Cross-sectional X-ray Nanodiffraction Research Articles

Multimethod cross-sectional characterization approaching the limits of diamond monophase multilayers

Chemical vapour deposited diamond draws significant scientific interest due to its wide range of mechanical and functional properties, which can be easily tuned by the applied deposition conditions. Here, hot filament chemical vapour deposition was used to synthesize four monophase multilayer diamond thin films with individual layer thickness down to ∼ 300 nm. Extensive structural characterization by scanning electron microscopy and Raman spectroscopy on the cross-section confirmed the different diamond morphologies originating from the different applied process parameters of the individual sublayers, where Raman spectroscopy could give a clear distinction between nano- and microcrystalline diamond down to a layer thickness of ∼ 1 μm. Cross-sectional X-ray nanodiffraction identified exclusively the diamond crystal structure throughout the cross-section of the multilayered diamond thin films. Cross-sectional phase and FWHM analysis allowed to discriminate between the diamond morphologies down to ∼ 500 nm, thus identifying the limitations of monophase diamond multilayers. While the microcrystalline diamond sublayers all exhibit pronounced structural gradients expressed in (i) increasing intensity of the 111 Debye-Scherrer ring, (ii) ⟨110⟩ fibre texture, and periodic variations of (iii) grain size and (iv) residual stress, the nanocrystalline diamond sublayers showed no pronounced cross-sectional variations. In summary, the experimental results provide insights into the cross-sectional evolution of microstructure and residual stress and the limitations of monophase multilayered diamond thin films.

Investigation of the microstructure of a graded ZrN/Ti0.33Al0.67N multilayer coating using cross-sectional characterization methods

An approach to enhance the performance of protective hard coatings for cutting applications is to modify the coating architecture by combining two inherently different materials in a multilayer. Besides the choice of the materials, the thickness of the individual layers strongly influences the coating microstructure and consequently also its properties. Within this work, a graded ZrN/Ti0.33Al0.67N multilayer coating with constant Ti0.33Al0.67N and stepwise increasing ZrN layer thickness was investigated in detail by a combinatorial approach of cross-sectional X-ray nanodiffraction, electron backscatter diffraction and transmission electron microscopy. The primary aim was to obtain a profound understanding of the microstructure of the coating as well as of the residual stress state. (Semi-)coherent grain growth was observed independently of the ZrN layer thickness. Changes in the multilayer architecture were found to affect not only the grain size, but also the residual stress state of the coating. While the grain size increased with increasing ZrN layer thickness, the residual stress decreased. This work contributes to a deeper understanding of the influence of the multilayer architecture on the microstructure and stress state of heteroepitactic multilayer coatings.

Impact of Si on the high-temperature oxidation of AlCr(Si)N coatings

• AlCr(Si)N coatings with an Al/Cr ratio of 70/30 and 0, 2.5 and 5 at.% Si were deposited • The oxidation behaviour up to 1460 °C was studied • Coatings were analysed by advanced cross-sectional X-ray nanodiffraction • The onset of oxidation was increased by the addition of Si from 1100 °C to 1260 °C • Complex elemental and micro-structural variations after oxidation were characterized The resistance of wear protective coatings against oxidation is crucial for their use at high temperatures. Here, three nanocomposite AlCr(Si)N coatings with a fixed Al/Cr atomic ratio of 70/30 and a varying Si-content of 0 at.%, 2.5 at.% and 5 at.% were analyzed by differential scanning calorimetry, thermogravimetric analysis and X-ray in order to understand the oxidation behavior depending on their Si-content. Additionally, a partially oxidized AlCrSiN coating with 5 at.% Si on a sapphire substrate was studied across the coating thickness by depth-resolved cross-sectional X-ray nanodiffraction and scanning trans-mission electron microscopy to investigate the elemental composition, morphology, phases and residual stress evolution of the oxide scale and the non-oxidized coating underneath. The results reveal enhanced oxidation properties of the AlCr(Si)N coatings with increasing Si-content, as demonstrated by a retarded onset of oxidation to higher temperatures from 1100 °C for AlCrN to 1260 °C for the Si-containing coatings and a simultaneous deceleration of the oxidation process. After annealing of the AlCrSiN sample with 5 at.% Si at an extraordinary high temperature of 1400 °C for 60 min in ambient air, three zones developed throughout the coating strongly differing in their composition and structure: (i) a dense oxide layer comprising an Al-rich and a Cr-rich zone formed at the very top, followed by (ii) a fine-grained transition zone with incomplete oxidation and (iii) a non-oxidized zone with a porous structure. The varying elemental composition of these zones is furthermore accompanied by micro-structural variations and a complex residual stress development revealed by cross-sectional X-ray nanodiffraction. The results provide a deeper understanding of the oxidation behavior of AlCr(Si)N coatings depending on their Si-content and the associated elemental, microstructural and residual stress evolution during high-temperature oxidation.

Evolution of structure, residual stress, thermal stability and wear resistance of nanocrystalline multilayered Al0.7Cr0.3N-Al0.67Ti0.33N coatings

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 Al 0.7 Cr 0.3 N/Al 0.67 Ti 0.33 N 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 Al 0.7 Cr 0.3 N/Al 0.67 Ti 0.33 N 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

Ion irradiation-induced localized stress relaxation in W thin film revealed by cross-sectional X-ray nanodiffraction

The influence of ion irradiation on residual stress and microstructure of thin films is not fully understood. Here, 5 MeV Si2+ ions were used to irradiate a 7 µm thick tungsten film prepared by magnetron sputtering. Cross-sectional X-ray nanodiffraction and electron microscopy analyses revealed a depth-localized relaxation of in-plane compressive residual stresses from −2.5 to −0.75 GPa after the irradiation, which is correlated with the calculated displacements per atom within a ~2 µm thick film region. The relaxation can be explained by the irradiation-induced removal of point defects from the crystal lattice, resulting in a reduction of strains of the 3rd order, manifested by a decrease of X-ray diffraction peak broadening, an increase of peak intensities and a decrease of lattice parameter. The results indicate that ion irradiation enables control over the residual stress state at distinct depths in the material.

Evolution of stress fields during crack growth and arrest in a brittle-ductile CrN-Cr clamped-cantilever analysed by X-ray nanodiffraction and modelling

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.

Nanoscale stress distributions and microstructural changes at scratch track cross-sections of a deformed brittle-ductile CrN-Cr bilayer

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.

Nanoscale residual stress and microstructure gradients across the cutting edge area of a TiN coating on WC[sbnd]Co

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.

Structure-stress relationships in nanocrystalline multilayered Al0.7Cr0.3N/Al0.9Cr0.1N coatings studied by cross-sectional X-ray nanodiffraction

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.

Evolution of structure and residual stress of a fcc/hex-AlCrN multi-layered system upon thermal loading revealed by cross-sectional X-ray nano-diffraction

Understanding the influence of process conditions and coating architecture on the microstructure and residual stress state of multi-layered coatings is essential for the development of novel thermally and mechanically stable coatings and requires advanced depth resolving characterization techniques. In this work, an arc-evaporated multi-layered coating, consisting of alternating Al70Cr30N and Al90Cr10N sublayers with an individual layer thickness between 120 nm and 380 nm, was investigated. The as-deposited state of the multi-layered coating and the state after vacuum annealing at 1000°C for 30 min was studied along its cross-section by synchrotron X-ray nano-diffraction using a beam with a diameter of 50 nm. The results revealed sublayers with alternating cubic and hexagonal phase, causing repeated interruption of the grain growth at the interfaces. The in-plane residual stress depth distribution across the coating thickness could be tuned in a wide range between pronounced compressive and slight tensile stress by combining the effects of the coating architecture and the modulated incident particle energy controlled by the substrate bias voltage ranging from −30 V to −250 V. This resulted in an oscillatory stress profile fluctuating between −2 GPa and −4.5 GPa or pronounced stress gradients with values between −4 GPa and 0.5 GPa. Finally, the decomposition routes of the metastable cubic Al70Cr30N phase could be controlled by the Al90Cr10N sublayers which acted as nucleation sites and governed the texture of the decomposition products as Cr2N. The results demonstrate that the cross sectional combinatorial approach, relying on a sophisticated multi-layer architecture combining various materials synthesized under tailored conditions, allowed for resolving structural variations and stress profiles in the individual layers within the complex architecture and pioneers the path for knowledge-based development of multi-layered coatings with predefined microstructure and a dedicated stress design.

In-situ Observation of Cross-Sectional Microstructural Changes and Stress Distributions in Fracturing TiN Thin Film during Nanoindentation.

Load-displacement curves measured during indentation experiments on thin films depend on non-homogeneous intrinsic film microstructure and residual stress gradients as well as on their changes during indenter penetration into the material. To date, microstructural changes and local stress concentrations resulting in plastic deformation and fracture were quantified exclusively using numerical models which suffer from poor knowledge of size dependent material properties and the unknown intrinsic gradients. Here, we report the first in-situ characterization of microstructural changes and multi-axial stress distributions in a wedge-indented 9 μm thick nanocrystalline TiN film volume performed using synchrotron cross-sectional X-ray nanodiffraction. During the indentation, needle-like TiN crystallites are tilted up to 15 degrees away from the indenter axis in the imprint area and strongly anisotropic diffraction peak broadening indicates strain variation within the X-ray nanoprobe caused by gradients of giant compressive stresses. The morphology of the multiaxial stress distributions with local concentrations up to −16.5 GPa correlate well with the observed fracture modes. The crack growth is influenced decisively by the film microstructure, especially by the micro- and nano-scopic interfaces. This novel experimental approach offers the capability to interpret indentation response and indenter imprint morphology of small graded nanostructured features.

Microstructure-controlled depth gradients of mechanical properties in thin nanocrystalline films: Towards structure-property gradient functionalization

Although the influence of the grain size on the mechanical properties of polycrystalline materials is well understood, the occurrence of depth-gradients of grain size, microstructure, and residual stresses in nanocrystalline thin films and their effect on the functional properties is a phenomenon which has not yet been studied in detail. Hence in this work, single-layered polycrystalline and mosaic epitaxial as well as multilayered CrN thin films were characterized by a combination of averaging as well as depth-resolved experimental techniques, such as cross-sectional X-ray nanodiffraction and small-angle cross-sectional nanoindentation. The results reveal the fundamental relationship between gradients of grain size, microstructure, and stresses, controlled by the film growth conditions, and gradients of hardness and elastic modulus. The effect of the compressive stress and structural defects associated with high particle energy on the mechanical properties of nanocrystalline thin films was found to be dominant over the grain size and crystallographic texture. These findings open the way to functionalize structure-property gradients in nanocrystalline thin films through microstructural design as demonstrated for multilayered CrN films.

Cross-sectional X-ray nano-diffraction and -reflectivity analysis of multilayered AlTiN–TiSiN thin films: Correlation between residual strain and bi-layer period

A multi-layered AlTiN–TiSiN thin film with a bi-layer period of ∼6nm is characterized by cross-sectional synchrotron X-ray nano-diffraction and -reflectivity using an X-ray beam size of 250×350nm2. The complementary approach allows for simultaneous determining gradients of residual strains and bi-layer thickness along the film depth and demonstrates a correlation between them. The observed dependency allows for a residual strain gradient design in multilayered thin films.

Resolving depth evolution of microstructure and hardness in sputtered CrN film

Hardness and elastic modulus of a sputtered nanocrystalline CrN thin film, prepared under varying ion bombardment conditions, were studied by nanoindentation using a depth-profiling technique and related to cross-sectional X-ray nanodiffraction data on the local microstructure. Changes in texture are shown to have almost no effect on the elastic modulus due to the isotropic response of the polycrystals. However, the locally varying growth conditions, which affect the crystal size and the number of defects in the film, determine the hardness values across the film thickness. Regions with highly distorted small crystals result in higher hardness in comparison with those with well-developed coarsened grains. This study confirms the notion of the existence of growth-related hardness gradients in single-phase nanocrystalline thin films.

Superior oxidation resistance, mechanical properties and residual stresses of an Al-rich nanolamellar Ti0.05Al0.95N coating prepared by CVD

The influence of microstructure on thermal and oxidation properties of Ti1-xAlxN hard coatings is not fully understood. In this work, an Al-rich Ti0.05Al0.95N coating with a unique self-organized microstructure of alternating soft hexagonal (w) AlN and hard cubic (fcc) TiN nanolamellae synthesized by a low-pressure chemical vapor deposition (CVD) process is analyzed in terms of oxidation resistance, microstructure and phase stability, hardness as well as residual stresses. Multiple coating samples on hard metal substrates were oxidized in ambient air for 1h at temperatures in the range of 700–1200°C. Exceptionally good oxidation resistance up to 1050°C was found, whereas above 1100°C a localized surface degradation caused by the substrate–air interaction with very characteristic oxide blisters was observed. A coating hardness of about 29GPa remained in all unaffected surface areas for temperatures up to 1050°C. This is interpreted by the specific lamellae microstructure that retained a nanocomposite character even after lamellae partly decomposed during annealing. Depth-resolved stress and phase characterization performed by cross-sectional X-ray nanodiffraction in the as-deposited Ti0.05Al0.95N revealed a strong compressive residual stress gradient of up to −2GPa at the coating surface which homogenized but remained still compressive after annealing at 1050°C. Finally, it is demonstrated that the unique dense nanocomposite microstructure is responsible for the superior oxidation behavior as well as high compressive stresses in the Ti0.05Al0.95N coating.