[en] Highlights: • Cr-CrN multilayers are deposited by MS-PVD under controlled condition; • Residual stress profiles were evaluated by FIB-DIC micro ring-core milling; • Scratch test shows dependence of adhesion on residual stress profile, without changes in overall hardness and microstructure Compressive residual stress in hard coatings can improve adhesion and in-service toughness, since they can inhibit crack nucleation and propagation. However, the role of through thickness residual stress profile is not fully understood. This is because of (a) lack of knowledge of stress evolution mechanisms and (b) limitations of experimental techniques used for stress profiling. The present work deals with design, deposition and characterization of Cr-CrN multilayer coatings, produced by Magnetron Sputtering Physical Vapour Deposition (MS-PVD), with the purpose to understand the effect of through thickness residual stress profile on coating adhesion. An automated optimisation algorithm was used to determine the desired residual stress through-thickness profile for a range of contact loading situations. On the basis of modelling activities, three different Cr-CrN multilayers were produced, with the aim of obtaining different stress gradients, as measured by incremental micro-scale focused ion beam (FIB) ring-core method, while keeping the same average stress value and same average hardness in the film. Results show a significant correlation between the observed residual stress profiles and scratch adhesion, where different optimal stress profiles are identified for different loading conditions. This is a major step with respect to previous literature, where scratch adhesion in hard coatings was only correlated to the average stress in the film, but not to the stress gradient within the film thickness. Here, we show that a lower interfacial compressive stress and a reduced through thickness stress gradient gives improved scratch adhesion, when using 10 μm and 200 μm sphero-conical indenters.

[en] Highlights: • A new method for the assessment of elastic moduli and residual stresses in bi-layer suspended micro-beams is presented • The method is based on micro-bending experiments performed by nanoindentation testing • An analytical procedure is adopted to extract the elastic modulus and residual stress of each layer • Nanoindentation method is validated by Focused Ion Beam - Digital Image Correlation (FIB-DIC) double-slot technique • The developed procedures can be useful for design of Microelectromechanical devices with improved performance In this paper, we present a detailed mechanical characterization of freestanding bilayer (Au-TiW) micro-cantilevers and double clamped beams, for applications as Radio Frequency (RF)-switches Micro-Electromechanical Systems (MEMS). The testing structures have been characterized by an optical profilometer and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS), in order to acquire information about their geometries, composition, and the gap between the substrate underneath. Then, the micro-beams are deflected by using a specifically designed nanoindentation procedure based dynamic stiffness measurement during bending in order to extract the elastic modulus and the residual stresses of both layers. Firstly, the classic beam theory has been implemented for bilayer cantilevers enabling the extraction of elastic moduli. Then, residual stresses are estimated by deflecting double clamped beams, while implementing new analytical models for a bilayer system. The obtained elastic moduli are consistent with the average ones obtained for a single layer micro-cantilever and with nanoindentation results for TiW and Au homogeneous films. The residual stresses are in agreement with the values obtained from the double slot Focused Ion Beam (FIB) and Digital Image Correlation (DIC) procedure, providing an alternative and portable way for the assessment of residual stresses on composite double clamped micro-beams.

No abstract available

[en] 3D-printed nano-architected ceramic metamaterials currently emerge as a class of lightweight materials with exceptional strength and stiffness. However, their application is hampered by the lack of knowledge on their mechanical reliability. Characteristics like the fracture strength and their dependency on environmental conditions are unknown. We herein present and discuss a nanoindentation pillar splitting method to measure fracture toughness, elastic modulus, and hardness of 3D-printed nano-ceramics. We show that two photon polymerization-derived pyrolytic carbon achieves improved fracture toughness over macroscopic forms of vitreous carbon, with values up to 3.1 MPam^0.5. However, experiments at different humidity levels reveal that only few, nanometer-sized, surface cavities can cause embrittlement from liquid diffusion, which promotes earlier crack propagation. While comparable effects are less relevant in macro-size ceramics, this study demonstrates that reliability and durability of micro- and nano-architected ceramic metamaterials and devices requires toughening design approaches that focus on size-dependent surface effects.

[en] An optimized nanoindentation pillar splitting technique is used for the fracture toughness measurement of spinel Li_xMn_2O_4 cathode material under different states of charge (SoC), along with the high-speed nanoindentation results for nanomechanical property mapping. High-speed nanoindentation enables for a robust and efficient evaluation of elastic modulus and hardness as a function of the SoC on strongly heterogeneous materials. The fracture toughness decreases linearly upon de-lithiation, with an overall reduction of 53% from 0% to 100% SoC. Decrease in fracture toughness is associated with the volume change, increase of defect density and stresses related to diffusion of lithium upon de-lithiation.

[en] Lath martensite structures in medium-carbon steels incorporate a significant amount of residual stresses that are mostly induced by the martensitic transformation process. Although former studies could identify these stresses using diffraction techniques, it was not possible to correlate the micro-scale distribution of the stress fields with respect to the morphological and the crystallographic parameters of the martensitic structure. In this study, we employ the micro-scale focused ion beam (FIB) ring-core milling technique for the measurement of local residual strain and stress distributions in fully martensitic microstructures. The aim is to study the residual stresses occurring within individual lath martensite crystals, and within areas of lath martensite which incorporate a parent austenite grain boundary. The relaxation strains obtained by the micrometer-sized ring-core milling, which correspond to the residual stresses prior to milling, are shown to exhibit an anisotropic distribution for each martensite variant. High extension relaxation strains (i.e. compressive stresses) prevail in the direction of the transformation-induced crystal shape deformation direction. Contraction strains (i.e. tensile residual stresses) are measured normal to the extension strains. In an area containing a prior austenite grain boundary, the residual stresses appeared – altogether – lower than in single crystal martensite laths. The significant residual tensile stresses identified in the martensite structures may support the formation of martensite micro-cracks, either in the as-quenched state or during deformation.

[en] Highlights: • A new analytical model of thermal diffusion and deformation during tempering is presented • Residual stresses in a Zr-based bulk metallic glass cylinder were mapped by Focused Ion Beam–Digital Image Correlation • The two methods show good agreement, confirming that both approaches can be used widely for a range of problems Quench processing is widely used in industry to impart the desired structural and mechanical properties by controlling microstructure and compositional gradients, e.g. to obtain supersaturated solid solutions in aluminium alloys, or to achieve martensitic hardening in steels. Rapid cooling, also referred to as quenching or tempering, is also the principal production route for bulk metallic glasses that exhibit high hardness and strength due to their amorphous structure that precludes plastic deformation by easy crystal slip. Importantly, rapid cooling is accompanied by the creation of residual stresses that also have a strong effect on the deformation behaviour. The present study aims to obtain insight into the residual stresses in cylindrical samples of Zr-based bulk metallic glass (BMG) by combining analytical modelling of thermal and mechanical problems with experimental measurements using Focused Ion Beam–Digital Image Correlation (FIB-DIC) ring-core milling. The results show good agreement between the two approaches, providing improved confidence in the validity of the two approaches considered here.

[en] Highlights: → New FIB-DIC method for the analysis of residual stress gradients in thin coatings. → Semi-automated incremental FIB ring-core milling successfully developed. → Developed procedure gives minimum re-deposition during FIBing. → Significant gradient of residual stress towards the coating/substrate interface detected. → Microstructural effects and elastic anisotropy were taken into account and corrected. - Abstract: A new methodology for the measurement of depth sensitive residual stress profiles of thin coatings with sub-micrometer resolution is presented. The two step method consists of incremental focused ion beam (FIB) ring-core milling, combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC). The through-thickness profile of the residual stress can be obtained by comparison of the experimentally measured surface strain with finite element modeling using Schajer's integral method. In this work, a chromium nitride (CrN) CAE-PVD 3.0 μm coating on steel substrate, and a gold MS-PVD 1.5 μm on silicon were selected for the experimental implementation. Incremental FIB milling was conducted using an optimized milling strategy that produces minimum re-deposition over the sample surface. Results showed an average residual stress of σ = -5.15 GPa in the CrN coating and σ = +194 MPa in the Au coating. These values are in reasonable agreement with estimates obtained by other conventional techniques. The depth profiles revealed an increasing residual stress from surface to the coating/surface interface for both coatings. This observation is likely related to stress relaxation during grain growth, which was observed in microstructural cross sections, as predicted by existing models for structure-stress evolution in PVD coatings. A correlation between the observed stress gradients and the in-service mechanical behavior of the coatings is proposed. Finally, critical aspects of the technique and the influence of microstructure and elastic anisotropy on stress analysis are analyzed and discussed.

[en] Residual stress evaluation in thin films at the sub-micron scale was achieved in the present study using a semi-destructive trench-cutting (ring-core) method. Focused Ion Beam was employed to introduce the strain relief by milling the slots around an “island” and also to record the images for strain change evaluation by digital image correlation analysis of micrographs. Finite element simulation was employed to predict the curves for strain relief as a function of milling depth, and compared with the experimental measurements, showing good agreement. An empirical mathematical description of the curves was proposed that allows efficient data analysis for residual stress evaluation.

[en] Highlights: • Nanomechanical properties of thermal barrier coatings were measured by high-speed nanoindentation and pillar splitting • During thermal cycling, delamination damage progresses slowly as long as the bond coat forms a dense, tough oxide scale • The fracture toughness of the thermally grown oxide scale drops when its thickness exceeds a 5-μm threshold • Severe delamination damage starts from the embrittled thermally grown oxide and propagates across the zirconia top coat • Such propagation is favoured by a simultaneous drop in top coat strength, due to accumulated microstructural alterations -- Abstract: This paper studies how the nano-mechanical properties of thermal barrier coatings (TBCs) vary during thermal cycling, as a way to shed new light on their failure mechanisms. In particular, high-throughput nanoindentation revealed the evolution of hardness and elastic modulus distributions of plasma-sprayed yttria-stabilized zirconia (YSZ) top layers. The evolution of fracture toughness of the YSZ layers and the thermally grown oxide (TGO) formed onto the vacuum plasma-sprayed NiCoCrAlY bond coat were investigated by nanoindentation micro-pillar splitting. The TGO fracture toughness increases up to ≈2.5–3.5 MPa√m at the early stages of thermal cycling, followed by a rapid decrease to ≈2.0 MPa√m after a critical TGO thickness of ≈5 μm is reached. Consequently, interface damage is initially limited to short cracks within the YSZ material. As TGO thickness exceeds the critical threshold, multiple cracks originate within the TGO and join through the YSZ to form long delamination cracks. Joining is favoured by a simultaneous loss in YSZ strength, testified by a decrease in the nanomechanical properties (hardness, elastic modulus) of both high- and low-porosity top coats. This is due to microstructural changes occurring because of the continuous interplay between sintering and thermal shock cracking in the YSZ layers.