical resolved shear stress for single crystals with orientation preferable for basal or, alternatively, non-basal slip systems. The presented results suggest that the non-basal slip plays dominant role in the room temperature plastic deformation of polycrystalline magnesium, which is in excellent agreement with the recent output of computer simulation of mechanical behavior of Mg and its alloys using viscoplastic self-consistent model [Acta Mater. 49 (2001) 4277].

Elastic and plastic properties of GaN determined by nano-indentation of bulk crystal

The major obstacle to the production of a blue laser is posed by difficulties with the preparation of defect-free GaN layers. A considerable amount of empirical work is presently being undertaken to achieve this goal. However, there is a lack of basic research on the reduction of residual stress and defects in these epilayers since the mechanical characteristics of GaN have not been measured yet. This is due to difficulties with experimental examination of thin films. This work addresses the mechanical properties of bulk GaN obtained by a high-pressure method. Young’s modulus (295 GPa), hardness (20 GPa), yield strength (15 GPa), and the stress–strain curve of GaN have been evaluated using nano-indentation. The cause of the sudden depth excursions during indentation of GaN epilayers has been clarified.

Deconfinement leads to changes in the nanoscale plasticity of silicon

Silicon crystals have an important role in the electronics industry, and silicon nanoparticles have applications in areas such as nanoelectromechanical systems, photonics and biotechnology. However, the elastic-plastic transition observed in silicon is not fully understood; in particular, it is not known if the plasticity of silicon is determined by dislocations or by transformations between phases. Here, based on compression experiments and molecular dynamics simulations, we show that the mechanical properties of bulk silicon and silicon nanoparticles are significantly different. We find that bulk silicon exists in a state of relative constraint, with its plasticity dominated by phase transformations, whereas silicon nanoparticles are less constrained and display dislocation-driven plasticity. This transition, which we call deconfinement, can also explain the absence of phase transformations in deformed silicon nanowedges. Furthermore, the phenomenon is in agreement with effects observed in shape-memory alloy nanopillars, and provides insight into the origin of incipient plasticity.

Precision of nanoindentation protocols for measurement of viscoelasticity in cortical and trabecular bone

Nanoindentation has recently gained attention as a characterization technique for mechanical properties of biological tissues, such as bone, on the sub-micron level. However, optimal methods to characterize viscoelastic properties of bones are yet to be established. This study aimed to compare the time-dependent viscoelastic properties of bone tissue obtained with different nanoindentation methods. Bovine cortical and trabecular bone samples (n=8) from the distal femur and proximal tibia were dehydrated, embedded and polished. The material properties determined using nanoindentation were hardness and reduced modulus, as well as time-dependent parameters based on creep, loading-rate, dissipated energy and semi-dynamic testing under load control. Each loading protocol was repeated 160 times and the reproducibility was assessed based on the coefficient of variation (CV). Additionally, three well-characterized polymers were tested and CV values were calculated for reference. The employed methods were able to characterize time-dependent viscoelastic properties of bone. However, their reproducibility varied highly (CV 9-40%). The creep constant increased with increasing dwell time. The reproducibility was best with a 30s creep period (CV 18%). The dissipated energy was stable after three repeated load cycles, and the reproducibility improved with each cycle (CV 23%). The viscoelastic properties determined with semi-dynamic test increased with increase in frequency. These measurements were most reproducible at high frequencies (CV 9-10%). Our results indicate that several methods are feasible for the determination of viscoelastic properties of bone material. The high frequency semi-dynamic test showed the highest precision within the tested nanoindentation protocols.

An electric current spike linked to nanoscale plasticity

The increase in semiconductor conductivity that occurs when a hard indenter is pressed into its surface has been recognized for years, and nanoindentation experiments have provided numerous insights into the mechanical properties of materials. In particular, such experiments have revealed so called pop-in events, where the indenter suddenly enters deeper into the material without any additional force being applied; these mark the onset of the elastic-plastic transition. Here, we report the observation of a current spike--a sharp increase in electrical current followed by immediate decay to zero at the end of the elastic deformation--during the nanoscale deformation of gallium arsenide. Such a spike has not been seen in previous nanoindentation experiments on semiconductors, and our results, supported by ab initio calculations, suggest a common origin for the electrical and mechanical responses of nanodeformed gallium arsenide. This leads us to the conclusion that a phase transition is the fundamental cause of nanoscale plasticity in gallium arsenide, and the discovery calls for a revision of the current dislocation-based understanding of nanoscale plasticity.

Rabbit cortical bone tissue increases its elastic stiffness but becomes less viscoelastic with age

Bone is dynamic tissue undergoing changes in its composition, structure and functional properties during growth. It has been proposed that especially changes in the collagen phase of bone are responsible for making the bone more fragile, and potentially less viscoelastic with age. Hence, robust methods to measure viscoelasticitiy are needed. This study aimed to characterize the development of the elastic and viscoelastic mechanical properties of rabbit bone during maturation and growth, as assessed by nanoindentation. The humeri from female New Zealand white rabbits of varying age (newborn, 11 days, 4 weeks, 3 and 6 months old, n=8 per group) were investigated. Mid-diaphyseal cortical bone samples were cut, dehydrated, embedded and polished. Nanoindentation probing, semi-dynamic testing with a frequency of 20 Hz and creep with a dwell time of 60 s were performed under load control to quantify the elastic and the time-dependent viscoelastic mechanical properties of bone. The elastic moduli were evaluated with all three methods and the viscoelastic parameters were assessed using the phase-shift and the creep time constant. The elastic stiffness of bone increased significantly with each consecutive age group, from 11 days to 6 months of age, based on the reduced modulus from the indentation probing, the storage modulus from the semi-dynamic test, and the first elastic parameter from the creep test. These elastic parameters correlated significantly (R=0.88-0.94, p<0.01). The values of viscoelastic parameters, the phase-shift and time creep constant, decreased significantly with age. The viscous properties determined by the creep and the semi-dynamic testing correlated significantly (R=0.90, p<0.01), however, no correlation was found between the phase-shift and the creep time constant. Additionally, the present results showed specific associations with tissue composition, as measured with Fourier Transform Infrared spectroscopy of the same samples. In summary, the present results reveal significant changes in material properties of rabbit cortical bone with age. The elastic modulus of bone tissue increased by approximately 60%, whereas the viscoelastic parameters decreased by 10% to 25% during the first 6 months of the rabbits life. Together, this indicates significant structural and functional maturation of the bone matrix during growth of the rabbit.

Nanoindentation experiments with different loading rate distinguish the mechanism of incipient plasticity

Recent findings in nanodeformation of semiconductors posed a dilemma whether the nanoscale plasticity starts with phase transformation or nucleation of dislocations in a stressed nanovolume. In this letter we demonstrate the results of nanoindentation experiments with different loading rate, which enable us to conclude on a mechanism of incipient plasticity. The recorded nanodeformation response of GaAs and Si contrasts that observed for either GaN or metallic Fe crystal, which supports the phase transformation nature of the GaAs incipient plasticity. The derived relationship between the energy barrier for defect nucleation and applied stress served as a verification of the obtained results.

Peculiar surface deformation of sapphire: Numerical simulation of nanoindentation

This report addresses the origin of peculiar anisotropic deformation of sapphire. The three-dimensional finite element simulation of the contact between spherical indenter and elastically anisotropic solid allowed us to analyze stress under the tip that penetrates in the (1010) and (0001) planes, and consequently, to localize those regions in which particular deformation mechanisms are likely to be activated. This approach contrasts the available models of “hardness anisotropy,” which routinely apply a modified uniaxial-stress approach to this essentially three-dimensional, nonisotropic contact problem. The calculated results are in agreement with the microscopic inspection of impressions; that is, the surface features reflect the distribution of stress. The computations made it also possible to evaluate the actual radius of the tip (nominally 5 μm ball).

Nanoindentation study on insight of plasticity related to dislocation density and crystal orientation in GaN

Yield shear stress dependence on dislocation density and crystal orientation was studied in GaN by nanoindentation examination. The yield shear stress decreased with increasing dislocation density, and it decreased with decreasing nanoindentation strain-rate. It reached and coincided at 11.5 GPa for both quasi-static deformed c-plane and m-plane GaN. Taking into account theoretical Peierls–Nabarro stress and yield stress for each slip system, these phenomena were concluded to be an evidence of heterogeneous mechanism associated plastic deformation in GaN crystal. Transmission electron microscopy and molecular dynamics simulation also supported the mechanism with obtained r-plane dislocation line.

Post-treatment of titanium nitride by ion implantation

Abstract The properties of titanium nitride and the effects of a post-treatment by ion implantation on coatings made of it are first considered in terms of data available from the scientific literature; 70 references are cited. Data obtained in the present work are then combined with these to offer an explanation of the process mechanisms and structural effects involved. The present work covers monolithic TiN coatings, deposited onto cemented carbide by chemical vapor deposition or steel by physical vapor deposition, and implanted with gas or metal ions at different doses and acceleration energies. The results considered together confirm that large changes in the residual stress and the strain distributions are introduced into the implanted zone (IZ) and extend well beyond forming an implantation affected zone (IAZ) which extends to a depth of several microns The surface of the IZ is amorphized softened by non-metallic implants but not by metallic ions which increase the hardness. The residual stress in the IZ is high, tensile or compressive depending on whether vacancy generation and atom peening effects dominate and is accompanied by concomitant high, irregular, distributions of strain caused by a high dislocation density and/or grain comminution and include high fractions of lattice vacancies. The forward momentum of the ions introduces a dense dislocation network and high residual stress in the IAZ corresponding to the so-called long range effect . The dislocation density increases and the residual stress becomes more compressive with increasing ion momentum.