Johann Michler

Determination of plastic properties of metals by instrumented indentation using different sharp indenters

Abstract Indentation testing is a common method to assess the mechanical properties of materials near their surface. The elasto-plastic properties may be determined from the force penetration curves measured in indentation using inverse methods. In this spirit, Dao et al. [1] (Acta Materialia, 49, 2001) have established a forward and a reverse analysis for engineering metals using the equivalent conical indenter of the Berkovich and the Vickers pyramids, which has an included angle θ of 70.3°. Extending Dao’s approach, we studied, based on a finite element analysis on elasto-plastic materials, the influence of the included angle of conical indenters ( θ =70.3, 60, 50 and 42.3°) and the friction coefficient on the force penetration curves. Based on this analysis, we suggest a more general method for determining the plastic properties of metals. The mechanical behaviour is modeled with the Young’s modulus, E , the yield strength, σ y , and the strain hardening exponent, n . We have shown that friction has a significant effect on the normal force measured on tips having included angles lower or equal to 50°. We have constructed, for each indenter geometry, a dimensionless function relating the characteristic parameters of the loading curve in indentation to the elasto-plastic parameters of metals. These functions have been generalized for any included angle. We show that the use of a second indenter with an included angle lower than θ =70.3° allows us to determine the strain hardening exponent with greater accuracy. Moreover, the sharper the indenter, the better the accuracy.

Hollow Urchin‐like ZnO thin Films by Electrochemical Deposition

Since the first report on ultraviolet lasing from ZnO nanowires (NWs), remarkable effort has been dedicated to the development of novel synthesis routes for 1D ZnO nanostructures. Ordered arrays of 1D ZnO NWs have a promising future as applications in electronic and optoelectronic devices, because they are expected to improve the performance of various nanodevices such as short-wavelength lasers, nanostructured solar cells, electroluminescent, and field-emission devices. What is now a relevant area of focus in nanoscience involves the preparation of higher-order assemblies, arrays, and superlattices of these 1D nanostructures. Recently, many efforts have focused on the integration of 1D nanoscale building blocks into 3D architectures. Hollow urchin-like ZnO NWs that combine properties of 3D and 1D materials may emerge as a more interesting alternative than simple arrays of NWs due to the higher specific surface and porosity, especially for application in dye and semiconductor-sensitized solar cells. To date, there are only two strategies to synthesize hollow urchin-like ZnO NWs. The first one is a wet-chemical route that uses a modified Kirkendall process, by which zinc powders that are spherical in shape are transformed into hollow urchin-like ZnO NWs dispersed in solution. The second strategy is based on the calcination of metallic Zn microsphere powders at relatively high temperature (500–750 8C). With these two approaches, ZnO nanostructures are often randomly distributed (in size and organization), which may limit their practical applications as building blocks in nanodevices. Nevertheless, it is essential for the fabrication of nanodevices to assemble NW-structured hollow spheres with a uniform size in ordered arrays, since such an organisation combines the merits of patterned arrays and nanometer-sized materials. Until now, a suitable technique is still missing for the fabrication of ordered arrays of hollow urchin-like ZnO NWs with tunable sizes. In this paper, we report on a novel approach to fabricate well-ordered hollow urchin-like single-crystal ZnO NWs with controlled NW and core dimensions. The method combines the formation of a polystyrene (PS) microsphere colloidal mono/ multilayer and the electrodeposition of ZnONWs, followed by the elimination of the PS microspheres, which play the role of a template. It is shown that the light scattering properties of such an ordered architecture exceed those of ZnO NW arrays. Applications as 3D building blocks in the field of nanostructured solar cells are discussed. Mono/multilayers of PS spheres covering conductive substrates have been used as templates to electrodeposit inverse opal structures. In such cases the nucleation of ZnO took place at the interstitial sites (on a conductive substrate) between the PS spheres leading to different morphologies depending on the employed method. Our strategy of electrodeposition differs from those previously described by the mode of nucleation and growth. In our case, the deposition of ZnO takes place, from the nucleation step, on the PS spheres and the conductive substrate, simultaneously. As a result, the spheres are homogenously covered by a thin film composed of single-crystal ZnO NWs connected together at their base. This approach provides a simple and versatile way to synthesize well-ordered mono/multilayers of ZnO hollow microspheres with the ability to control the sphere size in addition to the ZnO NW dimensions and morphology. A monolayer of commercially available carboxylate-modified PS spheres ( 4.3mm) is deposited directly on a transparent conductive oxide (TCO) substrate by using a self-assembly technique on a water surface. We have used the method of Zhou et al. with some modifications. A detailed description of our process is given in the experimental section. Figure 1a shows a tilted, low-magnification scanning electron microscopy (SEM) image of the self-assembled monolayer on TCO substrate. A well-organized monolayer of PS microspheres can be observed in addition to occasional point defects in some regions due to the presence of larger spheres in the commercial solution (circled region in Fig. 1a). This organization is observed throughout the entire TCO surface ( 1.5 cm). As a proof of that, the lower inset in Figure 1a shows an optical image of TCO/PS where the sample colour is perfectly homogeneous, reflecting the presence of only one PS domain (monolayer) on the substrate. The detailed organization of the spheres was investigated by a closer examination using high-magnification SEM (Fig. 1b and its inset), which shows a relatively large area of the self-assembled monolayer and a perfectly ordered array. The TCO/PS sample has then been immersed for 30min in 2 M ZnCl2 aqueous solution at room temperature and used as a working electrode in an electrochemical cell for the deposition of ZnO NWs. The electrolyte was an aqueous solution saturated by molecular O2, containing 5 10 4 M ZnCl2 (zinc precursor) and 0.1 M KCl

Fracture strength and Young's modulus of ZnO nanowires

The fracture strength of ZnO nanowires vertically grown on sapphire substrates was measured in tensile and bending experiments. Nanowires with diameters between 60 and 310 nm and a typical length of 2 μm were manipulated with an atomic force microscopy tip mounted on a nanomanipulator inside a scanning electron microscope. The fracture strain of (7.7 ± 0.8)% measured in the bending test was found to be close to the theoretical limit of 10% and revealed a strength about twice as high as in the tensile test. From the tensile experiments, the Youngs modulus could be measured to be within 30% of that of bulk ZnO, contrary to the lower values found in the literature.

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.

Axial p-n Junctions Realized in Silicon Nanowires by Ion Implantation

The electrical properties of vertically aligned silicon nanowires doped by ion implantation are studied in this paper by a combination of electron beam-induced current imaging and two terminal current-voltage measurements. By varying the implantation parameters in several process steps, uniform p- and n-doping profiles as well as p-n junctions along the nanowire axis are realized. The effective doping is demonstrated by electron beam-induced current imaging on single nanowires, and current-voltage measurements show their well-defined rectifying behavior.

Elevated temperature, nano-mechanical testing in situ in the scanning electron microscope.

A general nano-mechanical test platform capable of performing variable temperature and variable strain rate testing in situ in the scanning electron microscope is described. A variety of test geometries are possible in combination with focused ion beam machining or other fabrication techniques: indentation, micro-compression, cantilever bending, and scratch testing. The system is intrinsically displacement-controlled, which allows it to function directly as a micro-scale thermomechanical test frame. Stable, elevated temperature indentation∕micro-compression requires the indenter tip and the sample to be in thermal equilibrium to prevent thermal displacement drift due to thermal expansion. This is achieved through independent heating and temperature monitoring of both the indenter tip and sample. Furthermore, the apex temperature of the indenter tip is calibrated, which allows it to act as a referenced surface temperature probe during contact. A full description of the system is provided, and the effects of indenter geometry and of radiation on imaging conditions are discussed. The stabilization time and temperature distribution throughout the system as a function of temperature is characterized. The advantages of temperature monitoring and thermal calibration of the indenter tip are illustrated, which include the possibility of local thermal conductivity measurement. Finally, validation results using nanoindentation on fused silica and micro-compression of [100] silicon micro-pillars as a function of temperature up to 500 °C are presented, and procedures and considerations taken for these measurements are discussed. A brittle to ductile transition from fracture to splitting then plastic deformation is directly observed in the SEM for silicon as a function of temperature.

A new technique to determine the elastoplastic properties of thin metallic films using sharp indenters

In recent years, nanoindentation has established itself as one of the most convenient techniques to assess the mechanical properties of thin films by measuring the force–penetration (F–h) curve during loading and unloading. However, the mechanical understanding of the indentation process itself, which involves several non-linearities (inelastic material behaviour, large and non-homogeneous deformations), is very intricate and finite-element analysis is thus combined with experiments in order to develop methods allowing the determination of the flow stress. In this spirit, Dao et al. (Acta Materialia, 49, 2001, 3899) and Bucaille et al. (Acta Materialia, 51, 2003, 1663) conducted finite-element analysis using different sharp indenters and proposed functions relating F–h curves to elastoplastic parameters of metals. In the present work, these methods are applied on galvanically grown nickel films. Samples for both nanoindentation and microtensile tests were grown in the same batch and, therefore, allow their properties to be studied using two different techniques on identical materials. Nanoindentation tests were conducted with the Berkovich and cube corner pyramids at constant strain rate. Youngs modulus and stresses corresponding to representative strains, imposed by these indenters, of 3.3 and 12.6% were determined. These values are in good agreement with the stress–strain values measured in tension. We also showed the importance of taking into account the indentation size effect, the friction between the indenter and the material and the strain rate dependence of metal deformation behaviour. The application to thin metallic films is discussed.

Ductile-brittle transition in micropillar compression of GaAs at room temperature

Experiments have been carried out on how compressive failure of <100> axis GaAs micropillars at room temperature is influenced by their diameter. Slip was observed in all micropillars, often on intersecting slip planes. Cracks could nucleate at these intersections and then grow axially in the sample, with bursts of crack growth. However, GaAs micropillars with diameters less than approximately 1 µm did not split, nor was splitting observed where slip occurred on only one plane. The conditions under which such splitting can occur have been estimated by modifying an existing analysis. This predicts a ductile–brittle transition at a micropillar diameter of approximately 1 µm, consistent with experimental observations.

In Situ Electron Microscopy Mechanical Testing of Silicon Nanowires Using Electrostatically Actuated Tensile Stages

Two types of electrostatically actuated tensile stages for in situ electron microscopy mechanical testing of 1-D nanostructures were designed, microfabricated, and tested. Testing was carried out for mechanical characterization of silicon nanowires (SiNWs). The bulk micromachined stages consist of a comb-drive actuator and either a differential capacitive sensor or a clamped-clamped beam force sensor. High-aspect-ratio structures (height/gap = 20) were designed to increase the driving force of the geometrically optimized actuator and the sensitivity of the capacitive sensor. The actuator stiffness is kept low to enable high tensile force to be exerted in the specimen rather than in the suspensions of the comb drive. Individual SiNWs were mounted on the devices by in situ scanning electron microscopy nanomanipulation, and their tensile properties were determined to demonstrate the device capability. The phosphorus-doped SiNWs, which were grown in a bottom-up manner by the vapor-liquid-solid process, show an average Youngs modulus of (170.0 ± 2.4) GPa and a tensile strength of at least 4.2 GPa. Top-down electroless chemically etched SiNWs, with their long axis along the [100] direction, show a fracture strength of 5.4 GPa.

In situ micropillar compression reveals superior strength and ductility but an absence of damage in lamellar bone

Ageing societies suffer from an increasing incidence of bone fractures. Bone strength depends on the amount of mineral measured by clinical densitometry, but also on the micromechanical properties of the hierarchical organization of bone. Here, we investigate the mechanical response under monotonic and cyclic compression of both single osteonal lamellae and macroscopic samples containing numerous osteons. Micropillar compression tests in a scanning electron microscope, microindentation and macroscopic compression tests were performed on dry ovine bone to identify the elastic modulus, yield stress, plastic deformation, damage accumulation and failure mechanisms. We found that isolated lamellae exhibit a plastic behaviour, with higher yield stress and ductility but no damage. In agreement with a proposed rheological model, these experiments illustrate a transition from a ductile mechanical behaviour of bone at the microscale to a quasi-brittle response driven by the growth of cracks along interfaces or in the vicinity of pores at the macroscale.