arXiv subjects

M. Lorentzon

Publications and source records attributed to M. Lorentzon.

3 recordsLinked to original sources

arXiv2025

Three-fold Superstructured Superlattice HfN/HfAlN Thin Films for Enhanced Toughness

To simultaneously achieve high hardness and high toughness in protective coatings remains a fundamental challenge. Here, we harness the superlattice architecture to combine Koehler hardening while the coherent interfaces reduce the crack driving force and improve toughness, enabling coatings that are both hard and damage tolerant. We design and fabricate epitaxial HfN$_{1.33}$/Hf$_{0.76}$Al$_{0.24}$N$_{1.15}$ superlattices, deposited on MgO(001) substrates using low-energy, high-flux ion-assisted reactive magnetron sputtering. These superlattices with bilayer periods ranging from 6 to 20 nm, exhibit a unique three-fold superstructure, confirmed by X-ray diffraction and reciprocal space mapping (RSM). Each constituent forms distinct 3D checkerboard superstructures, with a period of 7.5 {\AA} for HfN and 12.5 A for HfAlN. RSMs further reveal low mosaicity, high crystalline quality, and in-plane compressive strains, indicating well preserved coherence across interfaces. Mechanical testing shows that the superlattices maintain the high hardness of HfAlN (\~36 GPa) independent of bilayer period, while surpassing the softer HfN (~27 GPa), consistent with interface-driven Koehler strengthening. Micropillar compression shows brittle fracture on the {110}<110> system, yet with distributed cracking and faster mechanical recovery compared to monolithic films, suggesting improved toughness. Cube-corner indentation further corroborate this behavior, with pile-up and suppressed fracture events. These results demonstrate that epitaxial HfN/HfAlN superlattices uniquely combine high hardness with improved toughness, enabled by their three-fold superstructured architecture. Leveraging the intrinsic high-temperature stability of HfN-based materials, this design offers a robust pathway toward next-generation protective coatings capable of maintaining performance under extreme conditions.

cond-mat.mtrl-sci↗

arXiv2025

Growth Mechanisms and Mechanical Response of 3D Superstructured Cubic and Hexagonal Hf$_{1-x}$Al$_x$N Thin Films

Transition metal aluminum nitrides are a technologically important class of multifunctional ceramics, however, the HfAlN system remains largely unexplored. We investigate phase stability, nanostructure design, and mechanical behavior of Hf$_{1-x}$Al$_x$N$_y$ thin films deposited on MgO(001) substrates using ion-assisted reactive magnetron sputtering. Compared to growth temperature and ion assistance, backscattered Ar neutrals are shown to have a dominant influence on the film structure. The Al-rich (x > 0.41) films form a nanocrystalline morphology consisting of Hf- and Al-rich nanodomains in a wurtzite-hexagonal(h) 0001 fiber-texture exhibiting about 22 GPa hardness, considerably higher than that of a binary AlN. For low Al contents, x < 0.30, surface-driven spinodal decomposition by energetic Ar neutrals during deposition in combination with quenching of sub-surface diffusion results in an unusual - and unique for nitrides - three-dimensional checkerboard superstructure of AlN- and HfN-rich nanodomains in the single-crystal rocksalt-cubic (c) phase. Lattice-resolved scanning transmission electron microscopy complemented with x-ray and electron diffraction reveals that the superstructure periodicity extends along <100> directions and the size increases linearly from 9 to 13 A with rising Al content. Consequently, the nanoindentation hardness increases sharply from 26 GPa for HfN$_y$, to \~38 GPa for c-Hf$_{1-x}$Al$_x$N$_y$, due to dislocation pinning at the superstructure strain fields. Micropillar compression of c-Hf$_{0.93}$Al$_{0.07}$N$_{1.15}$ shows a considerably higher yield stress compared to HfN$_y$ and controlled brittle fracture occurs via {110}<011> slip systems, attributed to superstructure inhibited dislocation motion. In contrast, nanocrystalline h-Hf$_{0.59}$Al$_{0.41}$N$_{1.23}$ exhibits a high yield stress and limited plasticity before strain burst failure.

cond-mat.mtrl-sci↗

arXiv2024

Eliminating the spin-down critical angle in polarizing neutron optics for expanding the polarization bandwidth

Polarized neutron scattering is a very important analysis technique for studies of magnetism, spintronics, and high-sensitivity measurements, among others, offering invaluable information. Yet, the efficiency of such experiments rely on the performance of the polarizing neutron optics to provide high reflectivity and polarization. Presently, state-of-the-art polarizers like Fe/Si supermirrors are not able to polarize neutrons at low scattering angles in a monochromatic beam or able to polarize neutrons with a variety of wavelengths as for a non-monochromatic beam. To overcome this limitation, it is suggested to use Co/Ti multilayers on Ti substrates owing to their favorable scattering length density characteristics. It is shown that this approach enables a wavelength bandwidth several times larger than achievable with state-of-the-art materials on Si or glass substrates. Consequently, enabling the possibility of polarizing neutrons across an extended wavelength range, including those with very high wavelengths.

cond-mat.mtrl-sci↗