L’élaboration des céramiques nanocomposites s’impose dans l’actualité comme une des voies la plus prometteuse pour l’obtention des matériaux céramiques ayant des propriétés remarquables. Toutefois, il s’agit d’une voie compliquée puisque la nanostructure et la densification des matériaux sont en général contradictoires. En effet, les relativement hautes températures et longues périodes de temps requises pour bien densifier les matériaux céramiques nanocomposites produisent le phénomène de grossissement des grains : dans ces conditions les phases présentes dans le composite cessent d’être nanostructurées. Dans ce contexte, l’utilisation de nouvelles techniques de frittage, telles que le « Spark Plasma Sintering » (SPS), peuvent aider à la consolidation de ce type de matériaux tout en conservant une microstructure fine. Ce travail de recherche a été consacré à l’élaboration des micro-nanocomposites alumine/5vol% SiC en utilisant trois techniques de frittage : conventionnelle ou naturelle (FN), le pressage à chaud (« Hot Pressing » ou HP) et principalement le frittage par « Spark Plasma Sintering ». Dans un premier temps, l’étape de dispersion des poudres d’alumine et de SiC a été optimisée afin de préparer des barbotines stables et homogènes et des poudres composites. Puis, les meilleures performances de la technique SPS par rapport aux autres techniques de frittage ont été mises en évidence. Ainsi, l’utilisation du frittage SPS permet une meilleure maîtrise de la microstructure : densité élevée, microstructure fine et la localisation des particules nanométriques principalement en position intergranulaire. / The development of ceramic nanocomposites is needed in the news as one of the most promising ways to obtain ceramic materials with remarkable properties. However, this is a complicated way since the nanostructure materials and densification are usually contradictory. Indeed, the relatively high temperatures and long periods of time required to fully densify the ceramic nanocomposite materials produce the phenomenon of grain growth: in these conditions the phases present in the composite cease to be nanostructured. In this context, the use of new sintering techniques, such as "Spark Plasma Sintering" (SPS), may help to consolidate this type of material while maintaining a fine microstructure. This research has been devoted to developing micro-SiC nanocomposites alumine/5vol% using three sintering techniques: conventional or natural (FN), hot pressing ("Hot Dry" or HP) and mainly the sintering by "Spark Plasma Sintering." As a first step, the step of dispersing powders of alumina and SiC was optimized to prepare stable and homogeneous slurry and composite powders. Then, the best performances of the technique compared to other SPS sintering techniques have been demonstrated. Thus, the use of SPS sintering allows better control of the microstructure: high density, fine microstructure and localization of nanoparticles mainly intergranular position.

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abstract: Advanced composites are being widely used in aerospace applications due to their high stiffness, strength and energy absorption capabilities. However, the assurance of structural reliability is a critical issue because a damage event will compromise the integrity of composite structures and lead to ultimate failure. In this dissertation a novel homogenization based multiscale modeling framework using semi-analytical micromechanics is presented to simulate the response of textile composites. The novelty of this approach lies in the three scale homogenization/localization framework bridging between the constituent (micro), the fiber tow scale (meso), weave scale (macro), and the global response. The multiscale framework, named Multiscale Generalized Method of Cells (MSGMC), continuously bridges between the micro to the global scale as opposed to approaches that are top-down and bottom-up. This framework is fully generalized and capable of modeling several different weave and braids without reformulation. Particular emphasis in this dissertation is placed on modeling the nonlinearity and failure of both polymer matrix and ceramic matrix composites. / Dissertation/Thesis / Ph.D. Aerospace Engineering 2011

Orientador: Inez Valéria Pagotto Yoshida / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-17T04:03:02Z (GMT). No. of bitstreams: 1 Lage_RaphaelRodrigues_M.pdf: 11995870 bytes, checksum: 4be63a61759fedf30e1ef7e9705c5570 (MD5) Previous issue date: 2010 / Resumo: Neste trabalho pretendeu-se obter compósitos de matriz cerâmica à base de oxicarbeto de silício, SiCxOy, carregados com cargas de diferentes naturezas (grafite (GCM), carbeto de silício (SCM) e siliceto de tungstênio (WCM)), de modo a explorar a influência destas cargas na condutividade das vitrocerâmicas obtidas. Os CMC foram preparados pela pirólise controlada de precursores poliméricos obtidos in situ pela reação de hidrossililação catalisada por um complexo de platina (II), do poli(metilsiloxano) (PMS) e 1,3,5,7- tetrametil-1,3,5,7-tetravinilciclotetrassiloxano (D4Vi), contendo as respectivas cargas previamente dispersas. Foi enfatizado o efeito da natureza das cargas e da matriz na composição e morfologia das fases resultantes nos materiais finais, correlacionando-as com as condutividades elétricas destes. Os compósitos obtidos e a matriz vítrea foram submetidos à caracterização estrutural por ressonância magnética nuclear de Si e C, quando possível, espectroscopia Raman e DRX. Medidas de densidade, volume de poros, e porosidade aberta também foram conduzidas. A morfologia dos materiais cerâmicos foi avaliada pela técnica de microscopia eletrônica de varredura. Por final, medidas de condutividade elétrica por meio da técnica de quatro pontas foram realizadas em todos os materiais que cumpriam os requisitos prévios para esta análise. Os compósitos SCM não apresentaram formação de novas fases, indicando a inércia da carga de SiC diante da matriz não cristalina. Por outro lado a não observação de perda de massa para os compósitos GCM e WCM, juntamente com o surgimento de novas fases, comprovado por DRX, indicaram a reatividade das cargas grafite e WSi2, especialmente à temperatura de 1500°C. A condutividade elétrica dos compósitos correspondeu à natureza das cargas sendo o compósito SCM semicondutor, e os compósitos WCM e GCM condutores. Para o compósito GCM a dependência dos valores de condutividade em relação à quantidade carga, indicou uma transição isolantecondutor por volta de 16,5% em massa de grafite, segundo cálculo efetuado pela equação de percolação / Abstract: In this study, it was intended to obtain ceramic matrix composites (CMC) based on silicon oxycarbide SiCxOy, filled with powders of different nature (graphite (GCM), silicon carbide (SCM), and tungsten silicide (WCM)), in order to explore the influence of these fillers on the electrical conductivity of the vitro-ceramic obtained. CMC was prepared by controlled pyrolysis of polymeric precursors obtained by hydrosilylation reaction, catalyzed by a platinum complex, from poly(methylsiloxane) (PMS) and 1,3,5,7-tetramethyl-1,3,5,7- tetravynilcyclotetrasiloxane (D4Vi), containing the respective fillers previously dispersed in the polymer. The effect of the nature of fillers and matrix was emphasized in the composition and morphology of the resulting phases in the final materials, correlating them with the electrical conductivity of those. The obtained composites were submitted to structural characterization by magnetic nuclear resonance (Si and C), when possible, Raman spectroscopy and X ray diffraction (XRD). Specific mass, pore volume and open porosity were also measured. The morphology of the ceramic materials was evaluated by scanning electronic microscopy (SEM). Finally, electrical conductivity measurements were performed by the four points probe technique. The SCM composites do not present formation of new phases, indicating the inertia of SiC filler. On other hand, there was no observation of mass loss for WCM and GCM, in agreement with the emergence of new phases, proved by XRD, suggests the reactivity of WSi2 and graphite fillers, specially at 1500°C. The electrical conductivity of composites was related to the nature of fillers, SCM was semiconductor, and WCM and GCM were electrical conductor composites. For the GCM composites, the dependence of electrical conductivity as function of graphite content, indicated an insulating-conductor transition around 16,5% in mass of graphite, according to the percolation equation / Mestrado / Quimica Inorganica / Mestre em Química

Les composites SiC/SiC sont envisagés comme matériaux pour des composants de cœur de réacteurs nucléaires du futur. Le dimensionnement de ces structures par la simulation numérique repose sur une modélisation du comportement mécanique de ces matériaux. Ces travaux visent à améliorer la compréhension de leurs mécanismes de déformation afin de construire une loi de comportement à même de prédire la réponse du matériau sous chargements complexes. Une caractérisation approfondie du comportement macroscopique de tubes SiC/SiC multicouches - similaires aux concepts de gaines de combustibles - a été entreprise, par des essais de traction-pression interne, traction-torsion et flexion multi-instrumentés, et a permis de constituer une importante base expérimentale pour la compréhension des mécanismes d'endommagement et l'identification de modèles. Des observations in-situ et après rupture ont permis de quantifier l'orientation des fissures matricielles observées en surface en fonction du type de chargement appliqué. Des mesures de champs de déplacement par Corrélation d'Images Numériques à l'échelle d'un motif du textile ont permis d'apporter des informations fines sur la cinématique de la surface du composite, telles que l'ouverture des fissures ou la déformation des fragments matriciels. Ces mesures ont également permis de mettre en évidence l'importance de la réorientation des torons dans la direction de chargement, mécanisme pouvant expliquer certaines spécificités du comportement macroscopique du fait de son couplage avec la fissuration matricielle. Enfin, ces différentes données expérimentales ont permis de construire un modèle phénoménologique, identifiable sur quatre essais uniaxiaux, permettant de prédire de manière satisfaisante le comportement macroscopique sous divers chargements bi-axés. L'accord de certaines quantités locales, telles que les caractéristiques de la fissuration en traction, ont également été vérifiées. / SiC/SiC composites are candidate materials for in-core components of future nuclear reactors. The analysis of these structures using numerical simulations requires material constitutive laws. The present work focuses on understanding the deformation mechanisms of these materials in order to build a constitutive model able to predict their stress-strain response under complex loadings. An extensive characterization of the mechanical behaviour of SiC/SiC multi-layered tubes – similar to fuel cladding concepts - was carried out, using tension-internal pressure, tension-torsion and bending tests, that allowed to build an important experimental basis for the understanding of the mechanisms and the identification of constitutive laws. In-situ and post-failure observations have allowed quantifying the orientation of surface matrix cracks as a function of the loading type. Full-field measurements using Digital Image Correlation at the tow scale brought precise information on the composite surface kinematics, such as the crack opening or the deformation of the matrix fragments. These measurements also evidence the importance of the tow reorientation, that could explain specific features of the macroscopic behaviour because of its coupling with matrix cracking. Finally, these experimental data allowed to build a constitutive model, identified on only four uniaxial tests, able to predict satisfactorily the macroscopic behaviour under several biaxial loadings. The correct prediction of local quantities, such as the characteristics of the matrix cracking in tension, has also been verified.

Ceramic Matrix Composites (CMC) are attractive material systems for structural applications where resistance to intermediate (700 0C-950 0C) and high temperatures (900 0C-1400 0C) is required and low density is desired. There are currently barriers to a more widespread adoption of CMCs which include less robust simulation tools, which this dissertation seeks to address. A novel unified reduced order homogenization model for initial quasi-static, creep, and fatigue loading of SiC/SiC CMCs at intermediate and high temperatures is proposed. Driven by a single set of parameters, the model can seamlessly transition between initial quasi-static, creep, and fatigue regimes while capturing the complex material response of SiC/SiC CMCs. The reduced order homogenization approach provides a robust and efficient computational platform for analyzing composite behavior. Continuum damage mechanics provides the basis for the initial brittle CMC behavior while a hybrid damage-viscoplasticity model combined with an oxidation driven crack sealing effect drives the time-dependent brittle-ductile material behavior at high temperatures. A temporal multiscale approach extends the spatial multiscale model into fatigue regime at high temperatures, avoiding the computational complexity of modeling each cycle individually. At intermediate temperatures, a one-dimensional model based on the slow crack growth model originally proposed by Iyengar and Curtin is generalized to three dimensions focusing on a woven composite architecture. For this oxidation-assisted rupture model, the constitutive equation in the axial tow direction is governed by the continuum damage mechanics variant of the slow crack-growth model and the availability of oxygen to fibers, which in turn depends on the initial matrix pores and subsequent matrix cracking. The model is verified on two SiC/SiC material systems, S200H and GEA SMI, in both initial quasi-static and time-dependent loading regimes at both high and intermediate temperatures.

Silicon carbide is a useful monolithic and matrix ceramic due to its excellent mechanical properties and corrosion/oxidation resistance at high temperature. This makes it an attractive material for use in advanced applications, such as aircraft engines and high-speed flight. In this study, additively manufactured monolithic SiC and Cf/SiC CMCs, processed via direct ink writing (DIW) of a 53 vol% colloidal suspension, achieved >96% theoretical density through pressureless sintering. When present, fibers are aligned in the direction of the print path. Five different print paths were studied, including a 0o path, 90o path, 0/90o path, 0/15/30/45/60/75/90o path, and 0/30/60/90/60/30/0o path. Four-point bend testing was performed to determine flexural strength and Weibull analysis was performed. Strengths were highest for the 0o print path. The characteristic strength, σo, of this print path was 375 MPa with a Weibull modulus of 7.4 for monolithic SiC and a σo of 361 MPa with a Weibull modulus of 10.7 for Cf/SiC. Weibull modulus was greater for Cf/SiC samples compared to identically printed monolithic SiC samples. SEM and optical microscopy were used to analyze printed parts which showed a high degree of fiber alignment in the direction of the print. Fiber pullout was observed on the fracture surface, as well as intragranular fracture.

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