Fedor F. Klimashin | Searcharxiv

arXiv2025

Interface-mediated softening and deformation mechanics in amorphous/ amorphous nanolaminates

Interfaces govern the unique mechanical response of amorphous multilayers. Here, we examine nanoindentation hardness and deformation behaviour of amorphous-amorphous Ta$_2$O$_5$/SiO$_2$ nanolaminates with bilayer thicknesses ranging from 2 nm to 334 nm. Whilst monolithic SiO$_2$ exhibits catastrophic failure through a single dominant shear band, multilayer architectures demonstrate varied deformation mechanisms. Hardness decreases with reduced bilayer thickness, from 7.7 GPa at 334 nm to 5.5 GPa at 2 nm spacing, contrasting with crystalline systems, which strengthen with decreasing spacing. Cross-sectional transmission electron microscopy reveals that fine bilayer spacings promote closely spaced vertical shear bands with bilayer compression, while coarser spacings show fewer, widely spaced shear bands with chemical intermixing. Scanning electron diffraction mapping demonstrates significant densification beneath indents. The high interface density facilitates strain accommodation that prevents catastrophic failure typical of brittle amorphous materials.

cond-mat.mtrl-sci↗

arXiv2016

Point defects stabilise cubic Mo-N and Ta-N

We employ \textit{ab initio} calculations to investigate energetics of point defects in metastable rocksalt cubic Ta-N and Mo-N. Our results reveal a strong tendency to off-stoichiometry, i.e. defected structures are predicted to be more stable than perfect ones with 1:1 metal-to-nitrogen stoichiometry, in agreement with previous literature reports. While Ta-N significantly favours metal vacancies, Mo-N exhibits similar energies of formation regardless of the vacancy type ($V_\text{Mo}$, $V_\text{N}$) as long as their concentration is below $\approx15\,\text{at.\%}$. The overall lowest energy of formation were obtained for $\text{Ta}_{0.78}\text{N}$ and $\text{Mo}_{0.91}\text{N}$, which are hence predicted to be the most stable compositions. To account for various experimental condition during synthesis, we further evaluated the phase stability as a function of chemical potential of individual species. The proposed phase diagrams reveal four stable compositions, $\text{Mo}_{0.84}\text{N}$, $\text{Mo}_{0.91}\text{N}$, $\text{MoN}_{0.69}$ and $\text{MoN}_{0.44}$, in the case of Mo-N and nine stable compositions in the case of Ta-N indicating the crucial role of metal under-stoichiometry, since $\text{Ta}_{0.75}\text{N}$ and $\text{Ta}_{0.78}\text{N}$ significantly dominate the diagram. These results are important for understanding and designing experiments using non-equilibrium deposition techniques. Finally, we discuss a role of defects ordering and estimate a cubic lattice parameter as a function of a defect contents.

cond-mat.mtrl-sci↗