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.
Display omitted
•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.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Display omitted
•The first experimental observation of bridge notch failure is presented.•The deviation of toughness from bridge-failure and the main cantilever failure is 7%.•Multiple toughness ...values can be gained from a single test improving the statistics.•Potential artifacts introduced by the Ga+ FIB pre-notches are reduced.
Focused ion beam (FIB) milling has been widely used to prepare micron-sized specimens for micromechanical testing, however, at the same time, unavoidable artifacts originating from the Ga+ ion beam might alter the obtained mechanical properties. Using a bridge notch geometry, which can promote the formation of a sharp natural crack after bridge-failure rather than creating a comparably blunt FIB notch was proposed as a strategy to reduce FIB-induced artifacts more than a decade ago. Even though bridge-failure is widely assumed and predicted by finite element method (FEM) simulations, it has never been observed and quantified experimentally. This study presents the first experimental observation of bridge notch failure and crack arrest before the entire through-thickness main notch (after crack arrest) propagates, which is possible by designing thin bridges and using a stiff experimental setup with superior load resolution. Consequently, we obtained up to three corresponding fracture toughness values from one bending cantilever and significantly less scattered data. Using previously reported geometry correction factors calculated by FEM, the fracture toughness estimated from the bridge-failure was compared with the one from the failure of the main through-thickness notch in CrN/AlN multi-layered and CrN hard coatings.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Display omitted
•The microstructure (point defects structure) varies with applied bias voltages.•Different point defect structures lead to distinct electronic structures.•The electrical conductivity ...and fracture toughness of TiN change with the point defect.•Point defect structures are directly correlated to the mechanical properties of TiN.
Defects significantly affect the mechanical properties of materials. However, quantitatively correlating the point defects with mechanical property could be a challenge. In this study, we explore the point defect effects on the structure and property of magnetron sputtered TiN nanocrystalline films (synthesized using different negative bias potential) via a combination of analytical techniques and density functional theory (DFT) calculations. We gain insights into the structural evolution and properties of nanocrystalline films at different length scales. It is found that nanocrystal microstructure and local electronic structure triggered by various point defects remarkably change. Along with the structural evolution and point defect changes, the electrical conductivity and the fracture toughness of TiN are improved. Furthermore, the fracture toughness, Young’s modulus, and cleavage energy and stresses for TiN films with different point defect structures are calculated. The experimental data is in excellent agreement with first-principle calculations. Our results suggest a direct correlation of the point defect structure with TiN films' mechanical properties.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Machining and forming tools exposed to challenging environments require protective coatings to extend their lifetime and reliability. Although transition metal nitrides possess excellent strength and ...resistance against chemical attacks, they lack ductility and are prone to premature failure. Here, by investigating structural and mechanical properties of MoN-TaN superlattices with different bilayer thickness, we develop coatings with high fracture toughness and hardness, stemming from the formation of a metastable tetragonally distorted phase of TaN up to layer thicknesses of 2.5 nm. Density functional theory calculations and experimental results further reveal a metal-vacancy stabilized cubic Ta
0.75
N phase with an increased Young’s modulus but significantly lower fracture toughness. We further discuss the influence of coherency strains on the fracture properties of superlattice thin films. The close interplay between our experimental and ab initio data demonstrates the impact of phase formation and stabilization on the mechanical properties of MoN-TaN superlattices.
Transition metal nitride coatings exhibit high hardness, but typically lack ductility and are therefore prone to failure. Here, the effect of bilayer thickness on the mechanical properties of MoN-TaN superlattices is investigated, leading to coatings with high fracture toughness.
Superlattice architecture represents an effective strategy to improve performance of hard protective coatings. Our model system, MoN/TaN, combines materials well-known for their high ductility as ...well as a strong driving force for vacancies. In this work, we reveal and interpret peculiar structure-stability-elasticity relations for MoN/TaN combining modelling and experimental approaches. Chemistry of the most stable structural variants depending on various deposition conditions is predicted by Density Functional Theory calculations using the concept of chemical potential. Importantly, no stability region exists for the defect-free superlattice. The X-ray Diffraction and Energy-dispersive X-ray Spectroscopy experiments show that MoN/TaN superlattices consist of distorted fcc building blocks and contain non-metallic vacancies in MoN layers, which perfectly agrees with our theoretical model for these particular deposition conditions. The vibrational spectra analysis together with the close overlap between the experimental indentation modulus and the calculated Young's modulus points towards MoN0.5/TaN as the most likely chemistry of our coatings.
Display omitted
•Cubic-based MoN/TaN superlattices exist in many vacancy-stabilised variants.•Under typical deposition conditions, the most favourable is MoN0.5/TaN.•MoN0.5/TaN is experimentally feasible and promises high ductility/toughness.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Transition metal nitride thin films traditionally possess a low intrinsic fracture toughness. Motivated by the recently discovered fracture toughness enhancing superlattice (SL) effect, as well as ...the remarkably high potential for toughness predicted by theoretical studies for TiN/WN superlattices, we synthesise a series of these materials by DC reactive magnetron sputtering. The SL coatings demonstrate a vacancy-stabilised cubic configuration throughout, as well as a marginal lattice mismatch between the TiN and WN layers. All investigated mechanical properties produced a distinct dependence on the bilayer period, featuring a hardness peak of 36.7 ± 0.2 GPa and a minimum of the indentation modulus of 387 ± 2 GPa. The toughness-related quantities of the SLs in particular show a significant enhancement compared to monolithic TiN and WN, including a tripling of the fracture energy. The fracture toughness is raised from 2.8 ± 0.1 (TiN) and 3.1 ± 0.1 (WN) to 4.6 ± 0.2 MPa√m by the SL arrangement. We relate this maximisation to the vastly disparate elastic moduli and compositional fluctuations. To complement our experimental data, we present Density Functional Theory-based models to disentangle the conspicuous trends observed for TiN/WN superlattices.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Although the microstructure evolution during TiN film growth has been extensively studied, how the atomic-resolution structure of the films changes with the deposition conditions remains unclear. ...Here, a series of TiN films were prepared on silicon substrates at different bias voltages (Vb) by magnetron sputtering. The microstructure and atomic resolution structures of the films were systematically investigated with transmission electron microscopy (TEM) and high resolution TEM (HRTEM), complemented by X-ray photoelectron spectroscopy measurement and density functional theory (DFT) calculations.
With increasing the bias voltage, the morphology of columnar grain evolves, and grain size and dislocation density increase, and films become much dense. HRTEM observations reveal that the boundaries between two neighboring grains are dominated by low-angle grain boundaries in the case of floating bias conditions. Numerous twins surprisingly form in the TiN films under certain bias voltage. The initiation of twins in TiN films can be attributed to the reduction of stacking fault energy (SFE) caused by non-stoichiometry to a certain degree, i.e., N/Ti atomic ratio, as validated by theoretical calculations. Moreover, the mechanisms of hardness alterations under different bias-voltages are discussed.
Display omitted
•The structure evolution of magnetron sputtered TiN films with bias voltage (Vb).•Grain size and dislocation density in TiN films increase with increasing bias voltage.•HRTEM reveals that the boundaries are dominated by low-angle grain boundaries.•Twins are observed in the TiN films under certain bias conditions.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
At present, the theoretical predictions of the mechanical properties of transition-metal nitride (TMN) superlattices (SLs) are primarily based on the intrinsic properties of perfect epitaxial ...nanolayers. However, due to a lack of understanding of the specific strengthening mechanism, the experimentally determined strength, e.g., hardness, of TMN SLs often deviates significantly from the theoretical predictions. Here, by coupling FIB (focused ion beam) sectioning with TEM, we observe the structural evolution of two representatives TiN/AlN SL coatings, i.e., a single-crystalline and a polycrystalline SL, under identical loads. We found that in comparison with the polycrystalline SL, the indented single-crystalline SL forms a larger ‘intermixed’ region, within which the layer structure transforms into a solid solution under loads. Close TEM characterization demonstrates that the single-crystalline SL deformation is of variety, including the distortion of SL interfaces, polycrystalline deformation (grain rotation) in solid solution, and SL slip deformation. By contrast, columnar grain boundary sliding is the primary deformation mechanism in the polycrystalline SL. And, a relatively large solid-solution zone in single-crystalline SL is attributed to the severe interfacial deformation. The current research unravels TMN SL deformation behavior at the atomic scale.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The mechanical properties of superlattice (SL) TMN (transition-metal nitrides) coatings with different as-deposited structures are often quite different. These differences in mechanical properties ...can be attributed to distinct deformation and strengthening mechanisms. Here, we discuss the strengthening mechanisms of single- and poly-crystalline SLs under nanoindentation loads. We observe that the dislocation behaviors during nanoindentation, such as dislocation accumulation and crossing interfaces, are responsible for the strengthening of single-crystalline SL coating, whereas no such pronounced strengthening is observed in the polycrystalline SL. We further reveal the monoclinic phase transformation occurring at the SL, solid solution zone, and crack tip region in the single-crystalline coating. Phase transformation alters the SL interface's structure, facilitating dislocation accumulation. Consequently, it raises the theoretical yield stress of single-crystalline coating. For polycrystalline coating, we observed a localized monoclinic phase present only near the crack tip. The current research unravels TMN SL strengthening mechanism at the atomic scale.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Mechanical properties of nanoscale multilayer coatings are to a large extent governed by the number of interfaces and their characteristics. While for a reduced layer thickness, increasing strength ...and toughness values have been reported, properties degrade for layer thicknesses of just several nanometers. Here, we report on an entirely overlooked phenomenon occurring during the indentation of nanolayers, presumably explaining the degradation of properties. Nanoindentation, commonly used to determine properties of hard coatings, is found to disrupt and intermix the multilayer structure due to the deformation imposed. Detailed electron microscopy studies and atomistic simulations provide evidence for intermixing in an epitaxial transition metal nitride superlattice thin film induced by nanoindentation. The formation of a solid solution reduces the interfacial density and leads to a sharp drop in the dislocation density. Our results confirm that plastic deformation causes the microstructure instability of nitride multilayer, which may further improve our understanding of multilayer strength mechanisms.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
