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WC-Co hard metal covered with thin ceramic coatings represents a composite structure with a supreme performance in many metalworking tool applications. The failure behaviour of this substrate-coating composite is significantly influenced by the plastic deformation of the hard metal, especially if the plastification is localized and associated with changes in the residual stress state. Today, there is no complete understanding of the effect of localized cyclic substrate plastification on the residual stress, and its interaction with the superimposed stress induced by external load. The current work sheds light on this interplay via an approach combining experimental observations and finite element simulation. To induce different levels of localized plastification, uncoated and AlCrN coated notched specimens were subjected to different levels of uniaxial cyclic compressive loads at a stress ratio R = σ min / σ max of minus infinity. The associated shift of residual stress towards tension at the notch root was determined in the hard metal for the same load situations experimentally via X-ray diffraction and in simulations. The applied finite element simulations included an experimentally parameterized material model to describe the cyclic deformation behaviour of the hard metal. A type of fatigue crack oriented parallel to the substrate-coating interface was observed close to a surface asperity via scanning electron microscopy in a cross section prepared by focused ion beam milling. The observed type of crack can be interpreted as an early stage of coating delamination. Simulation results indicate that the maximum applied compressive load induces tensile stresses acting perpendicular to the substrate-coating interface, when considering the surface roughness features present in the specimen. The magnitude of the observed tensile stresses, that drive the growth of the observed crack type, decreases rapidly due to the cyclic plastification of the hard metal substrate. Image 1 • Experimentally parameterized cyclic simulation predicts residual stress change in substrate. • Formation of cracks oriented parallel to substrate-coating interface. • Load-induced tensile stresses acting perpendicular to surface lead to crack formation below asperity. • Reduction of load-induced tensile stresses due to localized cyclic substrate plastification [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • 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. [ABSTRACT FROM AUTHOR]

In-situ wafer curvature measurements and ex-situ ion-beam layer removal method are used to evaluate residual stress depth profiles in a 1.8 μm thick Al x Ga 1 − x N heterostructure on Si(111) by evaluating substrate surface curvatures and deflections of stepwise ion-beam milled micro-cantilevers, respectively. Both approaches reveal oscillatory stress depth gradients which correlate excellent in their depth alternations. Differences are found locally in the magnitudes of the stress concentrations, especially in the regions with relatively large stresses, inhomogeneous microstructures and at relatively small thicknesses. The discrepancies are interpreted by local stress relaxations in the growing heterostructure, like dislocation formation and overgrowth by differently stressed regions. [ABSTRACT FROM AUTHOR]

A residual stress depth gradient is characterized in a 1.8 μm thick AlN/Al 0.25 Ga 0.75 N/GaN/Al 0.22 Ga 0.78 N heteroepitaxial structure grown using metallic-organic chemical vapour deposition on Si(111) substrate. The cross-sectional stress profile is evaluated with a step of 100 nm using ion beam layer removal (ILR) method based (i) on a sequential focused ion beam milling of a microcantilever, (ii) on an evaluation of a cantilever bending after every milling step and (iii) on a stress profile recalculation using finite element simulation. The profile shows tensile stress of ~ 1.5 GPa in AlN nucleation layer, stress changing from compressive to tensile in Al 0.25 Ga 0.75 N and GaN sublayers and relatively small stresses below 100 MPa in the top Al 0.22 Ga 0.72 N sublayer. The stress profile is qualitatively correlated with the results from precession electron diffraction which indicates approximately the same stress behavior. The cross-sectional stress magnitude and variation are interpreted by the mismatches of lattice constants and coefficients of thermal expansion as well as by growth mode changes during Al 0.25 Ga 0.75 N and GaN sublayer formation. The approach demonstrates the possibility to resolve nanoscale variation of residual stresses in heteroepitaxial structures using ILR method. [ABSTRACT FROM AUTHOR]

Thermochemical surface treatments such as nitriding result in the formation of complex near-surface gradients of phase fractions, residual stress and microstructure, which influence the structural and functional properties of the material decisively. In this work, a novel cross-sectional synchrotron microdiffraction method is used to characterize such gradients by analyzing cross-sections of nitrided Fe–Al and Fe–V alloy specimens down to a depth of 500 μm, with a depth resolution of less than 10 μm. The Debye–Scherrer diffraction data collected from the individual sample depths document very different nitride-precipitation mechanisms and resulting stress gradients. In nitrided Fe–Al samples, the delayed precipitation of largely incoherent AlN particles leads initially to the development of internal microcracks, followed by a pronounced increase of compressive stress until plastic deformation sets in, which finally results in the formation of regions with tensile and compressive stress. In contrast, the VN precipitation during nitriding of Fe–V alloys occurs very quickly and generates a desired high compressive stress at the surface of the nitrided part. The tiny and coherent VN precipitates increase the yield strength of the nitrided zone significantly. The evaluated corresponding ferrite-lattice parameter depth profiles can be quantitatively described as the outcome of (competing) effects of solute (Al, V) depletion, (excess) nitrogen dissolution and the emergence of a hydrostatic strain due to elastic accommodation of the precipitate/matrix misfit. The novel technique to expose depth gradients in real space, with micrometer resolution, opens the way to understand the development of microstructure and stress upon (thermochemical) surface treatment. [ABSTRACT FROM AUTHOR]

Residual stresses and their distribution within individual layers are a general concern in thin film technology. Here we use a recently developed ion beam layer removal method to determine the stress profile in a thin film system. The system consists of a thin tungsten and titanium nitride film deposited on a silicon substrate. The stresses are calculated from the deflection of a focused ion beam machined cantilever by means of Euler-Bernoulli beam theory and finite element simulations coupled with optimizing algorithms, and the results of the two methods are critically compared. Case studies taking into account manufacturing related variations in the cantilever geometry, different boundary conditions and relaxation during cantilever fabrication are performed. We find that the stress distribution in the thin film system is strongly influenced by the boundary conditions and the cantilever fabrication, while manufacturing related variations in the cantilever geometry only slightly influence the stress distribution. [ABSTRACT FROM AUTHOR]

Abstract: The influence of a local thermal treatment on the properties of Ti–Al–N coatings is not understood. In the present work, a Ti0.52Al0.48N coating on a WC–Co substrate was heated with a diode laser up to 900°C for 30s and radially symmetric lateral gradients of phases, residual stress and hardness were characterized ex-situ using position-resolved synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nanoindentation. The results reveal (i) a residual stress relaxation at the edge of the irradiated area and (ii) a compressive stress increase of few GPa in the irradiated area center due to the Ti–Al–N decomposition, in particular due to the formation of small wurtzite (w) AlN domains. The coating hardness increased from 35 to 47GPa towards the center of the heated spot. In the underlying heated substrate, a residual stress change from about −200 to 500MPa down to a depth of 6μm is observed. Complementary, in-situ high-temperature X-ray diffraction analysis of stresses in a homogeneously heated Ti0.52Al0.48N coating on a WC–Co substrate was performed in the range of 25–1003°C. The in-situ experiment revealed the origin of the observed thermally-activated residual stress oscillation across the laser heated spot. Finally, it is demonstrated that the coupling of laser heating to produce lateral thermal gradients and position-resolved experimental techniques opens the possibility to perform fast screening of structure–property relationships in complex materials. [Copyright &y& Elsevier]