A new technique to measure the true contact area using nanoindentation testing

Nanoindentation technique requires the determination of projected contact area under load for calculation of modulus and hardness of materials. This projected contact area is usually calculated by models which take into account the pile-up or sink-in phenomena around the tip. The most commonly used model was developed by Oliver and Pharr [1] which can precisely model the sink-in around the tip, but cannot account for pile-up. Another model developed by Loubet et al can be used [2]. It can take into account the pile-up and the sink-in phenomena and can precisely measure the projected contact area for a large range of materials, except for materials with high strain hardening exponent. Other techniques, like post mortem measurements, can be used. However these measurements do not take into account the elastic recovery during unloading. A new technique to estimate the true projected contact area will be presented. It consists of combining two models (The Dao et al. model and the Kermouche et al. model) that are used normally to calculate the representative stress and the representative strain in indentation. Consequently, the projected contact area calculation does not depend on any contact area model. Moreover, it can account for the pile-up or sink-in phenomenon and the strain hardening of the material, which is not possible with the actual models used. This new technique requires measuring indentations parameters like the maximum load, the contact stiffness and the loading curvature. It requires also the use of two tetrahedral indenters: a Berkovich tip and a tetrahedral tip where the included semi-angle is 50°. The method was tested on three different samples: glass, PMMA and 100C6 steel. For indentations on glass and PMMA samples, the projected contact area was precisely measured. For indentations on 100C6 steel sample, the method was adapted to take into account the Indentation Size Effect observed at small indentation depths. The projected contact area values measured with this new technique will be presented and compared to the values calculated with classical literature models. Also, the limits of the technique will be discusse

Global maps of soil temperature.

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km <sup>2</sup> resolution for 0-5 and 5-15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km <sup>2</sup> pixels (summarized from 8519 unique temperature sensors) across all the world's major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications

Variation of the Hydrodynamic Polymer Layer Thickness on Solid Surfaces with Molecular Weight and Concentration

An extensive study on the determination of the 'hydrodynamic' layer thickness of polymer solutions and melt polymers was conducted with a surface force apparatus. For different concentrations and polymer molecular weights, a 'hydrodynamic layer' of fluid was detected on each solid surface which did not contribute to the flow. These thicknesses, denoted as LH, are compared to the characteristic polymer dimensions and the hydrodynamic (R H ) and gyration (R g ) radii. It appears that in contrast to the molecular weight, polymer concentration has little effect on the relative hydrodynamic layer thickness (L H /2R H , L H /2R g ). Indeed, this ratio indicates that two coils of low molecular weight and one coil of high molecular weight are 'immobile' on the solid surfaces. The mechanism responsible could be entanglement of the free (unattached) chains in the bulk with immobilized chains on the surface

EFFET DE LA DISPERSION SUR LA PROPAGATION ACOUSTIQUE SOUS-MARINE

La propagation acoustique sous-marine est caractérisée par l'existence de chemins multiples bien marqués et très stables en position. Chaque trajet est défini par un terme d'amplitude et de phase. A courte distance, ces deux paramètres sont très fluctuants ce qui peut s'expliquer par le caractère aléatoire des réflexions de surface. A longue distance, les amplitudes varient lentement et la phase est très stable. Nous expliquons ce phénomène à partir de la dispersion et développons le calcul de ce déphasage.The underwater acoustic propagation is characterized by very identified, stable in range multipaths Each path is defined by a magnitude and phasis coefficient. At short range, this two parameters are fluctuant a lot, due to random surface reflexions. At long range, the magnitudes are slowly varying and the phasis is very stable. We explain this phenomenon by dispersion effect and calculate theoretically the phasis term

BORON CARBIDE COATINGS : CORRELATION BETWEEN MECHANICAL PROPERTIES AND LPCVD PARAMETERS VALUES

Des dépôts de carbure de bore ont été élaborés par L.P.C.V.D. Leurs caractéristiques (taille de grain, composition, texture) varient fortement avec la température du substrat et la composition du mélange gazeux. L'effet de ces deux paramètres sur la microdureté, le comportement élastoplastique et le module d'YOUNG ont été déterminés par des tests d'indentation instrumentés. Les variations des caractéristiques sont analysées.Boron carbide coatings have been prepared by L.P.C.V.D. Their characteristics (i.e. grain size, composition, texture) depend heavily on the substrate temperature and gas mixture composition. The effects of these two parameters on microhardness, elastoplastic behavior and YOUNG's modulus, have been determined by indentation tests and are discussed in terms of variations in the coating characteristics

A new long-term nanoindentation relaxation method to characterize the time-dependent behavior of thin ZrNi metallic glass films

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Gate-all-around technology: Taking advantage of ballistic transport?

International audienc