J.J. Roa

Calculation of Young's Modulus Value by Means of AFM

In the last years, Atomic Force Microscopy (AFM) has become a powerful tool not only to study the surface morphology but also the nanomechanics of all kind of samples. In this paper, the applicability of this technique is reviewed and its basic aspects of operation, advantages and drawbacks of using the AFM probe as a picoindenter (Force Spectroscopy mode, FS-AFM) are discussed. The patents concerning picoindentation measurements are discussed in the text and special attention is paid to measurements performed on hard materials as ceramics, as they have not been as thoroughly reviewed in the literature as in the case of soft matter. The possibilities of AFM in the nanomechanics field include the quantitative determination of the Youngs modulus (E) and the transition force from elastic to plastic deformation regimes, the measurement of adhesion forces and deformation mechanisms while applying vertical forces in the range from tens of pN to μN.

Corrosion damage in WC–Co cemented carbides: residual strength assessment and 3D FIB-FESEM tomography characterisation

Abstract The effect of corrosion damage on cemented carbides was investigated. The study included residual strength assessment and detailed fractographic inspection of corroded specimens as well as detailed 3D FIB-FESEM tomography characterisation. Experimental results point out a strong strength decrease associated with localised corrosion damage, i.e. corrosion pits acting as stress raisers, concentrated in the binder phase. These pits exhibit a variable and partial interconnectivity, as a function of depth from the surface, and are the result of heterogeneous dissolution of the metallic phase, specifically at the corrosion front. However, as corrosion advances the ratio between pit depth and thickness of damaged layer decreases. Thus, stress concentration effect ascribed to corrosion pits gets geometrically lessened, damage becomes effectively homogenised and relatively changes in residual strength as exposure time gets longer are found to be less pronounced.

Contact Mechanics at Nanometric Scale Using Nanoindentation Technique for Brittle and Ductile Materials

In the last years, Nanoindentation or Instrumented Indentation Technique has become a powerful tool to study the mechanical properties at micro/nanometric scale (commonly known as hardness, elastic modulus and the stress-strain curve). In this review, the different contact mechanisms (elastic and elasto-plastic) are discussed, the recent patents for each mechanism (elastic and elasto-plastic) are summarized in detail, and the basic equations employed to know the mechanical behaviour for brittle and ductile materials are described.

Effective silver-assisted welding of YBCO blocks: mechanical versus electrical properties

Superconducting welding of bulk YBCO is a key technology allowing the fabrication of large, complex-shaped pieces for applications such as levitation, bearings or large magnets. Ideally, the electrical and mechanical properties of welds should be comparable to that of the joint grains. In this paper, we have investigated the correlation between the microstructural, mechanical and critical current density performances of melt-textured [001]-tilt YBCO welds fabricated by the silver welding technique. The hardness reduction across the weld, measured by nanoindentation, correlates linearly with the decrease of intergranular critical current density, measured at 77 K and self-field by magnetic Hall mapping. Remarkably, we show that high quality zero-angle welds could be fabricated with unaltered current and hardness performances across the joint, paving the way for the implementation of silver welds in large-scale systems.

Yield strength, Shear Stress and Toughness of YBCO samples textured by Bridgman technique

Mechanical properties of the orthorhombic phase of YBa2Cu3O7-δ (Y-123) at room temperature have been investigated at different applied loads using nanoindentation technique. The study was carried out for several monodomains on the (001) planes for textured Bridgman samples with dispersed Y2BaCuO5 (Y-211) particles as pinning centers. The yield strength (σys), shear stress (τm) and toughness (KIC) of Y123/Y211composite was determined at different applied loads. First and second mechanical properties have been calculated though the Hertz equations and the last one with Lawn et al. equations. Finally, the ultra-low imprints obtained by nanoindentation have been correlated with parameters obtained by Field Emission Scanning Electron Microscope (FE-SEM).

Paramagnetic moments and time effects in melt-textured NdBaCuO system with Nd422 inclusions

We have performed magnetic measurements in two melt-textured NdBa2Cu3O7-d samples with Nd422 inclusions under magnetic fields from 0.05 up to 14 T, applied parallel to the ab planes. The measurements were made with a superconducting quantum interference device (SQUID) and a vibrating sample magnetometer (VSM). Paramagnetic moments could be observed during FCC (field-cooled cooling) and FCW (field-cooled warming) experiments. This effect, known as Paramagnetic Meissner Effect (PME), persisted up to 14 T and strong irreversibilities were observed among FCC and FCW experiments, revealing the presence of time effects. These time effects were confirmed by specific magnetic relaxation experiments in different cooling rates and temperatures, showing an anomalous and curious paramagnetic behavior. We explain our results based on the flux-compressed state generated within nonsuperconducting regions of the sample, such as the Nd422 inclusions dispersed into the superconducting matrix. These inclusions may produce a strong vortex pinning that stabilize the paramagnetic state, allowing the admission of extra vortices into the sample responsible for the positive moments during the relaxation experiments.

Implementation of advanced characterisation techniques for assessment of grinding effects on the surface integrity of WC–Co cemented carbides

ABSTRACT Grinding is a key step on the manufacturing process of WC–Co cemented carbides (hardmetals). In this work, an investigation of grinding effects on the surface integrity of hardmetals is conducted. It is done by combining diverse advanced characterisation techniques: X-ray diffraction, field emission-scanning electron microscopy, electron back scatter diffraction, focused ion beam – 3D tomography and transmission electron microscopy. The study is carried out in a fine-grained WC–Co grade. Besides ground state, polished surface finish condition is assessed for comparison purposes. It is evidenced that grinding induces significant alterations: 3D tomography illustrates microcracking exists down to 2.5 μm depth with a highly anisotropic distribution at the subsurface, large compressive residual stresses extending until subsurface levels of about 12 μm, and phase transformation of binder from the original fcc phase into the hcp one, as well as severe plastic deformation observed within the binder at the surface level. Invited keynote papers from EuroPM2017, Milan.

Dynamic Deformation of Metastable Austenitic Stainless Steels at the Nanometric Length Scale

Cyclic indentation was used to evaluate the dynamic deformation on metastable steels, particularly in an austenitic stainless steel, AISI 301LN. In this work, cyclic nanoindentation experiments were carried out and the obtained loading-unloading (or P-h) curves were analyzed in order to get a deeper knowledge on the time-dependent behavior, as well as the main deformation mechanisms. It was found that the cyclic P-h curves present a softening effect due to several repeatable features (pop-in events, ratcheting effect, etc.) mainly related to dynamic deformation. Also, observation by transmission electron microscopy highlighted that dislocation pile-up is the main responsible of the secondary pop-ins produced after certain cycles.

Functional behavior of the anomalous magnetic relaxation observed in melt-textured YBa2Cu3O7-δ samples showing the paramagnetic Meissner effect

This work was financed by the Brazilian MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). We acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL).

Functional behavior of the melt-textured YBa2Cu3O7-δ Meissner effectanomalous magnetic relaxation observed in samples showing the paramagnetic

This work was financed by the Brazilian MCTI/CNPq Universal 14/2012 (contract number 477506/2012-7). We acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL).