Os filmes cerâmicos de manganito de lantânio dopado com estrôncio (LSM) e de manganito de lantânio dopado com estrôncio/zircônia estabilizada com ítria (LSM/YSZ) são utilizados como catodos das células a combustível de óxido sólido de temperatura alta (CaCOSTA). Estes filmes cerâmicos porosos foram depositados sobre o substrato cerâmico denso de YSZ, utilizado como eletrólito, componente estrutural do módulo, assim conferindo uma configuração de meia-célula denominada auto-suporte. O estudo da meia-célula é fundamental, pois na interface catodo/eletrólito ocorre a reação de redução do oxigênio, conseqüentemente influenciando no desempenho da CaCOSTA. Neste sentido, o presente trabalho contribui para a síntese de pós de LSM e LSM/YSZ e para o processamento de filmes finos, utilizando a técnica de pulverização de pó úmido, adotada para a conformação dos filmes cerâmicos por permitir a obtenção de camadas porosas com espessuras variadas na ordem de micrômetros. Os pós de LSM foram sintetizados pela técnica de citratos e os pós de LSM/YSZ pela técnica de mistura de sólidos. Na etapa de conformação foram preparadas suspensões orgânicas de LSM e LSM/YSZ alimentada por gravidade em um aerógrafo manual. Para a conformação do substrato de YSZ utilizou-se uma prensa uniaxial hidráulica. Foram possíveis a obtenção das meia-células de óxido sólido catodo/eletrólito de estruturas cristalinas hexagonal para a fase LSM e cúbica para a fase YSZ. E as micrografias das meia-células mostram que o substrato YSZ é denso, suficiente para ser utilizado como eletrólito sólido, e os filmes de LSM e LSM/YSZ apresentam-se porosos com espessura de aproximadamente 30 μm e com boa aderência entre os catodos e o eletrólito. A presença do catodo compósito entre o catodo LSM e o substrato YSZ, possibilitou um aumento no desempenho eletroquímico na reação de redução do oxigênio. / The ceramic films of strontium-doped lanthanum manganite (LSM) and strontiumdoped lanthanum manganite/yttria-stabilized zirconia (LSM/YSZ) are used as cathodes of the high temperature solid oxide fuel cells (HTSOFC). These porous ceramic films had been deposited on the YSZ dense ceramic substrate, used as electrolyte, structural component of the module, thus conferring a configuration of half-cell called auto-support. The study of the half-cell it is basic, therefore in the interface cathode/electrolyte occurs the oxygen reduction reaction, consequently influencing in the performance of the HTSOFC. In this direction, the present work contributes for the processing of thin films, using the wet powder spraying technique, adopted for the conformation of the ceramic films for allowing the attainment of porous layers with thicknesses varied in the order of micrometers. The LSM powders were synthesized by the citrate technique and the LSM/YSZ powders synthesized by the solid mixture technique. In the stage of formation were prepared organic suspensions of LSM and LSM/YSZ fed by gravity in a manual aerograph. For the formation of the YSZ substrate was used a hydraulical uniaxial press. The attainment of solid oxide half-cells cathode/electrolyte was possible of crystalline structures hexagonal for phase LSM and cubic for phase YSZ. The half-cells micrographs show that the YSZ substrate is dense, enough to be used as solid electrolyte, and the LSM and LSM/YSZ films are presented porous with approximately 30 μm of thickness and good adherence between the cathodes and the electrolyte. The presence of composite cathode between the LSM cathode and YSZ substrate, presented an increase in the electrochemical performance in the oxygen reduction reaction.

O objetivo deste estudo foi avaliar comparativamente a estabilidade primária e a longevidade de implantes de zircônia (Y-TZP) e titânio (Grau IV) após ciclagem termomecânica (CTM). Foram obtidos 40 implantes de cada material que foram instalados em osso artificial e separados em grupos (n=10) segundo o material, a realização de ciclagem termomecânica e o ensaio realizado: Titânio - G1 (ensaio de torque de remoção); G2 (ensaio de arrancamento); G3 (CTM + ensaio de torque de remoção); G4 (CFTM + ensaio de arrancamento); e Zircônia - G5 (ensaio de torque de remoção); G6 (ensaio de arrancamento); G7 (CTM + ensaio de torque de remoção); G8 (CTM + ensaio de arrancamento). O equipamento de ciclagem mecânica foi configurado com carga de 133 N, frequência de 120 ciclos por minuto (2 Hz), totalizando 2.000.000 ciclos em cada espécime. A ciclagem térmica foi realizada com temperatura entre 5ºC, 37ºC e 55ºC. Após os ensaios mecânicos, os resultados foram submetidos à análise estatística (2-way ANOVA, teste de Bonferroni, p<0,05) e verificou-se que houve diferença estatisticamente significante (p<0,05) entre os implantes de titânio e zircônia, independente do tratamento utilizado, tanto para o torque de remoção quanto para a foça máxima de arrancamento. Os implantes de titânio mostraram maiores valores de torque de remoção e força de arrancamento comparado aos implantes de zircônia (p<0,05). Verificou-se também que a ciclagem termomecânica foi significante apenas para o ensaio de arrancamento quando utilizados implantes de titânio. Conclui-se que a estabilidade primária de implantes de titânio foi maior do que implantes de zircônia e que a ciclagem termomecânica foi um fator significativo apenas para a longevidade dos implantes de titânio. / The aim of this study was to assess comparatively the primary stability and longevity of zirconia (Y-TZP) and titanium (Grade IV) implants after thermomechanical cyclic (CTF). Forty implants to each material were obtained, installed in artificial bone and separated into groups (n=10) according to the material, the conducting of thermomechanical cyclic and the test submitted: Titanium - G1 (removal torque); G2 (pullout test); G3 (CTF + removal torque); G4 (TCF + pullout test); Zirconia - G5 (removal torque); G6 (pullout test); G7 (CTF + removal torque); G8 (CTF + pullout test). The mechanical cyclic testing machine was programmed to apply a load of 133 N, frequency of 120 cycles per minute (2 Hz) and a total of 2,000,000 cycles in each specimen. The thermocycling was set with a temperature between 5°C, 37ºC and 55ºC. After the mechanical tests, the data were subjected to statistical analysis (2-way ANOVA, Bonferroni test, p <0,05) and the results showed statistically significant difference (p <0.05) between zirconia and titanium implants, regardless of treatment used, for both removal torque and pullout tests. Titanium implants showed higher removal torque and pullout strength, which was statistically significant (p <0,05) compared with zirconia implants. It was also found that the thermomechanical cyclic was only significant for pullout test when used titanium implants. It was concluded that primary stability of titanium implants is higher than zirconia implants and the thermomechanical cyclic was a significant factor only for the longevity of titanium implants.

Cette thèse s’inscrit dans une démarche d’investigation des mécanismes d’endommagement de réfractaires électrofondus à très haute teneur en zircone. L’accent est en particulier mis sur la compréhension du phénomène de microfissuration, mécanisme de dégradation le plus dommageable pour ces matériaux. S’opérant au refroidissement, il conduit à la fracture des blocs électrofondus. Réalisés dans le cadre du programme NOREV (NOouveau REfractaires Verriers) soutenu par l’ANR, les travaux présentés sont le fruit d’une collaboration avec Saint Gobain CREE, le Centre des Matériaux P.M. FOURT de l’Ecole des Mines de Paris, le laboratoire GEMH de l’ENSCI de Limoges, ICAR et Euro Physical Acoustics. Ils font suite aux travaux réalisés lors du programme PROMETHEREF (2002-2005). Des expériences préliminaires ont permis de définir et d’affiner les orientations de l’étude. Prenant en considération l’anisotropie de dilatation de la zircone monoclinique et quadratique décrite dans la littérature, l’imagerie des électrons rétrodiffusés a notamment permis de révéler un lien entre l’arrangement de domaines cristallographiques et la fissuration observée. D’autres expérimentations conduites dans le domaine de l’émission acoustique ont mis en lumière son applicabilité et ses apports pour notre problématique ainsi que la nécessité de développer un algorithme de traitement adapté. Ainsi, trois axes de recherche ont été développés. Ils visent, par leur complémentarité, à permettre une analyse à plusieurs échelles de l’endommagement des matériaux de l’étude tout en s’appuyant sur leur comparaison. Ils permettent respectivement d’évaluer l’endommagement de manière indirecte, d’en examiner les origines à l’échelle microscopique et d’en déterminer l’occurrence en température à l’échelle globale de l’échantillon. Le premier volet consiste en une évaluation des propriétés mécaniques en températures soulignant les incidences de la fissuration. La mise en évidence de la fissuration sous-critique, l’évolution des propriétés élastiques ainsi que la détermination des propriétés à la rupture apportent autant d’éléments qui permettent ensuite d’enrichir l’étude fractographique. Le deuxième volet s’attache, après une interrogation quant à la présence simultanée de zircone monoclinique et quadratique, à déterminer les paramètres de maille et les coefficients de dilatation de ces deux structures pour les différents matériaux. La modélisation des mailles correspondantes se révèle alors d’un grand intérêt pour l’analyse de cartographies EBSD. Enfin, la validation et l’application d’un process de traitement novateur des données d’émission acoustique, intégrant un algorithme génétique, permet de quantifier l’endommagement et d’apporter des précisions quant à son action en température. Ces constatations sont confortées par des suivis par acousto-ultrasons. / This thesis falls under an approach of investigation on the damage mechanisms of high zirconia fused-cast refractories. The focus is particularly set on the comprehension of the phenomenon of microcracking responsible for the degradation of these materials. Taking place at cooling time, it leads to the fracture of the fused-cast blocks. Realized within the framework of the French research programme NOREV (NOouveau REfractaires Verriers) supported by the ANR, the work presented in this manuscript is the fruit of a collaboration with Saint Gobain CREE, the Centre des Materiaux P.M. FOURT of the Ecole des Mines de Paris, the laboratory GEMH of the ENSCI of Limoges, ICAR and Euro Physical Acoustics. It follows the works completed at the time of the previous programme (PROMETHEREF, 2002-2005). Preliminary experiments have enabled to define and specify the directions of this study. Taking into account the thermal expansion anisotropy of both monoclinic and tetragonal zirconia as described in the literature, the imagery of the backscatter electrons has in particular revealed a link between the presence of crystallographic domains and the observed cracking. Other experiments conducted in the field of acoustic emission have clarified its applicability and its contributions to our problems as well as the need for developing an adapted algorithm to process data. Thus, three research orientations have been developed. They aim, by their complementarity, to allow an analysis of the different scales of the damage on the study materials and is based on their comparison. They respectively allow to assess the damage in an indirect way, to examine its origins at a microscopic level and to determine its occurrence in temperature at the global scale of the sample. The first axis consists in the evaluation of the mechanical properties in temperatures emphasizing the incidences of cracking. The description of subcritical cracking, the evolution of the elastic properties as well as the determination of the fracture properties bring elements which enable to enrich the fractographic study. The second axis aims, after an interrogation as for the simultaneous presence of monoclinic and tetragonal zirconia, to determine the cell parameters and the thermal expansion coefficients of both structures for the different materials. The modeling of the corresponding cells then appears of great interest for the analysis of EBSD maps. Lastly, the validation and the application of an innovative process for the treatment of the acoustic emission data integrating a genetic algorithm enable to quantify the damage and to bring precise details as for its action in temperature. These observations are consolidated by acousto-ultrasounds follow-ups.

Les céramiques polycristallines pour application dentaire sont aujourd’hui majoritairement des zircones dopées à l’yttrium (Y-TZP). Ce matériau présente des avantages indéniables en terme de résistance à la rupture, de propriétés esthétiques ou encore de ténacité grâce au phénomène de renforcement par transformation de phase. Les problèmes de stabilité de la Y-TZP en présence d’eau peuvent être limités par un travail d’optimisation des poudres, mais la sensibilité intrinsèque du matériau vis-à-vis de l’eau ou des fluides biologiques demeure et représente un risque, spécialement dans le cadre d’applications cliniques. Les matériaux à base de zircone dopée au cérium (Ce-TZP) présentés dans cette étude ont été développés afin de répondre au triple objectif de ténacité, résistance et stabilité. A partir de la Ce-TZP, connue pour sa ténacité et sa stabilité en présence d’eau importantes, un travail d’optimisation de la microstructure a été réalisé afin d’obtenir une résistance à la rupture maximale. Différentes voies ont été explorées afin d’élaborer des microstructures permettant une augmentation de la résistance mécanique de la Ce-TZP, notamment le frittage SPS ou l'élaboration de composites par mélange de poudres commerciales. Parmi les résultats présentés, le plus marquant est sans doute l’élaboration de composites dans le système 10Ce-TZP/MgAl2O4, caractérisés par une combinaison de propriétés mécaniques inédite (Sigma R = 900 MPa, KIc >15MPa.m1/2), et une stabilité à très long terme en présence d’eau . La mise en forme de ce matériau par des procédés industriels de pressage a été rendue possible grâce à une étape de granulation par atomisation ultrasonique. Enfin, un axe de recherche a été dédié à l’élaboration de poudres composites à base de Ce-TZP en une seule étape, par synthèse chimique. Ces travaux plus prospectifs montrent qu’un mélange très intime de deux phases peut-être obtenu par des méthodes de chimie douce dans une poudre composite. Ces poudres permettent l’élaboration de matériaux nanostructurés, dont les propriétés pourraient dépasser celles des composites conventionnels. / Yttria-doped tetragonal zirconia ceramics (often referred as Y-TZP) are today of major interest in biomedical and particularly dental applications, due to their excellent combination of strength and aesthetic features. Nevertheless, the moderate toughness of 3Y-TZP, and its still possible low temperature degradation (LTD) leaves space for new materials development. The purpose of this study is to assess the potential benefit of using ceria-doped zirconia (Ce-TZP) based ceramics as an tough, strong and stable alternative to Y-TZP. Ce-TZP generally possesses high toughness, but moderate strength when compared to 3Y-TZP, which is related to a larger grain size. In order to improve the strength of Ce-TZP, three microstructural optimizations have been carried out. First, Spark Plasma Sintering (SPS) has been used, showing a good potential for the development of nanostructured materials, which can be dense and mainly tetragonal, but aesthetically incompatible with a dental application. Cerium reduction effects on color and phase repartition have been studied. Adding a second step of conventional sintering in air has led to fully dense submicron 12Ce-TZP with acceptable color, but unsufficient strengthening. In a second step, a conventional composite approach has been used, by mixing commercial powders. The most striking result is certainly the uncommon combination of mechanical properties (Sigma R = 900 MPa, KIc > 15MPa.m1/2) obtained in the system 10Ce-TZP/MgAl2O4, together with a long term stability in presence of water. Pilot scale processing of this material has been set up by mean of ultrasonic spray-drying. Finally, a third research axis has been devoted to the synthesis of composite powders based on Ce-TZP, in one step. This work has shown that a very close mix of two phases can be obtained par soft chemistry methods in a composite powder. Nanostructured materials can be obtained from these powders, which properties could exceed those of conventional composites.

Actuellement, la qualité de l’imagerie provenant de systèmes optiques spatiaux est limitée par la taille de leurs miroirs et la masse des structures supportant le miroir. Le développement de systèmes athermiques légers (un seul matériau) constitue le principal challenge dans l’amélioration de ces systèmes. De matériaux légers, résistants mécaniquement (E/ρ3 > 10, σf > 100 MPa) et stables thermiquement (< 2,0.e-6/K) doivent être développés. Dans ce cadre, notre travail porte sur l’élaboration de composites céramiques associant un matériau à coefficient de dilatation thermique (CTE) positif résistant mécaniquement (alumine ou zircone cériée) et un matériau à CTE très négatif (tungstate de zirconium ou β-eucryptite). L'étude du tungstate de zirconium a révélé plusieurs problèmes de décomposition et de réactions avec certaines matrices oxydes, menant à l’abandon de cet oxyde dans l’élaboration des composites. Dans le cas de la β-eucryptite, un phénomène de vermiculation a été mis en évidence, conduisant à la formation d’une porosité intragranulaire. L’optimisation des paramètres de frittage a permis de limiter cette porosité. L’étude du comportement thermique de la β-eucryptite confirme que son CTE très négatif provient principalement d’un phénomène de fissuration, généré par l’anisotropie de dilatation de sa maille cristalline. Cette fissuration est dépendante de la taille des grains mais également de la taille des agrégats de grains dans le cas des poudres. Ainsi, bien que le CTE intrinsèque de la β-eucryptite soit très faible (-0,4.e-6/K), son CTE extrinsèque peut atteindre des valeurs jusqu'à -10,9.e-6/K selon les conditions d’élaboration. Dans ce travail, deux stratégies d’élaboration de composites sont étudiées. Le premier cas consiste à diminuer le CTE des matrices oxydes à partir d’une poudre de β-eucryptite non microfissurée (-0,4.e-6/K) tandis que le second cas consiste à obtenir des matériaux à CTE très faible à partir d’une poudre de β-eucryptite microfissurée (-3,0.e-6/K). Lors de l’utilisation de la matrice en zircone cériée, le taux de dopage au cérium est optimisé afin de limiter la transformation de phase de la zircone. Cette transformation, induite par les contraintes de tension exercées par la β-eucryptite, affecte la linéarité du comportement thermique du composite. Dans les deux cas d’étude, les composites denses montrent une modification du CTE intrinsèque de la β-eucryptite passant de -0,4.e-6/K à plus de +3,2.e-6/K en raison des contraintes de compression appliquées par la matrice (alumine ou zircone cériée). La relaxation de ces contraintes nécessite une sous-densification des composites. A partir de ces observations, différents composites à CTE très faible sont élaborés. Toutefois, le sous-frittage des composites associé à la microfissuration de la β-eucryptite diminuent fortement les propriétés mécaniques des matériaux ainsi élaborés. / High resolution satellite imagery from space optical systems is mainly limited by the mirror size and the mass of structures supporting the mirror. Nowadays, the development of light athermal systems is the major challenge to improve these optical systems. So, light materials having good mechanical properties (E/ρ3 > 10, σf > 100 MPa) and thermal stability (< 2.0e-6/K) are required. Within this context, our project consists in processing new ceramic composites by combining positive thermal expansion coefficient (TEC) materials having good mechanical properties (alumina or ceria doped zirconia) and negative TEC materials (zirconium tungstate or β-eucryptite) The processing of zirconium tungstate-based materials showed several decomposition and chemical reactions with some oxide matrix leading to its giving up. In the case of β-eucryptite, vermicular phenomenon occurs during sintering leading to the formation of intragranular porosity. Sintering parameters optimization can limit this porosity. The study of the thermal behavior of pure β-eucryptite materials shows that the very negative TEC results from microcracking, generated by the TEC anisotropy of its crystal lattice. This microcracking depends on the grain size and the aggregate size in the case of powder materials. Despite the fact that the TEC of its lattice (called intrinsic TE C equals to -0.4e-6/K) is very low, its bulk (or extrinsic) TEC can reach values until -10.9e-6/K according to the processing conditions. In this work, two strategies for developing composites were studied. The first one consists in decreasing the matrix TEC using an uncracked β-eucryptite powder (-0.4e-6/K) while the second one consists in elaborating near zero TEC materials from a microcracked β-eucryptite powder (-3.0e-6/K). When ceria-doped zirconia is used, ceria content must be adjusted in order to limit zirconia phase transformation. This transformation is driven by tensile stresses induced by the β-eucryptite and modifies the composite thermal behavior linearity. In both studied cases, dense composites show a modification of the β-eucryptite intrinsic TEC from -0.4e-6/K to more than +3.2e-6/K as a consequence of compressive stresses applied by the oxide matrix. An uncompleted densification of composites is required to relax these stresses. Taking into account these observations, several very low TEC composites were elaborated. However, the uncompleted densification of composites and the β-eucryptite microcracking greatly decrease the mechanical properties of these materials.

Ce travail de thèse a pour objectif l’élaboration et la caractérisation de composites Alumine/Zircone obtenus par voies conventionnelles, et dédiés à un usage orthopédique. Ces composites présentent une biocompatibilité prouvée, d’excellentes propriétés mécaniques ainsi qu’une grande stabilité. Ils sont plus résistants, plus fiables que l’alumine ou la zircone seules et permettent d’envisager des composants de tailles et formes plus exigeantes mécaniquement. Actuellement ces composites semblent les plus adaptés pour la réalisation de prothèses orthopédiques mais peuvent encore être optimisés via la modification des microstructures. La première partie de ce travail a concerné l’étude de la réalisation industrielle de composants de grande taille à partir d’une poudre. Les différentes étapes de l’élaboration sont traitées : pressage des composants, frittage et usinage. Cette première partie est majoritairement consacrée à l’étude des gradients thermique dans une sphère lors du frittage. Nous montrons qu’il est possible de modéliser et de mesurer les gradients thermiques dans le matériau de manière très réaliste, ainsi que d’obtenir des ordres de grandeur des contraintes mécaniques. On pourra ainsi envisager de tester numériquement les cycles de frittage en fonction de la géométrie des pièces frittées. Nous étudions par ailleurs la possibilité de réaliser un usinage des composants après un traitement de préfrittage, qui permettrait de diminuer les coûts et simplifier l’élaboration de composants de grande taille. La seconde partie de ce mémoire a permis de montrer que différents types de microstructures, présentant des propriétés mécaniques différentes, peuvent être obtenues par simple mélange de poudre. Ceci est possible par l’utilisation d’un traitement thermique adapté, la variation du taux de zircone et grâce à l’ajout de dopants (Si, Ca et Mg) jouant sur la mobilité des joints de grains d’alumine. Lors de l’utilisation de Ca ou Mg, le taux de zircone et la température ont un effet prépondérant sur l’aspect des microstructures, permettant d’obtenir des micro/micro-composites (< 16vol% de zircone et >1500°C) et nano/nano-composites (25vol% de zircone et T < 1500°C). Seul l’ajout conjoint de Si et de Ca pour des échantillons contenant 2,5vol% de zircone permet de conduire à des micro/nano-composites avec une grande proportion de zircone intragranulaire. Les observations sur des composites avec un taux de zircone proche du taux de percolation (16vol%) permettent de mettre en évidence l’ensemble des types de renforcement observés dans la littérature, en fonction de la température et des dopants utilisés. La variété des microstructures obtenues permet de progresser dans le contrôle des microstructures des composites alumine-zircone, mais aussi d’envisager d’autres applications de ces composites en fonction des mécanismes de renforcement observés et de leurs propriétés mécaniques et structurales. / The aim of this work was to elaborate and characterize zirconia toughened alumina composites with different microstructures, using a simple process. These composites are obtained by colloidal process and are dedicated to orthopedic application. ZTA composites offer both higher strength and toughness than alumina, a lower sensitivity to ageing than zirconia, and also a proven biocompatibility. They open the door to component designs not reachable with other, more brittle materials. Nowadays, these composites are the safest for orthopedic implants application, but can still be improved. The first part of our study is dedicated to a numerical modeling of a large femoral head during sintering. It is so possible to obtain a realistic model of thermal and mechanical strain gradient. However, the modeling should be enhanced by a thorough study of the elastic-viscous-plastic behavior of the composite at high temperature. Then some experiments of sintering with various load or pressure and speed should be practice to determine precisely the sintering related strain. The possibility of machining in the pre-sintered state, with the aim of reducing machining costs and simplify the process of large components, is investigated. The second part is focused on the development of various microstructures with specific mechanical properties and reinforcement behaviors. Such structures were achieved by adjusting the amount of zirconia, controlling the grain growth with dopants to improve or inhibit the alumina grain growth (Si, Ca and Mg), and by adjusting the sintering thermal treatment. The use of calcium associated or not with magnesium seems useless as compared to the predominant influence of the zirconia content. Adjusting zirconia amount led to micro-composites (< 16vol% of zirconia and >1500°C) and nano-composites (25vol% of zirconia and < 1500°C). Only Si/Ca co-doped ZTA composites with small amount of zirconia (2.5vol %) leads to almost homogeneous micro/nano composites with a large proportion of intragranular zirconia particles at high temperature. Around the percolation threshold (16vol% of zirconia) all types of reinforcement mechanisms that could be observed in ZTA composites (referred to literature) can be observed, depending on the thermal treatment and the dopants used. The range of microstructures obtained in this study leads us to investigate other applications for these composites depending of its reinforcement behavior and its mechanical and structural properties.

Les dépôts de YSZ, élaborés par projection plasma, sont des céramiques réfractaires généralement utilisées pour les applications de barrières thermiques (TBC). Sa faible conductivité thermique associée à sa bonne résistance mécanique assure aux TBC de hautes performances et de bons rendements. La structure et la microstructure complexe sont à l'origine de ces propriétés mécaniques, et celles doivent être contrôlées. Tout comme les céramiques denses la YSZ se dégrade en température et sous vapeur d'eau.La dégradation des propriétés mécaniques dans le temps (module d'élasticité et contrainte à rupture)est accélérée par la température. Pour cette étude les propriétés ont été évaluées en flexion 3 points à température ambiante. Les observations structurales et microstructurales ont été réalisées respectivement par DRX et microscopie électronique à balayage au cours du vieillissement. Un model analytique a pu être proposé pour prédire le comportement du matériau dans le temps sous humidité. / Yttria Stabilized Zirconia (YSZ) coatings, deposited by plasma sprayed process, are refractory ceramics mostly used as the Thermal Barrier Coating (TBC) applications. The low YSZ thermal conductivity associated to the good mechanical resistance ensures a high performances and efficiencies of these TBC. The structure and the complex microstructure are responsible for the mechanical properties and must be controlled. Like brittle ceramic materials, the YSZ is affected by degradation at low temperature due to water vapor. Material ageing results from the progressive degradation of the mechanical properties (such as fracture strength and Young’s modulus), which seem to decrease in time and accelerate depending on temperature. In this study, the mechanical properties have been evaluated by means of three-point bending tests at room temperature. The observations of the structure and the microstructure are respectively investigated by X-ray diffraction and SEM-technique with material ageing. An analytical model is suggested in order to predict the evolution of the properties under humidity atmosphere.

La zircone 3Y-TZP présente un grand intérêt pour les applications dentaires, en implantologie orale, elle semble être un matériau extrêmement prometteur: elle allie une biocompatibilité à un aspect esthétique satisfaisant et présente aussi des propriétés mécaniques très supérieures aux autres céramiques. Ces bonnes propriétés mécaniques sont intimement liées à la microstructure du matériau, elle-même directement liée aux procédés d’élaboration comme nous l’a rappelé l’alarmante série de ruptures de plus de 800 têtes de prothèses de hanche en zircone au début des années 2000, due au phénomène de vieillissement de ce matériau. Cependant, les études cliniques menées à ce jour ne font toujours pas état des interrelations existant entre la microstructure, les propriétés mécaniques et la sensibilité au vieillissement. Il était donc primordial de valider et d’appliquer les connaissances acquises dans le domaine des sciences des matériaux pour l’application de la zircone 3Y-TZP en implantologie oral. C’est l’objectif de notre première publication. Nous avons également développé, dans notre deuxième publication, un protocole d’évaluation permettant dans un premier temps, d’évaluer l’effet de l’état de surface et de la microstructure sur la résistance mécanique d’implants neufs, puis de suivre leurs cinétiques de vieillissement tout en analysant l’évolution de la microstructure et son influence sur la résistance mécanique au cours du vieillissement. Tout cela permettant in fine, de prédire la durabilité d’un type d’implant. Nous avons par la suite, développé un programme informatique permettant la détection et la quantification du vieillissement pour un volume donné. Cette détection de la zone vieillie ou transformée est basée sur des modifications microstructurales caractéristiques engendrées lors du vieillissement. Enfin, nous avons pu mettre en évidence l’occurrence du phénomène de vieillissement in vivo, par l’analyse d’explants issus d’une étude clinique et démontrer leur probable implication dans ces cas d’échec. Le logiciel informatique de traitement d’image développé a été également, appliqué aux explants dans le but de mettre en lumière et d’expliciter l’occurrence du phénomène de vieillissement in vivo, afin de sensibiliser les acteurs de ce marcher aux problématiques rencontrées et d’optimiser de ce dispositif médical, à la lumière des connaissances actuelles. / 3Y-TZP zirconia is gaining interest in oral implantology, it seems to be a promising material with good biocompatibility, esthetic appearance and also the highest mechanical properties for a ceramic. These mechanical properties are closely related to the microstructure of the material itself directly related to production processes as we recalled the alarming series of breaks of more than 800 heads of zirconia hip replacements in the early 2000, due to the aging phenomenon of the material. However, clinical studies to date are still not state of the interrelationships between microstructure, mechanical properties and sensitivity to aging. It was therefore important to validate and apply the knowledge gained in the field of materials science for the application of 3Y-TZP zirconia oral implantology. This is the goal of our first publication. We also have developed in our second publication, a protocol to assess the effect of the surface modification and microstructure on the mechanical strength of new implants and follow their kinetics of aging and also, the evolution of the microstructure and its influence on the mechanical strength during aging, to predict the durability of a type of implant. We have subsequently developed a computer program for the detection and quantification of the aging for a given volume. This detection of the aged or transformed area is based on microstructural modifications produced during aging. Finally, we have been able to demonstrate the occurrence of the phenomenon of aging in vivo, by analyzing explants from a clinical study and demonstrate their involvement in the case of dental implant failure. The image processing developed was also applied to the explants in order to highlight and explain the occurrence of in vivo aging phenomenon. The objective is to optimize this medical device, in the light of current knowledge.

Ce travail de thèse a porté sur de nouveaux implants intervertébraux en céramique. Au cours du projet dans son ensemble (projet européen Longlife), un nouveau matériau et de nouveaux designs d’implants ont été développés, ainsi qu’un nouveau test destiné à simuler les sollicitations subies in vivo par les implants afin d’estimer leur durée de vie. Le nouveau matériau développé est un composite triphasé composé d’une matrice de zircone dopée à l’oxyde de cérium (pour sa résistance au vieillissement), d’une phase globulaire d’alumine α (pour affiner la microstructure) et d’une phase allongée composée d’aluminates de strontium (pour augmenter la ténacité). La première partie du travail a consisté à caractériser ce matériau afin de connaître son comportement en termes de résistance mécanique, stabilité thermique, et de résistance à la stérilisation. Une deuxième partie a été consacrée au développement d’un test multiphysique regroupant les différentes sollicitations attendues par une prothèse in vivo (fatigue axiale, micro-séparation, vieillissement et usure). Il a fallu pour cela s’appuyer sur des simulations numériques qui ont permis de développer le système. Les données de la littérature ont été utilisées afin de choisir les paramètres du test (durée, fréquence, milieu d’essai). Enfin, la dernière partie de ce travail a été la mise à l’épreuve de différents prototypes à travers le test multiphysique et leur caractérisation en cours d’essai. Les principaux résultats de ce travail de thèse sont les suivants : le composite montre un comportement pseudo-plastique sous charge, avec une nette transformation de phase avant rupture, ce qui est positif dans le cadre de son utilisation. De plus, il ne semble pas affecté par la stérilisation. Du point de vue des implants développés, peu passent le test multiphysique. Le design, ainsi que la géométrie (notamment la clearance des échantillons) sont des leviers d’amélioration qui permettront d’augmenter la fiabilité des implants. / This work deals with the development of new intervertebral prostheses, made with ceramics. A whole European project, Longlife, was dedicated to the development of such implants. To achieve this goal, several axes have been followed: the synthesis of a new material, the development of new designs of intervertebral bodies, and the set-up of a new test aimed at reproducing in vitro the different solicitations undergone by an intervertebral implant in vivo. The new material developed is a triphasic composite composed of a matrix of ceria-doped zirconia (insensitive to ageing), a secondary globular phase of α-alumina (to reduce the grain size), and a third, elongated phase composed of strontium aluminates platelets (in order to improve fracture toughness). The first part of this work was to characterize this new material in order to forecast its behaviour under mechanical solicitation, thermal stability and resistance to sterilization. Secondly, the set-up of the new test is exposed. Different steps were chosen (axial fatigue, micro-separation, ageing and wear) in order to reproduce the “real-life” solicitations. To achieve this goal, Finite Elements simulations were performed, allowing the development of specific specimen holders that mimic the fixation of the implants in the vertebrae. The parameters of the test (duration, frequency, medium) were chosen after a details survey of the literature and of standards. At the end, we tested different prototypes trough this new multiphysic assessment set up. As a main result of this thesis, the chosen ceramic composite exhibits a pseudo-plastic behaviour, with a large deformation due to phase transformation before fracture, which is a positive result in the framework of the forecast applications. Moreover, the material doesn’t seem degraded by the sterilization processes. Concerning the multiphysic test, only a few implants resisted it. The design of the implants is a key-point, as well as the geometry (in particular, clearance seems to be critical).

The submerged entry nozzle (SEN) is used to transport the molten steel from a tundish to a mould. The main purpose of its usage is to prevent oxygen and nitrogen pick-up by molten steel from the gas. Furthermore, to achieve the desired flow conditions in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. In addition, the steelmaking processes occur at high temperatures around 1873 K, so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during preheating and casting processes is necessary for the design of the steelmaking processes  The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during preheating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied. A large number of accretion samples formed inside alumina-graphite clogged SENs were examined using FEG-SEM-EDS and Feature analysis. The internal coated SENs were used for continuous casting of stainless steel grades alloyed with Rare Earth Metals (REM). The post-mortem study results clearly revealed the formation of a multi-layer accretion. A harmful effect of the SENs decarburization on the accretion thickness was also indicated. In addition, the results indicated a penetration of the formed alkaline-rich glaze into the alumina-graphite base refractory. More specifically, the alkaline-rich glaze reacts with graphite to form a carbon monoxide gas. Thereafter, dissociation of CO at the interface between SEN and molten metal takes place. This leads to reoxidation of dissolved alloying elements such as REM (Rare Earth Metal). This reoxidation forms the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to the formation of a high-viscous alumina-rich glaze during the SEN preheating process. This, in turn, creates a very uneven surface at the SEN internal surface. Furthermore, these uneven areas react with dissolved REM in molten steel to form REM aluminates, REM silicates and REM alumina-silicates. The formation of the large “in-situ” REM oxides and the reaction of the REM alloying elements with the previously mentioned SEN´s uneven areas may provide a large REM-rich surface in contact with the primary inclusions in molten steel. This may facilitate the attraction and agglomeration of the primary REM oxide inclusions on the SEN internal surface and thereafter the clogging. The study revealed the disadvantages of the glass/silicon powder coating applications and the SEN decarburization. The decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials from a commercial Submerged Entry Nozzle (SEN), were also investigated for different gas atmospheres consisting of CO2, O2 and Ar. The gas ratio values were kept the same as it is in a propane combustion flue gas at different Air-Fuel-Ratio (AFR) values for both Air-Fuel and Oxygen-Fuel combustion systems. Laboratory experiments were carried out under nonisothermal conditions followed by isothermal heating. The decarburization ratio (α) values of all three refractory types were determined by measuring the real time weight losses of the samples. The results showed the higher decarburization ratio (α) values increasing for MgO-C refractory when changing the Air-Fuel combustion to Oxygen-Fuel combustion at the same AFR value. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times compared to heating at lower temperatures during longer holding times for Al2O3-C samples. Diffusion models were proposed for estimation of the decarburization rate of an Al2O3-C refractory in the SEN. Two different methods were studied to prevent the SEN decarburization during preheating: The effect of an ZrSi2 antioxidant and the coexistence of an antioxidant additive and a (4B2O3 ·BaO) glass powder on carbon oxidation for non-isothermal and isothermal heating conditions in a controlled atmosphere. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina-graphite refractory base materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by a (4B2O3 ·BaO) glaze during the green body sintering led to an excellent carbon oxidation resistance. The effects of the plasma spray-PVD coating of the Yttria Stabilized Zirconia (YSZ) powder on the carbon oxidation of the Al2O3-C coated samples were investigated. Trials were performed at non-isothermal heating conditions in a controlled atmosphere. Also, the applied temperature profile for the laboratory trials were defined based on the industrial preheating trials. The controlled atmospheres consisted of CO2, O2 and Ar. The thicknesses of the decarburized layers were measured and examined using light optic microscopy, FEG-SEM and EDS. A 250-290 μm YSZ coating is suggested to be an appropriate coating, as it provides both an even surface as well as prevention of the decarburization even during heating in air. In addition, the interactions between the YSZ coated alumina-graphite refractory base materials in contact with a cerium alloyed molten stainless steel were surveyed. The YSZ coating provided a total prevention of the alumina reduction by cerium. Therefore, the prevention of the first clogging product formed on the surface of the SEN refractory base materials. Therefore, the YSZ plasma-PVD coating can be recommended for coating of the hot surface of the commercial SENs.