| 1. |
- Chen, Zhuo, et al.
(författare)
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Large mechanical properties enhancement in ceramics through vacancy-mediated unit cell disturbance
- 2023
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Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- Tailoring vacancies is a feasible way to improve the mechanical properties of ceramics. However, high concentrations of vacancies usually compromise the strength (or hardness). We show that a high elasticity and flexural strength could be achieved simultaneously using a nitride superlattice architecture with disordered anion vacancies up to 50%. Enhanced mechanical properties primarily result from a distinctive deformation mechanism in superlattice ceramics, i.e., unit-cell disturbances. Such a disturbance substantially relieves local high-stress concentration, thus enhancing deformability. No dislocation activity involved also rationalizes its high strength. The work renders a unique understanding of the deformation and strengthening/toughening mechanism in nitride ceramics.
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| 2. |
- Hu, Chun, et al.
(författare)
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Influence of co-sputtering AlB2 to TaB2 on stoichiometry of non-reactively sputtered boride thin films
- 2024
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Ingår i: Materials Research Letters. - : TAYLOR & FRANCIS INC. - 2166-3831. ; 12:8, s. 561-570
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal diboride thin films are promising functional materials for their outstanding mechanical properties and thermal stability. By combining experiment and simulations, we discuss angular distribution of the sputtered species, their scattering in the gas phase, re-sputtering and potential evaporation from the grown films for the complex evolution of film compositions, as well as energetic preference for vacancy formation and competing phases as factors for governing the phase constitution. By co-sputtering from two compound targets, we developed phase-pure crystalline (Ta,Al)B2 solid solution thin films and correlate the stoichiometry changes with the evolution of their microstructure, hardness, and elastic modulus. {GRAPHICAL ABSTRACT}
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| 3. |
- Janknecht, Rebecca, et al.
(författare)
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A Strategy to Enhance the B-Solubility and Mechanical Properties of Ti-B-N Thin Films
- 2024
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 271
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Tidskriftsartikel (refereegranskat)abstract
- The Ti–B–N system offers a wide range of possible meta(stable) phases, making it interesting for science and industry. However, the solubility for B within the face-centered cubic (fcc)-TiN lattice is rather limited and less studied, especially without forming B-rich phases. Therefore, we address how chemistries along the TiN–TiB2 or TiN–TiB tie-line influence this B-solubility. The variation between these two tie-lines is realized through non-reactive co-sputtering of a TiN, TiB2, and Ti target. We show that for variations along the TiN–TiB tie-line, even 8.9 at.% B (equivalent to 19.3 at.% non-metal fractions) can fully be incorporated into the fcc-TiNy lattice without forming other B-containing phases. The combination of detailed microstructural characterization through X-ray diffraction and transmission electron microscopy with ab initio calculations of fcc-Ti1-xNBx, fcc-TiN1-xBx, and fcc-TiN1-2xBx solid solutions indicates that B essentially substitutes N.The single-phase fcc-TiB0.17N0.69 (the highest B-containing sample along the TiN–TiB tie-line studied) exhibits the highest hardness H of 37.1±1.9 GPa combined with the highest fracture toughness KIC of 3.0±0.2 MPa·m1/2 among the samples studied. These are markedly above those of B-free TiN0.87 having H = 29.2±2.1 GPa and KIC = 2.7±<0.1 MPa·m1/2.
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| 4. |
- Koutna, Nikola, et al.
(författare)
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Atomistic mechanisms underlying plasticity and crack growth in ceramics : a case study of AlN/TiN superlattices
- 2022
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Ingår i: Acta Materialia. - : Pergamon-Elsevier Science Ltd. - 1359-6454 .- 1873-2453. ; 229
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Tidskriftsartikel (refereegranskat)abstract
- Interfaces between components of a material govern its mechanical strength and fracture resistance. While a great number of interfaces is present in nanolayered materials, such as superlattices, their fundamental role during mechanical loading lacks understanding. Here we combine ab initio and classical molecular dynamics simulations, nanoindentation, and transmission electron microscopy to reveal atomistic mechanisms underlying plasticity and crack growth in B1 AlN(001)/TiN(001) superlattices under loading. The system is a model for modern refractory ceramics used as protective coatings. The simulations demonstrate an anisotropic response to uniaxial tensile deformation in principal crystallographic directions due to different strain-activated plastic deformation mechanisms. Superlattices strained orthogonal to (001) interfaces show modest plasticity and cleave parallel to AlN/TiN layers. Contrarily, B1-to-B3 or B1-to-B4(B-k) phase transformations in AlN facilitate a remarkable toughness enhancement upon in plane [110] and [100] tensile elongation, respectively. We verify the predictions experimentally and conclude that strain-induced crack growth-via loss of interface coherency, dislocation-pinning at interfaces, or layer interpenetration followed by formation of slip bands-can be hindered by controlling the thicknesses of the superlattice nanolayered components.
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| 5. |
- Koutna, Nikola, et al.
(författare)
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Phase stability and mechanical property trends for MAB phases by high-throughput ab initio calculations
- 2024
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Ingår i: Materials & design. - : ELSEVIER SCI LTD. - 0264-1275 .- 1873-4197. ; 241
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Tidskriftsartikel (refereegranskat)abstract
- MAB phases (MABs) are atomically-thin laminates of ceramic/metallic-like layers, having made a breakthrough in the development of 2D materials. Though offering a vast chemical and phase space, relatively few MABs have been synthesised. To guide experiments, we perform high-throughput ab initio screening of MABs that combine group 4-7 transition metals (M); Al, Si, Ga, Ge, or In (A); and boron (B) focusing on their phase stability trends and mechanical properties. Considering the 1:1:1, 2:1:1, 2:1:2, 3:1:2, 3:1:3, and 3:1:4 M:A:B ratios and 10 phase prototypes, synthesisability of a single-phase compound for each elemental combination is estimated through formation energy spectra of competing dynamically stable MABs. Based on the volumetric proximity of energetically-close phases, we identify systems in which volume-changing deformations may facilitate transformation toughening. Subsequently, chemistry- and phase-structure-related trends in the elastic stiffness and ductility are predicted using elastic-constants-based descriptors. The analysis of directional Cauchy pressures and Young's moduli allows comparing mechanical response parallel and normal to M-B/A layers. The suggested promising MABs include Nb 3 AlB 4 , Cr 2 SiB 2 , Mn 2 SiB 2 or the already synthesised MoAlB.
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| 6. |
- Leiner, Thomas, et al.
(författare)
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On energetics of allotrope transformations in transition-metal diborides via plane-by-plane shearing
- 2023
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Ingår i: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 215
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal diborides crystallize in the α, γ, or ω type structure, in which pure transition metal layers alternate with pure boron layers stacked along the hexagonal [0001] axis. Here we view the prototypes as different stackings of the transition metal planes and suppose they can transform from one into another by a displacive transformation. Employing first-principles calculations, we simulate sliding of individual planes in the group IV-VII transition metal diborides along a transformation pathway connecting the α, γ, or ω structure. Chemistry-related trends are predicted in terms of energetic and structural changes along a transformation pathway, together with the mechanical and dynamical stability of the different stackings. Our results suggest that MnB2 and MoB2 possess the overall lowest sliding barriers among the investigated TMB2s. Furthermore, we discuss trends in strength and ductility indicators, including Youngs modulus or Cauchy pressure, derived from elastic constants.
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| 7. |
- Lin, Shuyao, et al.
(författare)
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Machine-learning potentials for nanoscale simulations of tensile deformation and fracture in ceramics
- 2024
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Ingår i: npj Computational Materials. - : NATURE PORTFOLIO. - 2057-3960. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- Machine-learning interatomic potentials (MLIPs) offer a powerful avenue for simulations beyond length and timescales of ab initio methods. Their development for investigation of mechanical properties and fracture, however, is far from trivial since extended defects-governing plasticity and crack nucleation in most materials-are too large to be included in the training set. Using TiB2 as a model ceramic material, we propose a training strategy for MLIPs suitable to simulate mechanical response of monocrystals until failure. Our MLIP accurately reproduces ab initio stresses and fracture mechanisms during room-temperature uniaxial tensile deformation of TiB2 at the atomic scale ( approximate to 103 atoms). More realistic tensile tests (low strain rate, Poisson's contraction) at the nanoscale ( approximate to 104-106 atoms) require MLIP up-fitting, i.e., learning from additional ab initio configurations. Consequently, we elucidate trends in theoretical strength, toughness, and crack initiation patterns under different loading directions. As our MLIP is specifically trained to modelling tensile deformation, we discuss its limitations for description of different loading conditions and lattice structures with various Ti/B stoichiometries. Finally, we show that our MLIP training procedure is applicable to diverse ceramic systems. This is demonstrated by developing MLIPs which are subsequently validated by simulations of uniaxial strain and fracture in TaB2, WB2, ReB2, TiN, and Ti2AlB2.
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| 8. |
- Schnitter, Claudia, 1989-
(författare)
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Epitaxial thin films of group 4 transition metal diborides
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Group 4 transition metal diboride films were deposited epitaxially onto different substrates, typically at 900 °C, using direct-current magnetron sputtering from compound targets of ZrB2, TiB2, and HfB2. Epitaxial ZrB2 has been deposited on Al2O3(0001), 4H-SiC(0001), and Si(100) describing the epitaxial relationships on Si(100) for the first time, where two relationships were observed from pole-figure analysis, namely A) in-plane: ZrB2[001] ∥ Si[110] and ZrB2[110] ∥ Si[110], out-of-plane: ZrB2(100) ∥ Si(100), and B) in-plane: ZrB2[11] ∥ Si[110] and the same rotated 90° around the 102 axis, out-of- plane: ZrB2(102) ∥ Si(100). Composition analysis by time-of-flight elastic recoil detection analysis revealed typically B-rich ZrB2 and TiB2, and stoichiometric to Hf-rich HfB2. For ZrB2, the application of an additional external magnetic field during growth influenced the B-to-Zr ratio towards being stoichiometric. Rocking curve measurements of ZrB2 deposited onto Si(100) reveal a higher crystal order in the 100-oriented domains, compared to the 102-oriented domains. In ZrB2 films annealed to temperatures in the range of 1100-1500 °C, rocking curve measurements of the symmetric 001 reflection as well as the asymmetric 101 reflection reveal increased order with increased temperature. This phenomenon occurred at lower temperature when the annealing was performed in H2 compared to Ar.The morphology in plan-view transmission electron microscopy reflects the composition of the film: TiB2.5 has B-rich areas around the grain boundaries, forming an almost continuous network around the grains. ZrBx films with x between 2.0 and 2.3 also contain B-rich regions, though to a smaller extent and mostly in areas where more than two grains adjoin each other. In Hf-rich HfB1.8, no B-rich areas were observed. Scanning transmission electron micrographs and a combination of B electron energy loss spectroscopy and energy dispersive X-ray analysis Hf distribution maps revealed Hf-rich areas in a close-to single crystalline matrix. The hardness of epitaxial HfB2 films is reported to 33 and 36 GPa for films deposited onto Al2O3 and SiC, respectively. These values are slightly higher than reported for bulk HfB2. ZrB2 films on SiC decrease in hardness to 38, 37, and 30 GPa, upon annealing in Ar up to 1100, 1300, and 1500 °C, respectively. In summary, the knowledge is expanded about epitaxially grown group 4 transition metal diborides in terms of their Chemical composition, crystallographic orientations, microstructure, electrical and mechanical properties, as well as their response to heat treatment.
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