Ceramic matrix composites for liner system of radioactive waste disposal cells

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Electrical resistivity monitoring of a SiC/[Si-B-C] composite under oxidizing environments

The introduction of Ceramic Matrix Composites parts in civil aeronautics requires a thorough understanding of their evolution under the oxidizing environments present within the engines. In this respect, a SiCf/PyC/[Si-B-C]m material has been tested in fatigue at 450 °C and 100 MPa (which is a typical stress during take-off) under two types of environmental conditions: ambient air, and moist air with an imposed water pressure of 10 kPa. Static fatigue and cyclic fatigue at a frequency of 1 Hz were both performed with these two conditions. As expected, the additional presence of moisture contributes to increase the degradation of the mechanical properties of the material, leading to shorter lifetimes and higher increases in electrical resistivity. It is shown that the pyrocarbon interphases are the main electrical conductors in this material: the electrical resistance can therefore be an accurate indicator of the damage state of these interphases, which are sensitive to the oxidizing environment. The global resistance increase presents two distinct phases of evolution in the four tests performed, with a transition around 35–40% of the time to failure. A model is proposed to account for this global resistance change, which proves to be in good agreement with experimental results. Moreover, the evolution of the electrical resistivity during the interposed unload-reload cycles can give key information about the state of the fiber/matrix interfaces which are critical for mechanical properties. Finally, electrical resistance monitoring seems to provide information on the damage state of the material complementary to acoustic emission results, allowing an unprecedented assessment of the evolution of the interphases state during ageing under oxidizing environments

Electrical resistivity monitoring of a SiC/PyC/[Si-B-C] composite under oxidizing conditions

L’introduction de Composites à Matrice Céramique en aéronautique civile nécessite une compréhension approfondie de leur comportement thermomécanique et le développement de techniques non-destructives permettant un suivi en temps réel de l’endommagement. A cet égard, un composite SiCf/PyC/[Si-B-C]m a été soumis à des essais de traction à température ambiante et des essais de vieillissement thermomécaniques avec une mesure en continu de sa résistance électrique. Il a été montré que l’interphase en pyrocarbone était le principal conducteur du courant dans ce composite. L’évolution de la résistance électrique en fonction des déformations a été modélisée et reliée analytiquement aux densités de fissuration et de décohésion, démontrant une corrélation de bonne qualité avec les résultats expérimentaux. Des essais de vieillissement effectués à 450°C ont mené à des augmentations de résistance bien plus élevées qu’à température ambiante, malgré des niveaux de contraintes fixées nettement plus bas. Cela a été relié à la dégradation de résistance par oxydation en plus de celle associée à l’usure mécanique. La présence d’humidité dans l’atmosphère amplifie l’oxydation de l’interphase et les augmentations de résistance. Le suivi d’endommagement par mesure de résistance électrique permet ainsi de quantifier efficacement l’état de dégradation de l’interphase, essentielles aux propriétés mécaniques du composite.The introduction of Ceramic Matrix Composites parts in civil aeronautical engines requires a thorough understanding of their thermo-mechanical behavior and the development of non-destructive techniques enabling a real time monitoring of damage. In this respect, a SiCf/PyC/[Si-B-C]m composite was submitted to room temperature tensile tests and high temperature thermo-mechanical ageing tests with electrical resistance monitoring. It was shown that the pyrocarbon interphase was the main electrical conductor in this composite. The evolution of the electrical resistance as a function of strains was modeled using a network of resistances, and it was analytically related to the crack and debonding densities, demonstrating a positive correlation with experimental results. Ageing tests performed at 450°C with a maximum stress of 100 MPa led to much higher resistance increases than at room temperature, despite lower stresses. This was related to the degradation of the interphase by oxidation in addition to mechanical wear. It was shown that the presence of humidity in the atmosphere strongly amplifies the oxidation of the interphase and the resistance increase. The electrical resistance monitoring technique is thus proved to efficiently monitor the damage level of the interphase, which is essential to the mechanical properties of the composite

Ablation des matériaux carbonés (lien entre la nanotexturation et la réactivité)

La problématique énoncée par l utilisation de matériaux composites C/C denses implique la connaissance et la maîtrise des processus de dégradation auxquels ils sont soumis. L utilisation de moyens de caractérisation in-situ de ces voies de dégradation constitue alors un atout considérable pour leur anticipation. Ainsi, l utilisation de la MEBE en Température associée à une caractérisation cristallographique par MET et une confrontation ex-situ par Analyse thermogravimétrique a abouti à l obtention de lois cinétiques caractérisant la propagation de l oxydation dans toutes les directions de l espace. A la suite de cette étape expérimentale, une approche numérique basée sur l utilisation d algorithmes de Monte-Carlo Cinétique, a alors été mise en place pour modéliser ces observations tant sur le plan atomique avec la modélisation de la loi cinétique d oxydation linéique, que meso et macroscopique par la simulation de la loi cinétique de perte de masse dans le cas particulier du HOPG.The problem stated by the use of composites C / C dense implies knowledge and control of degradation processes to which they are subjected. The use of in-situ characterization of these means of degradation pathways then is a considerable asset for their advance. Thus, the use of ESEM in temperature associated with a crystallographic characterization by TEM and ex situ confrontation by thermogravimetric analysis resulted in obtaining kinetic laws characterizing the propagation of oxidation in all directions. Following this experimental stage, a numerical approach based on the use of algorithms Kinetic Monte-Carlo, was then introduced to model these observations both at the atomic level with the modeling of the oxidation kinetics law linear, as meso-and macro-simulation by the kinetic law of mass loss in the case of HOPG.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Oxidation behavior of Cf / MC – SiC (with M = Hf, Zr) composites in an oxyacetylene torch environment under over oxygen concentration

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Oxidation behavior of Cf / MC – MB2 – SiC (with M = Hf, Zr) composites in an oxyacetylene torch environment

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Monitoring damage evolution of SiC f /[Si B C] m composites using electrical resistivity: Crack density-based electromechanical modeling

The introduction of Ceramic Matrix Composites parts in civil aeronautical engines requires a thorough understanding of their mechanical behavior. In this respect, a SiCf/PyC/[Sisingle bondBsingle bondC]m composite was submitted to room temperature tensile tests with electrical resistance monitoring and acoustic emission recording. The evolution of the electrical resistance as a function of strains was modeled using a network of resistances in series and parallel, with the introduction of locally higher resistances representing potential discontinuities in the material due to matrix microcracks. The evolution of the effective matrix crack density was deduced from the interposed unloading-reloading cycles. A high-quality correlation was found between the measured and predicted evolution of (i) the electrical resistance as a function of strains and (ii) their residual behavior observed upon unloading. A relationship is therefore established between the electrical resistance of the sample and the density of matrix cracks leading to a hindering of mechanical properties

Comportement en milieu oxydant d'un composite carbone/carbone pour applications structurales entre 150 et 400c dans l'aéronautique civile

L utilisation d un composite Carbone/Carbone 2D est envisagée pour des pièces de structure, travaillant entre 150 et 400C, dans l aéronautique civile. Dans ce domaine de température, la durabilité de ces matériaux n est pas connue car ils n ont jamais été développés pour de telles applications. Une première approche a permis de corréler la réactivité chimique des constituants élémentaires (fibres et matrices) à leur état d organisation structurale. Les vieillissements réalisés sur matériau composite ont ensuite mis en évidence qu un faible taux d oxydation pouvait conduire à un abaissement significatif des propriétés mécaniques résiduelles. Les fissures et les décohésions consécutives au procédé d élaboration conduisent à une oxydation préférentielle du composé le plus réactif et à la ruine prématurée du composite par délaminage. L'évolution des propriétés de ce matériau sur de longues durées est finalement discutée afin d évaluer sa capacité à remplacer les matériaux métalliques dans des pièces aéronautiques.A 2D Carbon/Carbon composite is envisaged for structural parts, operating between 150 and 400C, in civil aircraft. In this temperature range, the durability of these materials remains unknown because they have never been developed for this kind of applications. A first approach allowed us to correlate the chemical reactivity of the elemental constituents (fiber and matrix) to their structural organization. Then, thermal ageing tests performed on the composite material have demonstrated that a low rate of oxidation could be responsible to a significant reduction of residual mechanical properties. Cracks and fiber/matrix debonding resulting to the elaboration process create an extended pathway to a preferential oxidation of the most reactive compound. This latter is followed by a premature failure by delamination. The reduction of the material properties over long periods is finally discussed in order to evaluate its ability to replace metallic materials in aircraft structural parts.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Characterization of damage in C/SiC ceramic compositesby various monitoring means

Les propriétés mécaniques des composites C/SiC sont principalement fonction du procédé de fabrication, de la matrice, du type de fibres de Carbone et de l'architecture fibreuse constituant la préforme. L'objectif de ce travail est de mettre en relation la microstructure et le comportement mécanique de composites C/SiC d'origines différentes, testés dans plusieurs conditions. À cette fin, une caractérisation microstructurale a été effectuée au moyen de la microscopie optique, de la microscopie électronique à balayage et de la diffraction des rayons X. Les analyses microstructurales ont permis de comprendre les différences de comportement mécanique observées entre les matériaux et de faire le lien avec les différentes techniques d’élaboration. Les essais mécaniques ont été réalisés à température ambiante avec divers suivis de l’endommagement (émission acoustique, résistivité électrique et cycles interposés de déchargement/rechargement).Mechanical properties of C/SiC composites are mainly a function of the matrix manufacturing process, the C fibers used and the fibrous architecture into thepreform. The objective of this work is to relate the microstructure and mechanical behaviour of C/SiC composites of different origins, tested under several conditions. For this purpose, a microstructural characterization has been carried out using Optical Microscopy, Scanning Electron Microscopy and X-ray Diffraction. The microstructural identifications helped to understand the observed differences in mechanical behaviour between the materials and torelate them tothe different processing techniques. Mechanical tests were carried out at room temperature with various damage monitoring means (acoustic emission, electrical resistivity and interposed unloading/reloading cycles)

In-situ tensile tests under SEM and X-ray computed micro-tomography aimed at studying a self-healing matrix composite submitted to different thermomechanical cycles

Composites made of Ceramic for both fibres and matrix are three dimensional (3D) structured materials aimed at being used in aerospace applications. Material is submitted to mechanical stresses at high temperatures in oxidizing and corrosive environments for long durations. During thermomechanical cycles, damage, oxidation and healing-phenomenon appear and develop in the material. X-Ray Computed Micro-Tomography (μCT) and tensile test under Scanning Electron Microscopy (SEM) are solutions which allow an experimental study of these phenomenons. These techniques are performed for the study of the behaviour of the oxide (solid or liquid) in the crack of the material. The influence of the state of the oxide in the crack was analysed during tensile test under SEM or μCT. The observation of the crack shape allows to determine the influence of the oxide on the reclosure of the crack during the unloading