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.

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.

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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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