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In this work, the deformation and strength of an oxide/oxide ceramic matrix composite system under stress-rupture conditions were studied both experimentally and analytically. A rupture model for unidirectional composites which incorporates fiber strength statistics, fiber degradation, and matrix damage was derived. The model is based on a micromechanical analysis of the stress state in a fiber near a matrix crack and includes the effects of fiber pullout and global load sharing from broken to unbroken fibers. The parameters required to produce the deformation and lifetime predictions can all be obtained independently of stress-rupture testing through quasi-static tension tests and tests on the individual composite constituents. Thus the model is truly predictive in nature. The predictions from the model were compared to the results of an extensive experimental program. The model captures the trends in steady-state creep and tertiary creep but the lifetime predictions are extremely conservative. The model was further extended to the behavior of cross-ply or woven materials through the use of numeric representations of the fiber stresses as the fibers bridge matrix cracks. Comparison to experiments on woven materials demonstrated the relationship between the behavior of the unidirectional and cross-ply geometries. Finally, an empirical method for predicting the durability of materials which exhibit multiple damage modes is examined and compared to results of accurate Monte Carlo simulations. Such an empirical method is necessary for the durability analysis of large structural members with varying stress and temperature fields over individual components. These analyses typically require the use of finite element methods, but the extensive computations required in micromechanical models render them impractical. The simple method examined in this work, however, is shown to have applicability only over a narrow range of material properties. / Ph. D.

Increasing use of fiber reinforced ceramic matrix composites (CMC's) materials is needed, especially for hostile environments such as elevated temperatures. However, some fundamental issues regarding how these materials should be made for optimized performance are far from being settled. This study focuses on the modeling of the tensile behavior of unidirectional CMC using statistical methods and micro-mechanical analysis, based on laboratory observations. The model can be used to examine the effect of performance-influencing parameters on the strength of unidirectional CMC, thus shed light on how such material should be put together. The tensile strength model was then modified such that the behavior of unidirectioal CMC under cyclic tensile load can be studied. Results from the tensile strength model suggest that the Weibull modulus, <i>m</i>, of the strength of the reinforcing fibers and the fiber/matrix interfacial shear stress both have significant effect on the strength and toughness of the unidirectional composite: a higher <i>m</i> value and a lower interfacial shear stress result in a lower strength; a lower value of <i>m</i> and a higher interfacial shear stress results in a higher strength but lower toughness. Calculations from the tensile fatigue model suggest that a lower <i>m</i> value results in a longer fatigue life. / Ph. D.

The forced—flow thermal gradient chemical vapor infiltration (FCVI) process for fabricating ceramic matrix composites (CMCs) was modeled and analyzed based on the finite element method (FEM). The modeling study was focused on the fabrication of silicon carbide (SiC) matrix composites from methyltrichlorosilane (MTS) precursors because of their high strength, high modulus and excellent oxidation resistance properties at high temperatures. Unlike other available FCVI models, which use lumped reaction schemes, both gas phase and surface reactions of the FCVI process were explicitly considered for the present FEM FCVI model. The kinetics of SiC deposition from MTS precursor were derived by analyzing our own deposition rate data as well as reported results. The SiC deposition process was modeled using the following reactions — (1) : gas phase decomposition of MTS molecules into two major intermediates, one containing silicon and the other containing carbon; (2) : adsorption of the intermediates onto the surface sites of the growing film; (3) : reaction of the adsorbed intermediates to form silicon carbide. The equilibrium constant for the gas phase decomposition process was divided into the forward and backward reaction constants as 2.0E+25 exp[(—448.2 kJ/mol)/RT] and 1.1E+32 exp[(-416.2 kJ/mol)/RT], respectively. Equilibrium constants for the surface adsorption reactions of silicon—carrying and carbon—carrying intermediates were determined to be 0.5E+11 exp[(—21.6 kJ/mol)/RT] and 7.1E+09 exp[(—33.1 kJ/mol)/RT], while the rate constant for the surface reaction of the intermediates was 4.6E+05 exp[(—265.1 kJ/mol)/RT]. Effects of the deposition temperature and vapor pressure variations on the density profiles of the composite preform were studied based on this FEM FCVI model. It was found that the advantages of the commonly used ambient—pressure FCVI process (APFCVI) are likely to be limited by the equipment and the accumulation of gaseous components around the entrance sides, which could render the deposition process to be mass transport limited. A conceptual multi-step FCVI process was proposed to alleviate this problem and obtain products of good final density profiles within reasonable processing times. This multi—step FCVI process involved deposition under ambient—pressure to improve the density profiles and shorten the processing times. This was followed by the sub ambient—pressure FCVI process (LPFCVI) process to overcome the mass transfer limitations caused by the entrance accumulation effect and possible limitations on the equipment. A balance between processing time and final density profile can be achieved through the use of this multi-step FCVI process. Advantages of this process has been demonstrated by studying the densification process in large size specimens. / Ph. D.

The impedance response of silicon carbide whisker-alumina composites is investigated utilizing novel stereological techniques along with a microstructural simulation. The stereological techniques developed allow for a measurement of the trivariate length, radius and orientation distribution of whiskers in the composite from measurements made on two-dimensional sectioning planes. The measured distributions are then utilized in a Monte Carlo simulation that predicts connectivity in the composite for a given volume fraction. It is assumed in the simulation that connectivity factors dominate the electrical response, not interfacial phenomena. The results of the simulation are compared with impedance spectra taken from real samples, and conclusions are drawn regarding the nature of the impedance response.

Ceramic-matrix composites of alumina and silicon carbide whiskers have recently found novel commercial application as electromagnetic absorbers. However, a detailed understanding of how materials issues influence the composite electrical response, which underpins this application, has been absent until now. In this project, such composites were electrically measured over a wide range of conditions and modeled in terms of various aspects of the microstructure in order to understand how they work. For this purpose, three types of composites were made by different methods from the same set of ceramic powder blends loaded with different volume fractions of whiskers. In doing so, the interfaces between whiskers, the preferred orientations of whiskers, and the structure of electrically-connected whisker clusters were varied; the whisker aspect-ratio distributions were the same for all methods. At the electrode interfaces, Schottky barriers at the junctions of the electrically-percolating wide-bandgap semiconductor whiskers on the surface were responsible for a significant portion of the total measured impedance. The associated electrical response was studied on the microscopic and macroscopic level, and the gap between these different scales was bridged. Also, a modeling approach was developed for the non-linear behavior of the composite which results from these barriers. In regards to the whiskers within the composite bulk, the effects of various factors on the wide-band frequency dependence of the dielectric response and dc conductivity were explained and contextualized for the electromagnetic absorber application. Such factors include whisker preferred orientation, electrical percolation and cluster structure, the interfaces between electrically-connected SiC whiskers, and porosity. A quantitative correlation between the anisotropy of the microstructure and that of the conductivity was found, and was understood in terms of the interfacial SiC-Al2O3-SiC conduction mechanism. This behavior was shown to differ from the behavior commonly observed for other disordered mixtures of relatively conductive particles dispersed inside insulating polymer hosts. A description of this new mechanism was developed based on an observed correlation between the temperature dependencies of the static and radio-frequency electrical responses. Also, the aforementioned non-linear response model was expanded upon to describe conduction through and across electrically-percolated clusters. The model demonstrates how loading and interface behavior influence the topology and the strength of the non-linear response of the clusters.

Le travail exposé dans ce mémoire propose un examen, selon une approche multiéchelle, de la relation entre l’évolution de l’endommagement et la perte de conductibilité thermique de Compositesà Matrice Céramique. Les recherches sont menées à la fois sur le plan expérimental et sur le plan théorique. La démarche mise en oeuvre consiste à examiner deux échelles significatives (Microet Meso) auxquelles agissent des mécanismes d’endommagement différents et à évaluer pardes techniques d’homogénéisation l’effet sur les propriétés thermiques effectives.Une attention particulière a été donnée à l’élaboration d’une démarche expérimentale approfondieassociant des moyens de caractérisation mécanique, thermique et microstructurale. Aux deuxéchelles étudiées, un banc expérimental a été conçu pour réaliser des mesures thermiques sur des CMC sollicités mécaniquement. La diffusivité thermique longitudinale du mini composite est estimée par thermographie à détection synchrone. Des variantes de la méthode flash en face arrière sont mises en oeuvre pour l’étude du composite tissé. Par ailleurs, la progression de l’endommagementest déduite de l’enregistrement des signaux acoustiques et d’observations microstructurales post-mortem. Les résultats expérimentaux sont systématiquement comparés à des simulations. A l’échelle Micro, un modèle micromécanique est proposé afin de simuler la perte de conductivité thermique d’un mini composite en traction. A l’échelle Méso, une stratégie multiéchelle de calcul numérique de l’effet de l’endommagement sur les propriétés thermiques d’un CMC tissé est présentée. / In this work the relationship between the evolution of damage and the loss of thermal propertiesof Ceramic Matrix Composites is investigated by a multiscale approach. Research are conductedboth experimentally and theoretically. The implemented approach is to consider two significantscales (micro and meso) where different damage mechanisms are operating and then assess theeffect on the effective thermal properties by homogenization techniques.Particular attention has been given to the development of a thorough experimental work combiningvarious characterization tools (mechanical, thermal and microstructural). At the two aforementionedscales, an experimental setup was designed to perform thermal measurements onCMC under tensile test. Thermal diffusivity of minicomposites is estimated using Lock-in thermography.Also, tranverse diffusivity mapping as well as global in-plane diffusivity of woven CMCare determined by suitable rear face flash methods. The evolution of damage is then derived fromacoustic emission activity along with postmortem microstructural observations. Experimental resultsare systematically compared to simulations. At microscale, a micromechanical-based modelis used to simulate the loss of thermal conductivity of a minicomposite under tensile test. At mesoscale,a multiscale Finite ElementModel is proposed to compute the effect of damage on thermalproperties of woven CMC.

Le développement des moteurs aéronautiques du futur a permis d’accentuer les recherches concernant les matériaux composites thermostructuraux SiC/SiC. La voie classique d’élaboration de ces matériaux consiste en l’infiltration de la matrice via un procédé par voie gazeuse. Due à leur porosité résiduelle importante, ces matériaux possèdent une faible conductivité thermique. Cette dernière peut générer de forts gradients thermiques pouvant entrainer une rupture prématurée de la pièce. Afin de réduire cette porosité, un procédé alternatif peut être utilisé : l’infiltration par du silicium liquide (procédé MI : Melt Infiltration). L’objectif de ce travail est de comprendre l’ensemble des mécanismes intervenant au cours de l’infiltration du silicium au sein d’une préforme fibreuse composée de fibres SiC Hi-Nicalon S. Ce procédé nécessite une étape en amont de l’imprégnation du silicium consistant en l’introduction de particules de SiC au sein de la préforme.La première partie de ce travail a consisté d’une part, en la définition de l’architecture poreuse des matériaux et, d’autre part, en la réalisation de tests de montée capillaire en utilisant des fluides organiques modèles. A l’aide de l’équation de Washburn, il est ainsi possible d’identifier des tailles de pores caractéristiques au sein de la préforme fibreuse et de la matrice granulaire et de prévoir le courbes d’ascension capillaire du silicium liquide au sein des matériaux. La seconde partie de ce travail décrit la mise en place d’un four permettant de réaliser le suivi in-situ de la prise de masse en silicium au cours du procédé MI. Une comparaison entre les résultats expérimentaux et les courbes prévisionnelles obtenues à l’aide de l’équation de Washburn a ainsi pu être effectuée. La dernière partie de ce travail a consisté en la réalisation d’essai d’imprégnation partielle afin d’identifier les mécanismes de montée capillaire du silicium liquide durant le procédé MI. / The development of aeronautic engines increased the need in high temperature SiC/SiC composite researches. A standard way to proceed is to infiltrate the matrix by chemical vapor infiltration. Due to their high porosity, their thermal conductivity is generally low. This could lead to strong thermal gradients and an early failure in a harsh environment. To reduce porosity, an alternative process can be used: the infiltration of molten silicon (MI: Melt Infiltration). The aim of this work is to understand all mechanisms occurring during the infiltration of silicon in a fibrous preform composed of SiC Hi-Nicalon S fibers. This process needs a first step which consists in the introduction of SiC particles into the preform before the MI process.First, this work focused on the definition of the porous structure of studied materials and capillarity tests using wetting organic solvent. With the use of Washburn’s law, it was possible to identify pore sizes within the fibrous preform and the granular matrix, and so to predict the capillarity ascent graphs of molten silicon into our material. A second part was devoted to the conception of an infiltration furnace which allows in situ following of the samples weight gain. The correlation between graphs obtained with the Washburn model and the experimental process could be established. Finally, the last part of this work presents partial infiltrations of molten silicon into studied materials which permit to identify capillary mechanisms occurring during the MI process.

Made available in DSpace on 2017-07-21T20:42:40Z (GMT). No. of bitstreams: 1 NILSON.pdf: 5389834 bytes, checksum: 149d2b8675ec375e2d7f3614952460ef (MD5) Previous issue date: 2007-02-27 / Ceramic Matrix Composites (CMC) are the most recent materials in field composites. The main factors for the good acting of these materials are the choice: of the matrix material, of the reinforcement, of the processing and chemical composition. Ceramic materials possess elastic module very loud, low density, and can support temperatures very discharges, even so they present low toughness. The introduction of a ductile metallic phase in the ceramic can improve its toughness to for frature. The SiC stands among the materials used in the ceramic matrix. Among the metals stands out the aluminum. The infiltration under pressure of a melted (Squeeze Casting) it is one of the methods more used in the production of CMC, being the focus of many searchs. The mechanical properties of a composite varies in function of the ligants addition and addictive in for pre-form. In this work it was obtained CMC of SiC-Talc/AI by "Squeeze Casting" method and investigated the influence of the talc as addictive in the mechanical properties of the ceramic matrix with controlled sizes of particles, intending improvement its mechanical properties of composites obtained. Rectangular test bodies of SiC were made with proportions from 10 to 50% in talc mass. The samples went burned partially at 1100°C, characterized and later infiltrated with aluminum melted at 850°C under pressure. The composites were characterized by optical microscopy and scanning electronic microscopy (SEM), where the different sizes of particles of matrix and the complete infiltration of the channels by melted metal was observed. They were also analyzed by X-ray difraction, showing the presence of phases SiC, Al, Si, SiO2, and MgAl2O4. The result of the flexural module at three points obtained for the proportions of 10 to 50% of talc show the influence of the proportions of addictive in the values of the mechanical properties of these composites. / Os compósitos de matriz cerâmica (CMC) são os mais recentes campo dos compósitos. Os principais fatores para o bom desempenho destes materiais são a escolha: da matriz, do reforço, do processamento e composição química. Materiais cerâmicos possuem módulo elástico muito alto, baixa densidade, e podem suportar temperaturas muito altas, porém apresentam baixa tenacidade. A introdução de uma fase metálica dúctil nas cerâmicas pode melhorar sua tenacidade à fratura. O SiC destaca-se entre os materiais utilizados na matriz cerâmica. Dentre os metais destaca-se o alumínio. A infiltração sob pressão de um fundido (“Squeeze Casting”) é um dos métodos mais utilizados na fabricação de CMC, sendo o foco de muitas pesquisas. As propriedades mecânicas de um compósito variam em função da adição de ligantes e aditivos na pré-forma. Neste trabalho foi obtido CMC de SiC-Talco/Al pelo método "Squeeze Casting" e investigado a influência do talco como aditivo nas propriedades mecânicas da matriz cerâmica com tamanhos controlados de partículas, pretendendo-se obter redução de custo da matéria-prima total do compósito e melhoria de suas propriedades mecânicas. Foram confeccionados corpos de prova retangulares de SiC com proporções de 10 a 50% em massa de talco. As amostras foram parcialmente sinterizadas a 1100ºC, caracterizadas e posteriormente infiltradas com alumínio fundido a 850ºC sob pressão. Os compósitos foram caracterizados por microscopia óptica (MO) e microscopia eletrônica de varredura (MEV), onde se observou os diferentes tamanhos de partículas da matriz a infiltração dos canais pelo metal fundido. Também foram analisadas por difração de Raios X, mostrando a presença das fases SiC, Al, Si, SiO2, e MgAl2O4. O resultado dos ensaios de flexão em três pontos obtidos para as proporções de 10 a 50% de talco mostram a influência das proporções de aditivo nos valores das propriedades mecânicas destes compósitos.