Mechanisms of elastic softening in highly anisotropic carbons under in-plane compression/indentation

International audienceWe present a combined experimental and computational study of the elastic behavior of a series of highly anisotropic pyrocarbons, with crystallite sizes La in the 2–10 nm range, under a-axis compressive load. The materials include a rough laminar and a regenerative laminar pyrocarbon, as-prepared by chemical vapor deposition and after various heat treatments up to 2600 °C, for which a-axis nanoindentation experiments have been performed, showing a significant decrease in the indentation modulus and hardness with increasing La (or heat treatment temperature). To rationalize this behavior, molecular dynamics simulations of the uniaxial compression of accurate atomistic models of the materials as well as pristine graphite were performed, unraveling significant out-of-plane deformations in the models with increasing compressive strain, leading to elastic softening. More precisely, significant kinks were observed around extended screw dislocation-like defects in the most disordered pyrocarbon at rather large strain levels (∼ 3%). Conversely, graphite rather shows the formation of extended buckles, starting at very low strain values. Finite element modelling shows that such kinking/buckling transitions should take place in a large area under the indenter tip within usual nanoindentation conditions. Both finite element calculation and analytical approximation of the indentation modulus predict the correct trend of decreasing modulus with increasing La when applied with the elastic tensors computed after the buckling/kinking transitions, certainly proving the importance of the latter in the observed experimental indentation moduli

Synthesis and characterization of monolithic CVD-SiC tubes

International audience† The authors dedicate this article to the memory of our colleague P. Weisbecker who has recently passed away Abstract : The high open porosity of SiC/SiC composites is problematic for uses in advanced thermal or confinement systems. A strong and gas-tight SiC sheath would helpfully complete the composite structure. The aim of this work was to prepare and characterize long, free standing and high strength SiC tubes prepared through a rapid near net shape manufacturing process. The tubes were produced by continuous chemical vapor deposition (CVD) at atmospheric pressure using CH 3 SiHCl 2 /C 3 H 6 /H 2 mixtures. The composition, morphology and microstructure of the CVD-SiC tubes were analyzed in details. The Si/C ratio and crystalline state through wall thickness are related to the progress of gas phase reactions during continuous deposition. The silicon excess in the CVD-SiC coating can be reduced by preheating gases and adding C 3 H 6. C-ring specimens cut from the obtained tubes were submitted to compressive tests at room and high temperature to assess the failure properties and thermomechanical behavior

HIGH TEMPERATURE MECHANICAL CHARACTERIZATION OF MICRO-AND MESO-INTERFACES IN A 3D CARBON-CARBON COMPOSITE

International audience3D Carbon-Carbon (3D C/C) composites are key elements in aerospace applications thanks to their beneficial property-to-mass ratio at high temperature. Yet, they display complex mechanical behavior, especially at high temperature, because of the local anisotropy of their constituents and the intricate behavior at the interfaces in-between. The objective of this study is to characterize the interfacial properties of a 3D C/C. This has been carried out at two scales: i) the meso-interfaces, between the yarns composing the material; ii) the micro-interfaces between the fibers and the carbon matrix into the yarns. Monotonic and cycled push-out tests brought characteristic values of these interfaces, such as strength, dissipated energy and friction. The damage mechanisms and the effect of the temperature up to 1473K are also reported. These experimental values will eventually feed a numerical multiscale modeling approach aiming at reproducing the mechanical behavior of such C/C materials

Green luminescence in silica glass: A possible indicator of subsurface fracture

We investigate the nature of defects triggering laser damage in fused silica in subsurface fracturesin nanosecond near ultraviolet regime. Mechanical, laser induced surface flaws as well as pristinesilica surface were characterized by optical microscopy and luminescence confocal microscopybefore and after acid etching. In all cases, photoluminescence decreases with etching timeassessing the existence of defects close to the surface. Spectral analysis of the evolution of thesesignals during etching allows new interpretations of the nature of precursors inducing damage.Green luminescence around 2.25 eV is seen as a potential subsurface fracture indicator leading tolaser damage

Relationship between both thickness and degree of crystallisation of BN interphases and the mechanical behaviour of SiC/SiC composites

International audienceSiC/BN/SiC-Si single-ply composites with Hi-Nicalon STM fibres were synthesised by chemical vapour infiltration and the liquid route. These are model composites representing the behaviour of industrial composites intended to be used as structural parts in the hot sections of aeronautical engines. Four interphases with two different degrees of crystallisation and, in each case, two different thicknesses were tested. The interfacial shear stress measured by fibre push-out test is significantly higher when the crystallisation degree of the BN interphase is low, yet in this case it decreases with the thickness of the interphase. This higher interfacial bonding is also associated, albeit less significantly, with better macromechanical properties as measured by tensile and shear tests. The cracks observed in the composite with different microscopy techniques are also less long and open. Depending on the interphases, the debonds are more or less pronounced and occur preferentially at different locations in the interfacial system

Monitoring Tantalum nitride thin films structure by reactive HiPIMS magnetron sputtering : from microstructure to properties

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Monitoring Tantalum nitride thin films structure by reactive HiPIMS magnetron sputtering : from microstructure to properties

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Hardness and Young's modulus behavior of Al composites reinforced by nanometric TiB2 elaborated by mechanosynthesis

Different milling conditions have been tried to produce nanoparticles of TiB2 to be used as reinforcement in Al matrix composite materials. The milling of TiB2 compounds leads to a decrease of both the crystallite size and the particle size. Moreover, we demonstrate that the use of optimized conditions with a vibratory miller allows - in a short period of time - the direct synthesis of nanoparticles of TiB2 starting from the elemental powders (Ti and B). Then TiB2 reinforced Al composite was fabricated using a simple powder metallurgy process followed by free sintering under controlled atmosphere. The influence of (i) the nanometric TiB2 particles grain size and (ii) their volume fraction on the hardness and Young's modulus, measured from nanoindentation tests, has been analyzed

Enhancement of mechanical properties and chemical durability of Soda‐lime silicate glasses treated by DC gas discharges

We report for the first time a study on non‐contact thermal poling of soda lime silicate glasses using DC gas discharge. In this work, the formation of a glow discharge is evidenced during the thermal poling treatment (longer than 30 minutes). The hardness and the chemical durability of glasses poled under different conditions (contact or non‐contact) and atmospheres (nitrogen or air) are measured and compared to that of un‐poled reference glass. The results reveal enhanced mechanical and chemical properties for samples poled under nitrogen as compare to air poled or soda lime silicate glass samples. A structural and chemical analysis of surface of the glass using IR‐reflectance measurement and ToF‐SIMS is also presented. The formation of a “silica‐like” layer on the surface of nitrogen poled glasses is observed, which is likely associated with the enhancement of surface properties. On the other hand, the introduction of protons beneath the surface of glasses poled under air leads to the formation of a hydrated alkaline earth silica layer. Based on the observations a mechanism behind the sustainability of the plasma under DC conditions is proposed

Nanotexture of Pyrolytic Carbon Matrices: a review of characterization and modeling studies

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