Jozef Keckes

Decomposition pathways in age hardening of Ti-Al-N films

The ability to increase the thermal stability of protective coatings under work load gives rise to scientific and industrial interest in age hardening of complex nitride coating systems such as ceramic-like Ti1−xAlxN. However, the decomposition pathway of these systems from single-phase cubic to the thermodynamically stable binary nitrides (cubic TiN and wurtzite AlN), which are essential for age hardening, are not yet fully understood. In particular, the role of decomposition kinetics still requires more detailed investigation. In the present work, the combined effect of annealing time and temperature upon the nano-structural development of Ti0.46Al0.54N thin films is studied, with a thermal exposure of either 1 min or 120 min in 100 °C steps from 500 °C to 1400 °C. The impact of chemical changes at the atomic scale on the development of micro-strain and mechanical properties is studied by post-annealing investigations using X-ray diffraction, nanoindentation, 3D-atom probe tomography and high-resolution...

Mechanical model for the deformation of the wood cell wall

Abstract The molecular mechanisms responsible for the deformation of wood, as well as the mechanical interaction of cell-wall components, such as cellulose, lignin and hemicelluloses, are not well understood. In a recently published experiment [1], we have shown that wood foils and single cells of compression wood of spruce (Picea abies [L.] Karst.) could deform permanently under tensile load via a stick and slip mechanism at the molecular level occurring during shear of the matrix between cellulose microfibrils. The shear originates from the fact that microfibrils are spiralling around the central lumen of the wood cell which can be considered as a hollow tube. During stretching, the microfibril angle between the cell axis and the direction of the cellulose was found to decrease in synchrotron X-ray diffraction experiments, giving rise to shear deformation of the matrix and to a recovery mechanism after irreversible deformation. The corresponding stick and slip mechanism is treated here in the framework ...

Effect of growth rate on mean microfibril angle and cross-sectional shape of tracheids of Norway spruce

The variation of the mean microfibril angle (MFA) and the shape of the cross-section of lumen with the distance from the pith in fast grown Norway spruce were studied by X-ray scattering and optical microscopy. The samples were from stems of a clone of Norway spruce [ Picea abies (L.) Karst.] grown in a fertile site at Nurmijärvi, southern Finland Both the mean MFA and the circularity index of the lumen of the fast-grown trees decreased more gradually as the distance from the pith increased than those in reference trees grown in a medium fertility site. However, in mature wood the mean MFA reached the same level in fast-grown trees as in reference trees (5°–10°) but the cross-sections of the cells remained more circular in fast-grown trees than in reference trees. The dependence of the mean MFA on the distance from the pith was similar for earlywood and latewood, but the values of the mean MFA of latewood were systematically smaller than those of earlywood. Two different X-ray diffraction geometries were compared from the points of view of biology and data analysis.

A heating stage up to 1173 K for X-ray diffraction studies in the whole orientation space

A multi-purpose heating attachment designed primarily for X-ray four-circle diffractometers but applicable also for classical powder diffraction is presented. When working in reflection geometry, the air-cooled heating stage allows diffraction studies to be performed on plate-like samples up to 1173 K in the whole orientation space. This paper gives a detailed description of the assembly and important technical specifications for the performance of experiments. The heating characteristics of the heating stage, the displacement of the sample from the goniometer centre as a result of thermal expansion and the influence of the protecting dome on the diffraction experiment are presented. The simple technical construction, the low weight, the small size and good heating performance make this equipment a general purpose heating attachment for X-ray diffraction experiments in reflection geometry.

Methodology for studying strain inhomogeneities in polycrystalline thin films during in situ thermal loading using coherent x-ray diffraction

Coherent x-ray diffraction is used to investigate the mechanical properties of a single grain within a polycrystalline thin film in situ during a thermal cycle. Both the experimental approach and finite element simulation are described. Coherent diffraction from a single grain has been monitored in situ at different temperatures. This experiment offers unique perspectives for the study of the mechanical properties of nano-objects.

X-ray analysis of residual stress gradients in TiN coatings by a Laplace space approach and cross-sectional nanodiffraction: a critical comparison

Residual stresses in as-deposited and blasted TiN coatings are characterized using a Laplace approach and using the novel cross-sectional X-ray nanodiffraction technique. A comparison of real and Laplace space techniques demonstrates the advantages of the nanodiffraction method, with a possibility to analyse local gradients of stress, texture, crystallite size and phase in thin films and coatings.

A two-dimensional waveguide beam for X-ray nanodiffraction

The micro- and nanofocus X-ray scattering (MINAXS) beamline of PETRA III is equipped with two consecutively arranged endstations, the last of which is the nanofocus endstation. The first in-beam commissioning of the experimental equipment was successfully performed at the end of 2010, using two-dimensionally confining hard X-ray silicon waveguides with cross sections of 50 nm × 50 µm to 50 nm × 2 µm for nanobeam generation. A full characterization of the waveguide-generated beams was performed, giving values for the beam geometries, the transmission efficiencies of the waveguides and absolute fluxes. Along with these results a detailed description of the setup is presented in this paper. A first high-resolution nanodiffraction experiment on a nanocrystalline TiN hard coating was performed to verify the resolution of the nanodiffraction setup and to reveal the local gradients across the blasted TiN coating. In conclusion, the main concern is the availability of the nanobeam, how it was generated and the fact that a beam out of a two-dimensionally confining waveguide was used for diffraction experiments for the first time.

Towards strain gauges based on a self-assembled nanoparticle monolayer--SAXS study.

An in situ small-angle x-ray scattering study of the nanoparticle displacement in a self-assembled monolayer as a function of a supporting membrane strain is presented. The average nanoparticle spacing is 6.7 nm in the unstrained state and increases in the applied force direction, following linearly the membrane strain which reaches the maximum value of 11%. The experimental results suggest a continuous mutual shift of the nanoparticles and their gradual separation with the growing stress rather than nanoparticle islands formation. No measurable shift of the nanoparticles was observed in the direction perpendicular to the applied stress.

Texture development in polycrystalline CrN coatings: the role of growth conditions and a Cr interlayer

The control of the texture of transition-metal nitride coatings by varying the energy of incident ions and the coating thickness and by introducing a highly textured interlayer was demonstrated on polycrystalline CrN coatings grown by reactive magnetron sputtering. Development of the preferred (1 1 1) orientation was observed at moderate ion bombardment as the adatom net flux from the (2 0 0) to (1 1 1)-oriented grains dominated. A reversal of this flux at higher ion energies associated with collisional dissociation of ions at the film surface resulted in the development of a (2 0 0) texture. Coatings composed of grains with mixed (2 0 0) and (2 2 0) orientations were developed under intense ion bombardment due to preferential re-sputtering. The preferred orientation also changes with increasing coating thickness from (2 0 0) towards (1 1 1) through competitive growth. In contrast, a complete (2 0 0)-oriented structure of the CrN layer is formed when it is grown on a Cr interlayer with strong preferred (2 0 0) orientation.

Properties of chemically and mechanically isolated fibres of spruce (Picea abies [L.] Karst.). Part 3: Mechanical characterisation

Abstract Microtensile investigations were carried out on chemically and mechanically isolated fibres that had similar dimensions before isolation. The properties calculated for the fibres were nearly identical for the two isolation methods. However, cell wall cross-sections of chemically isolated fibres shrank much more transversely than those of the mechanically isolated fibres.