The fundamental understanding of the relation between the architecture and the development of structure and residual stress in multilayer coatings is a challenging task. In this work, cross-sectional ...X-ray nanodiffraction with a spatial resolution of 50 nm, performed at the cross-section of a Al0.7Cr0.3N/Al0.67Ti0.33N multilayer coating was used to investigate its depth gradients of the phase composition and residual stress. It could be demonstrated that the cubic structure can be stabilized in a substrate bias range between −100 and −200 V developing stress values above −5 GPa. Below −100 V and above −200 V a dual phase microstructure developed with reduced stress values compared to the pure cubic regime. Based on the results of the cross-sectional X-ray nanodiffraction investigation, five multilayer architectures, differing in the sublayer thickness, interface design and microstructure were developed to test the wear resistance by ball-on-disc experiments at temperatures up to 700 °C. It could be demonstrated that a sublayer thickness between 70 and 100 nm has been proven to be an ideal thickness for such an Al0.7Cr0.3N/Al0.67Ti0.33N multilayer to decrease the wear rate by a factor of 10 at 600 °C compared to their monolithic counterparts.
•Cross-sectional X-ray nanodiffraction with a spatial resolution of 50 nm•Investigation of depth gradients of the microstructure and residual stress•Demonstration of various approaches to (de)stabilize the metastable cubic phase•Increase in wear resistance due to bi-layer and substrate bias voltage variations
The dependence of decomposition routes on intrinsic microstructure and stress in nanocrystalline transition metal nitrides is not yet fully understood. In this contribution, three Al
Cr
N thin films ...with residual stress magnitudes of -3510, -4660 and -5930 MPa in the as-deposited state were in-situ characterized in the range of 25-1100 °C using in-situ synchrotron high-temperature high-energy grazing-incidence-transmission X-ray diffraction and temperature evolutions of phases, coefficients of thermal expansion, structural defects, texture as well as residual, thermal and intrinsic stresses were evaluated. The multi-parameter experimental data indicate a complex intrinsic stress and phase changes governed by a microstructure recovery and phase transformations taking place above the deposition temperature. Though the decomposition temperatures of metastable cubic Al
Cr
N phase in the range of 698-914 °C are inversely proportional to the magnitudes of deposition temperatures, the decomposition process itself starts at the same stress level of ~-4300 MPa in all three films. This phenomenon indicates that the particular compressive stress level functions as an energy threshold at which the diffusion driven formation of hexagonal Al(Cr)N phase is initiated, provided sufficient temperature is applied. In summary, the unique synchrotron experimental setup indicated that residual stresses play a decisive role in the decomposition routes of nanocrystalline transition metal nitrides.
In order to understand the fracture resistance of nanocrystalline thin films, it is necessary to assess nanoscopic multiaxial stress fields accompanying crack growth during irreversible deformation. ...Here, a clamped cantilever with dimensions of 200 × 23.7 × 40 μm3 was machined by focused ion beam milling from a thin film composed of four alternating CrN and Cr layers. The cantilever was loaded to 460 mN in two steps and multiaxial strain distributions were determined by in situ cross-sectional X-ray nanodiffraction. Characterization in as-deposited state revealed the depth variation of fibre texture and residual stress across the layers. The in situ experiment indicated a strong influence of the residual stresses on the cross-sectional stress fields evolution and crack arrest capability at the CrN-Cr interface. In detail, an effective negative stress intensity of −5.9 ± 0.4 MPa m½ arose as a consequence of the residual stress state. Crack growth in the notched Cr layer occurred at a critical stress intensity of 2.8 ± 0.5 MPa m½. The results were complemented by two-dimensional numerical simulation to gain further insight into the elastic-plastic deformation evolution. The quantitative experimental and modelling results elucidate the stepwise nature of fracture advancement across the alternating brittle and ductile layers and their interfaces.
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•Multi-axial residual stress distributions across a notched CrN-Cr clamped micro-cantilever were determined experimentally.•Residual stresses in Cr introduce an effective stress intensity of −5.9 ± 0.4 MPa m½, forming a plastic zone around the notch.•Upon crack growth through Cr to the adjacent CrN-Cr interface, the crack is arrested there and its stress fields disappears.
In this work, cross-sectional position-resolved X-ray nanodiffraction with a beam diameter of ~50 nm, was used to characterize the depth evolution of microstructure, texture and residual stress ...across an Al0.7Cr0.3N/Al0.9Cr0.1N multilayer coating cross-section deposited by cathodic arc evaporation. The method allowed to resolve variations in microstructure and stress state in all individual sublayers of the multilayer coating which was synthesized to include three different design approaches separated in individual sections. By this cross-sectional combinatorial approach, phase (de)stabilization in an alternating cubic and hexagonal multilayer structure (section 1), incident particle energy-dependent microstructure depth-gradients in the cubic phases (section 2) and phase (de)stabilization related to a gradual phase change from cubic to hexagonal (section 3) were investigated. While the in-plane residual stresses in the cubic AlCrN phase (sections 1 and 2) slightly fluctuated between −3 and −3.5 GPa, the compressive stress state in the hexagonal AlCrN phase showed a layer thickness dependency with values up to −6.5 GPa for sublayer thicknesses below 100 nm and down to −1 GPa for sublayer thicknesses above 600 nm. The presented results document that the cross-sectional X-ray nanodiffraction is a highly effective characterization method to investigate coatings with optimised architecture and dedicated stress design.
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•Development of a cross-sectional combinatorial approach based on AlCrN•Highly position resolved analysis across the coating thickness•Evaluation of microstructure, texture and stress gradients•Control of microstructure dependent on templating effects of adjacent sublayers•Control of the crystallographic structure dependent on the incident particle energy
In order to interpret the mechanical response of thin films subjected to scratch tests, it is necessary to elucidate local stress distributions and microstructural changes accompanying deformation ...across the scratch track area. Here, 50 nm synchrotron cross-sectional X-ray nanodiffraction and electron microscopy are used to characterize nanoscale multiaxial residual stress gradients and irreversible microstructural-morphological changes across a brittle-ductile film consisting of 1.2 and 2 μm thick CrN and Cr sublayers. The experimental results reveal a complex alternation of the original columnar grain microstructure and a formation of pronounced lateral and depth stress gradients, which are complemented by a finite element model. After scratching, steep gradients of in-plane, out-of-plane and shear stress distributions were revealed, ranging from −6 to 1.5 and − 1.5 to 1.5 GPa in CrN and Cr, respectively, which are furthermore correlated with microstructural changes and residual curvatures. The scratch test results in intergranular grain sliding and the formation of nanoscopic intragranular defects within CrN, while Cr sublayer's thickness reduction and pile-up formation are accompanied by a bending of columnar crystallites and localized plastic deformation. In summary, the quantitative stress data elucidate the stabilizing role of the Cr sublayer, which suppresses the bilayer's catastrophic fracture during scratch tests.
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•Experimentally determined multi-axial residual stress distributions across a scratched Cr-CrN bilayer film reveal deformation mechanisms.•The ductile Cr sublayer provides toughening through plastic deformation while the hard/brittle CrN toplayer absorbs severe stresses, functioning synergistically.•Small-angle X-ray scattering and X-ray peak width analyses elucidate microstructural changes unaccounted for by finite element modelling.
Superhard nanocomposite coatings are currently of great interest for wear protection of tools. Within this work, after some consideration of the design and failure of nanocomposites, results on ...nanocomposite Ti–B–N and Ti–B–C films are presented and discussed. Coatings with different compositions in the quasi-binary systems TiN–TiB
2 and TiC–TiB
2 were deposited onto austenitic stainless steel and molybdenum sheets by means of unbalanced d.c. magnetron co-sputtering using segmented TiN/TiB
2 and TiC/TiB
2 targets. Coating chemical, structural and mechanical properties were investigated using electron-probe microanalysis (EPMA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), and depth-sensing nanoindentation. Coatings with elemental compositions of about 35–45
at.% Ti, 18–44
at.% B and 20–36
at.% N or 22–40
at.% C consist of a nanocrystalline arrangement of TiB
2 and TiN or TiC, respectively, with crystallite sizes of 1–5
nm. Coating hardness varies between 50 and 70
GPa and elastic moduli are close to 500
GPa.
Despite their high hardness and indentation modulus, nanostructured crystalline ceramic thin films produced by physical vapour deposition usually lack sufficient fracture strength and toughness. This ...brittleness is often caused by underdense columnar grain boundaries of low cohesive energy, which serve as preferential paths for crack propagation. In this study, mechanical and structural properties of arc-evaporated Al0.9Cr0.1N thin films were analyzed using micromechanical tests, electron microscopy, atom probe tomography and in situ high-energy high-temperature grazing incidence transmission X-ray diffraction. Vacuum annealing at 1100°C resulted in the formation of regularly-distributed globular cubic Cr(Al)N and elongated cubic CrN precipitates at intracrystalline Cr-enriched sublayers and at columnar grain boundaries with sizes of ∼5 and ∼30 nm, respectively. Consequently, in situ micromechanical testing before and after the heat treatment revealed simultaneous enhancement of Young's modulus, fracture stress and fracture toughness by ∼35, 60 and 10%, respectively. The annealing-induced concomitant improvement of toughness and strength was inferred to precipitations observed within grains as well as at grain boundaries enhancing the cohesive energy of the grain boundaries and thereby altering the crack propagation pathway from inter- to transcrystalline. The here reported experimental data unveil the hitherto untapped potential of precipitation-based grain boundary design for the improvement of mechanical properties of transition metal nitride thin films.
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The lifetime of coated cutting tools is decisively influenced by residual stress gradients across cutting edges. Here, cross-sectional X-ray nanodiffraction with a beam size of 35 × 25 nm2 was ...used to retrieve gradient properties of a ~2 µm thick TiN coating on WCCo substrate. The planar regions next to the edge exhibit gradual and constant stress profiles with anisotropic defect build-ups on the flank and rake faces, respectively. Directly at the edge, nonlinear lateral and cross-sectional compressive residual stress gradients up to about −3 GPa were observed. The coating's 〈111〉 fibre texture orientation correlates with the orientation of principal stress components.
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