Juraj Todt

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.

Thermal expansion of rock-salt cubic AlN

We combine continuum mechanics modeling and wafer curvature experiments to characterize the thermal expansion coefficient of AlN in its metastable cubic rock-salt (B1) structure. The latter was stabilized as nm thin layers by coherency strains in CrN/AlN epitaxial multilayers deposited on Si (100) substrates using reactive magnetron sputtering. The extraction of the B1-AlN thermal expansion coefficient, from experimentally recorded temperature dependent wafer curvature data, is formulated as an inverse problem using continuum mechanics modeling. The results are cross-validated by density functional theory calculations.

In-situ Observation of Cross-Sectional Microstructural Changes and Stress Distributions in Fracturing TiN Thin Film during Nanoindentation

Load-displacement curves measured during indentation experiments on thin films depend on non-homogeneous intrinsic film microstructure and residual stress gradients as well as on their changes during indenter penetration into the material. To date, microstructural changes and local stress concentrations resulting in plastic deformation and fracture were quantified exclusively using numerical models which suffer from poor knowledge of size dependent material properties and the unknown intrinsic gradients. Here, we report the first in-situ characterization of microstructural changes and multi-axial stress distributions in a wedge-indented 9 μm thick nanocrystalline TiN film volume performed using synchrotron cross-sectional X-ray nanodiffraction. During the indentation, needle-like TiN crystallites are tilted up to 15 degrees away from the indenter axis in the imprint area and strongly anisotropic diffraction peak broadening indicates strain variation within the X-ray nanoprobe caused by gradients of giant compressive stresses. The morphology of the multiaxial stress distributions with local concentrations up to −16.5 GPa correlate well with the observed fracture modes. The crack growth is influenced decisively by the film microstructure, especially by the micro- and nano-scopic interfaces. This novel experimental approach offers the capability to interpret indentation response and indenter imprint morphology of small graded nanostructured features.

X-ray nanodiffraction analysis of stress oscillations in a W thin film on through-silicon via

X-ray nanodiffraction is used to evaluate axial and tangential residual stress distributions in a W thin film deposited on the scalloped inner wall of a through-silicon via. The results reveal oscillatory stress distributions which correlate well with the scallop wavelength and morphology.

Combinatorial refinement of thin-film microstructure, properties and process conditions: iterative nanoscale search for self-assembled TiAlN nanolamellae

A novel iterative combinatorial nanoscale search based on the application of cross-sectional synchrotron X-ray nanodiffraction and cross-sectional nanoindentation is used to refine the relationship between deposition conditions, microstructure and properties of nanostructured TiAlN thin films. Using three iterative steps, a nanolamellar TiAlN thin film with a maximal hardness of ∼36 GPa is developed.

Mono-textured nanocrystalline thin films with pronounced stress-gradients: On the role of grain boundaries in the stress evolution

The origins of residual stress gradients in nanocrystalline thin films, especially the role of grain size and texture gradients, are still not fully understood. In this work, the stress evolution in exemplary nanocrystalline TiN thin films with one and two fiber texture components as well as in homogeneous amorphous SiOx films is analyzed using wafer curvature as well as laboratory and synchrotron cross-sectional nanobeam X-ray diffraction techniques. The stress evolution across the film thickness is attributed to the evolutionary nature of microstructural development at the individual growth stages. While the effect of the smooth crystallographic texture changes during growth is only of minor importance, as this does not significantly affect the dominant stress formation mechanisms, the change in the grain size accompanied by a change of the volume fraction of grain boundaries plays a decisive role in the stress development across the film thickness. This is demonstrated on the monotextured thin films, w...

Integrated experimental and computational approach for residual stress investigation near through-silicon vias

The performance of three-dimensional integrated circuits is decisively influenced by the thermo-mechanical behavior of through-silicon vias (TSVs), which are subjected to stresses formed during fabrication process as well as cyclic operation as a result of coefficients of thermal expansion (CTEs) mismatch between the silicon substrate, passivation layers, and metallic conduction paths. In this work, we adopted an integrated approach combining micro-Raman, wafer curvature experiments, and finite element (FE) modeling to study the triaxial residual stresses in silicon in the vicinity of W-coated hollow TSVs. A comparison of the experimental and calculated Raman shifts from a TSV cross section allowed a validation of the FE model, which was then extended to a non-sliced TSV. In the next step, the calculated bulk strains were compared with the ones measured using synchrotron X-ray micro-diffraction in order to specifically assess the stress decrease in Si as a function of the distance from the TSV wall within...

Influence of Annealing on Microstructure and Mechanical Properties of a Nanocrystalline CrCoNi Medium-Entropy Alloy

An equiatomic CrCoNi medium-entropy alloy was subjected to high-pressure torsion. This process led to a refinement of the microstructure to a grain size of about 50 nm, combined with a strong increase in the materials hardness. Subsequently, the thermodynamic stability of the medium entropy alloy was evaluated by isothermal and isochronal heat treatments. Annealed samples were investigated by scanning and transmission electron microscopy as well as X-ray diffraction, and were subjected to tensile tests to establish microstructure-property relationships. Furthermore, a comparison of mechanical properties with a grade 316L stainless steel was performed in order to evaluate if the CrCoNi alloy is competitive with commercially available structural materials in the nanocrystalline state. A minority phase embedded in the face-centered cubic matrix of the CrCoNi alloy could be observed in multiple annealed states, as well as the as-received and high-pressure torsion processed material. For 200 h of annealing at 500 °C, it was determined that the minority phase has a hexagonal-closed-packed crystal structure. A possible explanation for the formation of the phase is a preferential segregation of Co to stacking faults.

Nanostructured Coatings for Tooling Applications

In this contribution the authors will present a short summit of the state of the art in the field of nanostructured coatings and will then emphasise on the most recent developments concerning the possible changes in the coating architecture of CVD AlTiN, which can be achieved by changing some of the process parameters

An investigation on shear banding and crystallographic texture of Ag–Cu alloys deformed by high-pressure torsion

Ag–Cu alloys of two different initial microstructures—a cast eutectic alloy (AgCu-E) and an equivolume Ag–Cu powder mixture (AgCu-P)—were deformed by high-pressure torsion. The codeformation of Ag and Cu grains led to uniform refinement and a nanolamellar microstructure for both alloys. However, the lamellar structure in AgCu-P alloys was broken at intermediate shear strains (γ > 150) by extensive shear banding. On the other hand, no shear banding was observed for AgCu-E alloy at similar microstructural refinement. At higher strains deformation induced intermixing of Ag and Cu atoms was observed. Further, three-dimensional diffraction analysis of AgCu-E alloy showed that in contrast to conventional single phase alloys, the Ag and Cu phases develop similar crystallographic texture.