Papers by Author: Vilma Buršíková

Abstract: The local mechanical properties of Fe₇₈Al₂₂ alloy were studied using nanoindentation techniques. Sharp Berkovich indenter was used to perform load-controlled nanoindentation experiments on the studied sample. Hardness and elastic modulus maps were created on the basis of the indentation tests carried out in different grains. The focus of the work was to study the dependence of mechanical properties on the grain orientation. The results were in good agreement with quantum-mechanical calculations of anisotropic elastic properties of the studied alloy. It was explained that the maximum detected elastic modulus values are likely for grains with [111] crystallographic orientations which we theoretically identified as the hard ones.

Abstract: With the improvement of depth-sensing instruments nanoindentation has become a standard tool for the study and design of thin film systems, nanocomposites and other nanostructured materials and devices. Mechanical properties, such as elastic modulus and hardness can be measured at scales in the depth range of tens of nanometers. The correct determination of the mechanical properties depends on the proper evaluation of the real contact area. While two standard methods are commonly used, indentations on a reference sample and measurement by atomic force microscopy (AFM), there are many caveats and the issue remains tricky, especially for large depths. In this contribution we present an example where the standard calibration of the tip area function via a reference sample cannot be used for the desired tip and the AFM method must be used instead.

Abstract: Skutterudites are an important class of thermoelectric p- and n-type materials and they have already achieved fair efficiencies for the conversion of heat to electricity. Nevertheless researchers try to further enhance the figure of merit, ZT, by various ways. In this work we study microstructure and mechanical properties of two thermoelectric materials: an industrial n-type (Mm,Sm)_yCo₄Sb₁₂ skutterudite and an industrial p-type DD_yFe₃CoSb₁₂ skutterudite, both mixed with 1 wt.% of Ta_0.8Zr_0.2B. Thin lamellae were prepared from the compacted materials using a focused ion beam. Analytical transmission electron microscopy was used on lamellae to study details of microstructure. A fine dispersion of precipitates was found both at nanograin boundaries and in their interiors. Quasistatic and dynamic nanoindentation tests were carried out on planar polished sections in the range of applied loads from 0.01 to 10 mN. The results were complemented with quantitative modulus mapping of local mechanical properties with 10-nm resolution.

Abstract: The aim of the present work was to study the mechanical properties of thin nanocomposite Mo-B-C coatings consisting of nanocrystalline Mo₂BC embedded in amorphous Mo-B-C matrix. Magnetron sputtering of three targets, B₄C, C and Mo, was used for coatings preparation. The Mo-B-C coatings were deposited on high speed steel substrates. The fracture resistance of Mo-B-C coatings was studied by both indentation and dynamic impact tests. The impact tests enabled us to predict the load limit causing the coating destruction.

Abstract: Several W-B-C layers were prepared by magnetron sputtering. The microstructure of thin layers was observed by means of scanning and transmission electron microscopy on cross sections prepared using a focused ion beam. Both undisturbed layers and the volume under indentation prints were inspected. The W-B-C layers are fine nanostructured materials about 2 μm thick and indents with loads up to 1 N do not cause any visible defects (cracks, delamination etc). The results were correlated with mechanical properties characterized by means of nanoindentation experiments in both the static and the dynamic loading regime using a Berkovich indenter. Elastic modulus, indentation hardness and fracture resistance of prepared nanostructured coatings were evaluated and discussed.

Abstract: Nanostructure coatings based on boroncarbide were prepared using magnetron sputtering technique. The mechanical properties of the coatings were tested using quasi-static and dynamic nanoindentation tests with Berkovich and cube-corner indenters. The fracture resistance of the coatings and the coating/substrate interface was studied. Moreover, the dynamic wear and scratch resistance of the coatings was evaluated by means of impact and scratch tests.

Abstract: X-B-C (X=Mo, Ta) layers prepared by magnetron sputtering were tested. Mechanical properties were characterized by means of nanoindentation experiments in both the static and the dynamic loading regime. The results were correlated with observations of the microstructure under indentation prints by means of scanning and transmission electron microscopy on cross sections prepared using a focussed ion beam. An excellent fracture resistance of prepared nanostructured coatings was found.

Abstract: In this work we have studied local mechanical properties of CoCrFeSiB ribbons prepared by planar flow casting (PFC). Recently improved PFC technology enabled us to use two melts simultaneously and to prepare a bilayered Co₆₉Fe₂Cr₇Si₈B₁₄/Co₅₉Fe₁₂Cr₇Si₈B₁₄ ribbon with a good homogeneity of the layers and well defined interface. Single-layered ribbons of the two compositions above were prepared as well. The microstructure and the composition profiles were studied by means of analytical electron microscopy. Nanoindentation tests were carried out on the cross sections of the bilayered ribbon as well as of single-layered ribbons. The in-situ SPM imaging was used in order to achieve nanometer precision positioning of the indents and to get information about the surface topography before and after the tests.

Abstract: In this work the influence of the thermal treatment on the local mechanical properties of model diluted Cu-Co alloys with Co content of 4 at.% is investigated. The samples underwent annealing at 1273 K followed by water quenching. The further thermal treatment at 1073 K of the oversaturated solid solution generated a fine distribution of Co-rich precipitates. Parameters of microstructure were evaluated by means of analytical electron microscopy. The nanoscale mechanical properties of precipitates, areas adjacent to the precipitates and precipitate-free zones were studied using large area grid indentation tests. Moreover, the modulus mapping capability was applied to obtain quantitative maps of the storage and loss stiffness and modulus.

Abstract: The aim of the present work is to study the nanoindentation induced defects in as-deposited and annealed nanocomposite n-TiC/a-C:H coatings. Furthermore, the resistance of the coatings against indentation induced defect creation was studied using TEM analysis of lamellas prepared using a focused ion beam for both as-deposited and annealed samples.