To quickly screen for single-phased multi-principal-element materials, a so-called entropy forming ability (EFA) parameter is sometimes used as a descriptor. The larger the EFA, the larger is the ...propensity to form a single-phase structure. We have investigated this EFA descriptor with atomic relaxations in special-quasi-random structures and discovered that the EFA correlates inversely with the lattice distortion. Large effective atomic size differences lead to multi-phase compounds, and little size differences to single-phase compounds. Instead of configurational entropy, we therefore demonstrate the applicability of the Hume-Rothery rules to phase stability of solid solutions even in compositionally complex ceramics.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Superlattice (SL) thin films composed of refractory ceramics unite extremely high hardness and enhanced fracture toughness; a material combination which is often mutually exclusive. While the ...hardness enhancement is well described by existing models based on dislocation mobility, the underlying mechanisms behind the increase in fracture toughness are yet to be unraveled. Here, we provide a model based on linear elasticity theory to predict the fracture toughness in (semi-)epitaxial nanolayers. As representative of cubic transition metal nitrides, a TiN/CrN superlattice structure on MgO (100) is studied. The density of misfit dislocations is estimated by minimizing the overall strain energy, each time a new layer is added on the nanolayered stack. The partly relaxed coherency stresses are then used to calculate the apparent fracture toughness (Kapp) by applying the weight function method. The results show that Kapp increases steeply with increasing bilayer period for very thin SLs, before the values decline more gently along with the formation of misfit dislocations. The characteristic Kapp vs. bilayer-period-dependence nicely matches experimental trends. Importantly, all critical stress intensity values of the SLs clearly exceed the intrinsic fracture toughness of the layer materials, evincing the importance of coherency stresses for increasing the crack growth resistance.
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•An algorithm based on elasticity theory is proposed to estimate the dislocation densities in ceramic superlattice thin films.•The bilayer period significantly influences the misfit dislocation densities and the associated coherency stresses.•Predictions of the apparent fracture toughness of TiN/CrN superlattices match experimental trends.•The model reveals the potential of a superlattice structure to increase the crack growth resistance of a thin film.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•(Al,Mo,Ta,V,W)N0.79 and -N0.33 show columnar microstructures.•The oxynitride features a renucleated finer, randomly oriented microstructure.•N-deficiency of (Al,Mo,Ta,V,W)N is ...beneficial for hardness and phase stability.•The oxynitride is harder than the N-rich nitride in as-deposited state.•The oxynitride cracks open during annealing due to formation of binary oxides.
(Al,Mo,Ta,V,W)-N0.79, -N0.33, and -N0.88O0.12 coatings were sputtered in N2-rich, N2-lean, and N2+O2 containing atmospheres. The face-centered cubic structured coatings feature as-deposited hardness values of 32.3, 39.7, and 34.5 GPa, respectively. The (Al,Mo,Ta,V,W) N0.79 and (Al,Mo,Ta,V,W)N0.33 consist of highly oriented columns, plus some very large grains in the latter coating. During vacuum annealing at 800 °C for 30 h, the (Al,Mo,Ta,V,W)N0.79 loses N down to 25 at.%, while the (Al,Mo,Ta,V,W)N0.33 remains stable. Their alignment in chemical composition also caused an approach of their hardness values with 35.2 and 38.1 GPa, respectively. The (Al,Mo,Ta,V,W)N0.88O0.12 exhibits partly tilted and randomly oriented smaller columnar grains than the nitrides, and the hardness drops from 34.5 to only 14.1GPa when vacuum annealed due to massive phase-transformations toward individual oxides and the connected crack formation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Zig-zag structured porous γ-Mo2N films were prepared by glancing angle sputtering.•A solid-state symmetric zig-zag γ-Mo2N supercapacitor device was produced.•The device achieves both ...superior high energy density and power density.•The device exhibits excellent cycling stability and desirable mechanical flexibility.•The open porous nanostructure accelerates ion diffusion and electron transfer.
The performance of electrochemical capacitors strongly depends on their accessible surface area, chemical stability, and electrical conductivity. Simple columnar and zig-zag γ-Mo2N thin-film electrodes were prepared by magnetron sputtering. The latter is obtained by glancing angle deposition (GLAD), which is known for open porous structures due to the ballistic shadowing effect. As intended, the zig-zag structured γ-Mo2N electrode presents an outstanding area capacitance of 248 mF/cm2 at a scan speed of 50 mV/s, which is 4 times higher than that of the simple columnar one. Both of them exhibit excellent cycling stability of 95 % over 20,000 cycles (at 200 mV/s). The symmetrical solid-state supercapacitor prepared with the zig-zag structured γ-Mo2N thin film delivers an excellent power density of 107.1 W/cm3 at 33.8 mWh/cm3, and its volumetric capacitance is ∼ 3.5 times higher than the simple columnar structured γ-Mo2N device. Bending tests of such solid-state γ-Mo2N supercapacitors proved their mechanical flexibility to a bending angel of even 107°. Based on these studies we can conclude that the highly porous zig-zag structured γ-Mo2N-based electrodes prepared by GLAD combine outstanding electrochemical energy storage capabilities with excellent mechanical flexibility.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Ab-initio calculations show good compatibility of B incorporation in fcc-(Hf,Ta,Ti,V,Zr) N.•Reactive sputtering leads to a mixture of fcc and BN phase with corresponding hardness ...decrease.•Non-reactive sputtering leads to single-phase fcc coatings with high Zr and C contents and up to 46 GPa hardness.•Vacuum annealing at 1200 °C leads to no phase change in non-reactively sputtered boronnitride coatings.
Phase stability and mechanical properties of multimetal-boronnitride (Hf,Ta,Ti,V,Zr)-B-N is investigated by ab initio computations and experimental methods. (Hf,Ta,Ti,V,Zr)-B-N shows a strong energetic preference for the fcc NaCl-type structure over other structures up to a B:N ratio of 3.5. Reactively deposited (Hf,Ta,Ti,V,Zr)-B-N coatings show formation of X-ray amorphous BN, accompanied by a drastic hardness decrease with increasing B content. But non-reactively sputtered (Hf,Ta,Ti,V,Zr)-B-N coatings exhibit a single-phase fcc solid solution, up to the maximum B:N ratio of 1.12 studied, in good agreement with calculations. All non-reactively sputtered multimetal-boronnitride coatings contain a high Zr metal-fraction and ≈8at% C, stemming from impurities in the target. The single-phase coatings reach superhardness up to 46.3 GPa. Even after vacuum annealing to 1200°C, the hardness of the coating with a B:N ratio of 1.03 is still 43.7 GPa, while that of ZrN0.72C0.28 decreased from 36.3 to 30.2 GPa. Our results demonstrate the importance of the deposition technique to deposit single-phased coatings with exceptional hardness and thermal stability.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Sufficient Ni content leads to an interconnected Ni-network in the CrN–Ni coating.•Nanoporous CrN can be obtained by HCl etching of CrN–Ni.•Nanoporous CrN has an ~ 80% higher ...specific capacitance than CrN.•Guideline to prepare porous transition metal nitrides with a high specific surface area.
Transition metal nitrides (TMN) have received widespread consideration as supercapacitor materials for a variety of energy storage applications. An important requirement to such electrode materials is a very high specific surface area, which is typically obtained through a percolating pore-network. Here, we use arc ion plating to prepare CrN-Ni composite coatings with 0, 30.4, 54.2, and 77.6 at.% Ni. Detailed X-ray diffraction and X-ray photoelectron spectroscopy showed that the coatings consist of CrN phases next to metallic Ni. Post-deposition treatments with HCl solutions lead to selective leaching, causing a porous nanostructure for the Ni-containing CrN coatings. From there, the coating with 54.2 at.% Ni in its as-deposited state provided the highest (electrode-geometric-area)-specific capacitance after the HCl treatment, giving 58.5 mF·cm−2 at 1.0 mA·cm−2 in a 0.5 M H2SO4 aqueous electrolyte. This value is about 80 times higher than for the as-deposited coatings or the Ni-free CrN. Consequently, our strategy to combine TMNs with metallic Ni and its subsequent treatment with HCl is highly effective in enlarging the specific surface area and adsorption sites. It allows to significantly enhance the energy storage performance of TMN thin film supercapacitors.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Based on density functional theory, we recently suggested that metastable α-WB2 is a promising candidate combining very high hardness with high toughness. These calculations further suggested that ...the addition of Tantalum supports the crystallization of α-structured W1−xTaxB2−z, with only minor reduction in toughness. Thus, various Ta containing WB2-based coatings have been synthesized using physical vapor deposition. With increasing Ta content, the hardness increases from ~41 GPa (WB2) to ~45 GPa (W0.74Ta0.26B2). In situ micromechanical cantilever bending tests exhibit fracture toughness KIC values of 3.7 to 3.0 MPa√m for increasing Ta content (single-phased up to 26 at.% Ta).
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In this study, the effects of the microstructural properties on the deformation and damage mechanism of CrN/TiN multilayer coatings deposited on Custom 450 steel using the unbalanced reactive ...magnetron sputtering PVD process were studied. All coatings were fabricated with an overall thickness of 1.5 µm, but different bilayer periods (Λ). Structural and mechanical properties of coatings were investigated by XRD analysis and nanoindentation experiment, respectively. Indentation tests at three loads of 100, 300, and 450 mN were performed on the coatings’ surface and then, cross-sections of fractured imprints were analyzed with SEM and TEM. Measuring the length of the cracks induced by indentation loads and analyzing the load-displacement curves, apparent fracture energy values of multilayer coatings were calculated. We observed that multilayer systems with bilayer periods of 4.5–15 nm possess superlattice structure, which also results in higher values for Young’s modulus and hardness as well as higher fracture energy. Comparison of cross-sectional SEM and TEM observations showed that coatings with smaller bilayer periods tend to deform by shear sliding mechanism due to the existence of the long-grown columns, while short dispersed grains—growing in the coatings with a larger bilayer period—led to deformation via local grain boundary sliding and grain rotation.
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