Residual stress characterization in DLC coating by focused ion beam milling and finite element modeling

Low friction Diamond-Like-Carbon coatings have broad applications to reduce wear and improve lifetime products [1]. Cost efficient PECVD deposited DLC is especially used in the automotive industry. These fabrication process lead to large intrinsic compressive stresses and eventually in service delamination, blisters with cracks may occur. Hence, reliable measurement technics and mechanical models devoted to the local stress distribution reveal important for optimizing the mechanical properties of the materials. Main characterization technics use X-Ray diffraction or a wafer curvature analysis [2]. However, they are limited to crystalline material and only provide an average and homogeneous stress distribution. More recently, new methods based on Focused Ion Beam milling allowed determining residual stress at a very local scale. It consists in strain relaxation SEM observations of micro-cantilevers or micro-pillars prepared by controlled material removal [3,4,5]. This present work is dedicated to the micro beam characterization and FEM modelling of the residual stresses in DLC coating on M2 steel substrate and an intermediate underlayer. Different micro-beams were milled and their maximum deflection measured Fig1a. Simultaneously a FEM analysis was carried out to study the parameters that influence le micro-beam flection. By a combination of the FIB-experimental method and the numerical investigation, we will aim to determine the residual stress on DLC coating either on the sublayers according to the geometry of the cantilever and its deflection. Please click Additional Files below to see the full abstract

Multi-mechanical in situ testing for automotive industry DLC/interlayer/M2-Steel coatings

Wear resistance enhanced mechanical components with lower friction can help to answer to automotive industry challenges such as high performance, improved reliability and environmental friendly production. Diamond Like Carbon (DLC) deposited on mechanical components efficiently decrease the friction coefficient and insure wear protection [1]. DLC hard coatings, produced by Plasma Enhanced Chemical Vapor Deposition (PECVD), exhibit high compressive residual stress [2]. In order to improve coating adhesion, an appropriate interlayer is pre-deposited on mechanical components. However, in severe conditions, local coating delamination initiated by blistering is still occasionally detected [3]. To understand wear induced damage in DLC coated material and to optimize the coating/interlayer/substrate system design, very local and reliable mechanical data are required. The present work is dedicated to the investigation of mechanical properties of industrial DLC coatings. Several advanced experimental techniques were used to characterize DLC /interlayer/M2Steel samples. Please click Additional Files below to see the full abstract

Microshear mechanical properties measurements on tribolayers

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High strain rates micromechanical behavior of materials: A coupled experimental and numerical approach

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Nanomechanical testing for crystal plasticity constitutive framework identification at high strain rates

Shot-Peening (SP) is a surface mechanical treatment that consists in propelling hard particles, called shot, onto a ductile metallic surface at high velocity to induce subsurface residual compressive stresses. It is widely used in the industry to increase fatigue life and wear resistance of treated parts. Shot-peening induced macroscopic residual stresses (e.g. Type I) predictions using finite element analysis or analytical method is today already well assessed. However, recent works [1] revealed that spherical indentation in specific crystal orientations could induce subsurface intragranular tensile stresses. In the shot-peening context, such intra-granular (e.g. Type III) residual stresses could influence structure’s High Cycle Fatigue (HCF) behavior and macroscopic residual stresses stability over the load cycles It would also favor early stage plasticity and crack initiation. Shot-peening simulations at the crystal scale would therefore provide essential quantitative inputs for treated parts fatigue life prediction. Such simulations require to select relevant constitutive frameworks representing the crystal behavior at high strain rate (up to 106 s-1) and accounting for repeated impact induced cyclic effects. Also, identification of such behavior will require mechanical tests at the crystal scale under process-representative test conditions. In the present work, a new methodology for crystal plasticity inverse identification for large strain rate ranges is developed. It relies on high-strain rate micropillar compression tests performed with a recently developed nano-indenter test apparatus [2], at strain rates up to 102 s-1. Micropercussion induced residual imprints are also experimentally generated to provide material behavior inputs at higher strain rates. Both tests are combined for inverse identification of two different crystal plasticity constitutive frameworks for copper. Unicity and stability of the given coefficients are studied using cost function plots and an identifiability indicator developed by Renner et al. [3]. Further works will focus on high strain rates Berkovich indentation tests to complete the developed methodology. Experimental data will also be generated at higher strain rates and for repeated impacts, using a currently developed impact shot gun that will propel shots at shot-peening velocity with a spatial accuracy of . [1] S. Breumier, A. Villani, C. Maurice et M. &. K. G. Lévesque, «Effect of crystal orientation on indentation-induced residual stress field: simulation and experimental validation,» Materials & Design, vol. 169, 2019. [2] G. Guillonneau, M. Mieszala, J. Wehrs, J. Schwiedrzik, S. Grop, D. Frey, L. Philippe, J.-M. Breguet, J. Michler et J. Wheeler, «Nanomechanical testing at high strain rates: New instrumentation for nanoindentation and microcompression,» Materials & Design, vol. 148, pp. 39-48, 2018. [3] E. Renner, Y. Gaillard, F. Richard, F. Amiot et P. Delobelle, «Sensitivity of the residual topography to single crystal plasticity parameters in Berkovich nanoindentation on FCC nickel,» International Journal of Plasticity, vol. 77, pp. 118 - 140, 2016

A new method to measure shear surface mechanical properties

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Mechanical characterization of a tribollayer created by high temperature fretting wear in a ceramic/metal alloy contact

In aeronautics, the blade disk contact, between ceramic and Haynes 25 (cobalt-based alloy) surfaces, is submitted to fretting oscillations, at high temperature, the fretting being an oscillatory movement (at the micrometer order) between two surfaces in contact. This contact has been modeled in the laboratory, showing high friction and wear at temperatures lower than 500°C whereas a sudden decrease of the friction coefficient, and negligible wear is observed above this threshold temperature. The cause of high friction and wear at low temperature was explained in previous paper [1]. At temperatures higher than 500°C, low friction and wear are linked to the formation of a third body, named glaze layer, or tribolayer, this layer being created by compacted and sintered debris, and adhering on both parts, the thickness being between 5-20µm. Its structure and chemical composition was studied in a previous paper [2]. However, understanding the mechanical properties responsible of the glaze layer lubricious properties is still a challenge. In this presentation, the mechanical properties of the Glaze Layer, measured as a function of the temperature, will be presented, and compared to its tribological properties. The HS25/ceramic fretting contact has been studied, in flat/flat and cross cylinders’ configuration, at temperatures higher than 500°C, in order to create the glaze layer. The mechanical characterization of the tribolayer and the HS25 was performed through nanoindentation and in situ SEM microcompression experiments as a function of the temperature, in the temperature domain where the glaze layer is performant and in the temperature domain where friction and wear are important, the tests being performed in the cross section. The first part of the presentation will be focused on the tribological contacts presentation, and micromechanical devices used description. Then, the glaze layer microstructure, chemical composition and mechanical properties will be detailed and discussed. Finally, a comparison between the mechanical properties and tribological properties of the glaze layer will be detailed [3]. This work was supported by the LABEX MANUTECH-SISE (ANR-10-LABX-0075) of University of Lyon, within the program “Investissements d’Avenir” (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR). [1] A. Dreano, S. Fouvry, G. Guillonneau, A tribo-oxidation abrasive wear model to quantify the wear rate of a cobalt-based alloy subjected to fretting in low-to-medium temperature conditions, Tribol. Int. 125 (2018) 128–140. doi:10.1016/j.triboint.2018.04.032. [2] A. Viat, M.-I. De Barros Bouchet, B. Vacher, T. Le Mogne, S. Fouvry, J.-F. Henne, Nanocrystalline glaze layer in ceramic-metallic interface under fretting wear, Surface and Coatings Technology. 308 (2016) 307–315. doi:10.1016/j.surfcoat.2016.07.100. [3] A. Viat, G. Guillonneau, S. Fouvry, G. Kermouche, S. Sao Joao, J. Wehrs, J. Michler, J.-F. Henne, Brittle to ductile transition of tribomaterial in relation to wear response at high temperatures, Wear. 392 (2017) 60–68. doi:10.1016/j.wear.2017.09.015

Global burden of 288 causes of death and life expectancy decomposition in 204 countries and territories and 811 subnational locations, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021

BACKGROUND Regular, detailed reporting on population health by underlying cause of death is fundamental for public health decision making. Cause-specific estimates of mortality and the subsequent effects on life expectancy worldwide are valuable metrics to gauge progress in reducing mortality rates. These estimates are particularly important following large-scale mortality spikes, such as the COVID-19 pandemic. When systematically analysed, mortality rates and life expectancy allow comparisons of the consequences of causes of death globally and over time, providing a nuanced understanding of the effect of these causes on global populations. METHODS The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 cause-of-death analysis estimated mortality and years of life lost (YLLs) from 288 causes of death by age-sex-location-year in 204 countries and territories and 811 subnational locations for each year from 1990 until 2021. The analysis used 56 604 data sources, including data from vital registration and verbal autopsy as well as surveys, censuses, surveillance systems, and cancer registries, among others. As with previous GBD rounds, cause-specific death rates for most causes were estimated using the Cause of Death Ensemble model-a modelling tool developed for GBD to assess the out-of-sample predictive validity of different statistical models and covariate permutations and combine those results to produce cause-specific mortality estimates-with alternative strategies adapted to model causes with insufficient data, substantial changes in reporting over the study period, or unusual epidemiology. YLLs were computed as the product of the number of deaths for each cause-age-sex-location-year and the standard life expectancy at each age. As part of the modelling process, uncertainty intervals (UIs) were generated using the 2·5th and 97·5th percentiles from a 1000-draw distribution for each metric. We decomposed life expectancy by cause of death, location, and year to show cause-specific effects on life expectancy from 1990 to 2021. We also used the coefficient of variation and the fraction of population affected by 90% of deaths to highlight concentrations of mortality. Findings are reported in counts and age-standardised rates. Methodological improvements for cause-of-death estimates in GBD 2021 include the expansion of under-5-years age group to include four new age groups, enhanced methods to account for stochastic variation of sparse data, and the inclusion of COVID-19 and other pandemic-related mortality-which includes excess mortality associated with the pandemic, excluding COVID-19, lower respiratory infections, measles, malaria, and pertussis. For this analysis, 199 new country-years of vital registration cause-of-death data, 5 country-years of surveillance data, 21 country-years of verbal autopsy data, and 94 country-years of other data types were added to those used in previous GBD rounds. FINDINGS The leading causes of age-standardised deaths globally were the same in 2019 as they were in 1990; in descending order, these were, ischaemic heart disease, stroke, chronic obstructive pulmonary disease, and lower respiratory infections. In 2021, however, COVID-19 replaced stroke as the second-leading age-standardised cause of death, with 94·0 deaths (95% UI 89·2-100·0) per 100 000 population. The COVID-19 pandemic shifted the rankings of the leading five causes, lowering stroke to the third-leading and chronic obstructive pulmonary disease to the fourth-leading position. In 2021, the highest age-standardised death rates from COVID-19 occurred in sub-Saharan Africa (271·0 deaths [250·1-290·7] per 100 000 population) and Latin America and the Caribbean (195·4 deaths [182·1-211·4] per 100 000 population). The lowest age-standardised death rates from COVID-19 were in the high-income super-region (48·1 deaths [47·4-48·8] per 100 000 population) and southeast Asia, east Asia, and Oceania (23·2 deaths [16·3-37·2] per 100 000 population). Globally, life expectancy steadily improved between 1990 and 2019 for 18 of the 22 investigated causes. Decomposition of global and regional life expectancy showed the positive effect that reductions in deaths from enteric infections, lower respiratory infections, stroke, and neonatal deaths, among others have contributed to improved survival over the study period. However, a net reduction of 1·6 years occurred in global life expectancy between 2019 and 2021, primarily due to increased death rates from COVID-19 and other pandemic-related mortality. Life expectancy was highly variable between super-regions over the study period, with southeast Asia, east Asia, and Oceania gaining 8·3 years (6·7-9·9) overall, while having the smallest reduction in life expectancy due to COVID-19 (0·4 years). The largest reduction in life expectancy due to COVID-19 occurred in Latin America and the Caribbean (3·6 years). Additionally, 53 of the 288 causes of death were highly concentrated in locations with less than 50% of the global population as of 2021, and these causes of death became progressively more concentrated since 1990, when only 44 causes showed this pattern. The concentration phenomenon is discussed heuristically with respect to enteric and lower respiratory infections, malaria, HIV/AIDS, neonatal disorders, tuberculosis, and measles. INTERPRETATION Long-standing gains in life expectancy and reductions in many of the leading causes of death have been disrupted by the COVID-19 pandemic, the adverse effects of which were spread unevenly among populations. Despite the pandemic, there has been continued progress in combatting several notable causes of death, leading to improved global life expectancy over the study period. Each of the seven GBD super-regions showed an overall improvement from 1990 and 2021, obscuring the negative effect in the years of the pandemic. Additionally, our findings regarding regional variation in causes of death driving increases in life expectancy hold clear policy utility. Analyses of shifting mortality trends reveal that several causes, once widespread globally, are now increasingly concentrated geographically. These changes in mortality concentration, alongside further investigation of changing risks, interventions, and relevant policy, present an important opportunity to deepen our understanding of mortality-reduction strategies. Examining patterns in mortality concentration might reveal areas where successful public health interventions have been implemented. Translating these successes to locations where certain causes of death remain entrenched can inform policies that work to improve life expectancy for people everywhere. FUNDING Bill & Melinda Gates Foundation

Interface structure of Ni nanoparticles on MgO (100) : a combined HRTEM and molecular dynamic study

International audienceNi clusters with an average size of 4 nm, supported on MgO micro-cubes were studied by high resolution electron microscopy (HRTEM) and image simulations by the multislice technique. Regular defects were evidenced in the metal clusters at the interface. Molecular dynamic calculations of a 4 nm cluster indicates the same type of defects

Compressive behaviour of carbon fibres micropillars by in situ SEM nanocompression

International audienceAs carbon fibres are heterogeneous, anisotropic and small in size, the determination of their mechanical properties is rather difficult. Here, the compressive behaviour of two different carbon fibres is studied using in situ compression tests on micro-pillars with scanning electron microscopy (SEM). The mode of failure is axial splitting. Large hysteretic loops are observed, associated with crack development, but no or small permanent deformation is visible. The compressive properties (modulus, strength) of these pillars are lower than the tensile properties of the fibres (128 GPa and 2.38 for UTS50 respectively and 110 GPa and 2.36 GPa for HR40 respectively). The mechanisms involved are studied and compared with those of various other experimental techniques. The core–shell structure of the fibres is at the origin of these inferior properties (the pillar is associated with the core). The nano-buckling scenario of crystalline carbon stacks constrained by the shear stiffness of the fibre is in agreement with our moduli and strength results