Nous étudions la réponse élastoplastique des verres silicatés à de fortes contraintes par diffusion Brillouin de la lumière. Des cartographies micro-Brillouin 3D du champ de densité résiduelle sont obtenues dans l'empreinte plastique laissée par une indentation Vickers et comparés à des modélisations par éléments finis. L'analyse conjointe des mesures réalisées en enclumes diamants sur la silice dans le domaine de déformation élastique et des données de la littérature fait apparaître que le durcissement anormal des modules élastiques avec la température est d'origine dynamique. La température à laquelle le durcissement est mis en évidence augmente avec la pression hydrostatique appliquée. Nous observons également que la densification progressive de la silice diminue fortement l'amplitude du maximum dans le frottement interne observé à 2 GPa de même qu'elle supprime l'anomalie dans la compressibilité. / We study the elastoplastic response of silicate glasses at high stresses with Brillouin light scattering. 3D micro-Brillouin mapping residual density field are obtained in the plastic region left by a Vickers indentation. Maps are compared with finite element modeling. The joint analysis of new high-pressure measurements in a diamond anvil cell on silica in the elastic domain and literature data revealed that the abnormal hardening of elastic moduli with temperature is of dynamical origin. The onset temperature of the hardening increases with increasing applied hydrostatic pressure. We also observe that densification of silica strongly reduces the amplitude of the maximum in internal friction observed at 2 GPa as well as it suppresses the compressibility anomaly.

Ce travail est dédié à la compréhension du comportement mécanique des verres silicates à micro-échelle. Il est consacré à l'étude de la densification permanente des verres silicates et aux changements structuraux induits par différents méthodes. D'abord la densification par presse « Belt » en fonction de la température et de la pression est étudiée. Une inhomogénéité macroscopique à l'échelle de l'échantillon dépendant de la température et de la pression est observée. Ces observations sont interprétées par une diminution de la viscosité du verre avec la pression. Ensuite, une étude dans le domaine plastique du verre de silice par différents techniques « hautes pressions » est réalisée. Des modifications structurales sont observées durant la compression, en particulier une diminution de l'angle Si-O-Si intertétraèdre et une augmentation des petits cycles à 3 trétraèdres. De plus, il est montré que dans une compression hydrostatique la limite élasto-plastique d'un verre de silice prédensifiée ne dépend pas du chemin de densification. Le comportement du verre de silice prédensifié puis indenté évolue d'un verre anormal, vers un verre normal à mesure que la densité augmente. Enfin, l'indentation d'un verre silico-sodo-calcique sous indentation Vickers montre que la cartographie de la densification est similaire pour les deux charges (1 et 2 Kg) et que la distribution de la densification n'est pas affectée par la préparation des échantillons. Ces résultats sont comparés avec des résultats numériques obtenus par modélisation par éléments finis / This work was dedicated to the comprehension of mechanical behavior of silica glass at micro scale. It is focused on the study of the permanent densification of silica glass and structural changes induced by different methods. First, “Belt” densification is studied as a function of temperature and pressure. A macroscopic inhomogeneity at the sample scale was observed and it was dependent upon temperature. These observations were due to glass viscosity decrease with pressure. The plastic domain of silica glass with different “high pressure” techniques was analyzed. Structural modifications were observed during compression especially as Si-O-Si intertetrahedral angle decrease and an increase of small rings. Furthermore, it is demonstrated that during a hydrostatic compression, the elasto-plastic limit of predensified silica glass does not depend on densification path. We observed that the behavior of the predensified and indented silica glass evolves from normal to abnormal glass when density increases. Finally, the Vickers indentation of sodo-lime-silicate glass shows that densification cartography is similar for the two charges (1 and 2 Kg) and that densification distribution was not affected by sample preparation method. These results were compared with numerical calculations obtained by finite element modeling

Solid Oxide Fuel Cells (SOFCs) are emerging as a potential breakthrough energy conversion technology for clean and efficient production of electricity and heat from hydrogen and hydrocarbon fuels. Sc0.1Ce0.01ZrO2 electrolytes for Solid Oxide Fuel Cells are very promising materials because their high ionic conductivity in the intermediate temperature range 700°C-800°C. The vibration response of cubic and rhombohedral (β) 10 mol%Sc2O3 - 1 mol%CeO2 - ZrO2(Sc0.1Ce0.01ZrO2 ) both at room and high-temperatures is reported. The in-situ heating experiments and ex-situ indentation experiments were performed to characterize the vibrational behavior of these important materials. A temperature and stress-assisted phase transition from cubic to rhombohedral phase was detected during in-situ Raman spectroscopy experiments. While heating and indentation experiments performed separately did not cause the transition of the cubic phase into the rhombohedral structure under the performed experimental conditions and only broadened or strained peaks of the cubic phase could be detected, the heating of the indented (strained) surface leaded to the formation of the rhombohedral Sc0.1Ce0.01ZrO2. Both temperature range and strained zone were estimated by in situ heating and 2D mapping, where a formation of rhombohedral or retention of cubic phase has been promoted. The mechanical properties, such as Young’s modulus, Vickers hardness, indentation fracture resistance, room and high temperature four point bending strength and SEVNB fracture toughness along with the stress – strain deformation behavior in compression, of 10 mol% Sc2O3 – 1 mol % CeO2 - ZrO2 (ScCeZrO2) ceramics have been studied. The chosen composition of the ScCeZrO2 has very high ionic conductivity and, therefore, is very promising oxygen ion conducting electrolyte for the intermediate temperature Solid Oxide Fuel Cells. Therefore, its mechanical behavior is of importance and is presented in this study.

Scandia doped zirconia has been considered a candidate for electrolyte material in intermediate temperature Solid Oxide Fuel Cells (SOFCs) due to its high ionic conductivity, chemical stability and good electrochemical performance. The aim of this study is to determine the mechanical properties of SCZ, ie. zirconia (ZrO₂) doped with Scandia (Sc₂O₃) and small amount of ceria (CeO₂) that are important for reliability and durability of the components manufactured from SCZ. The SCZ was prepared from powder by uniaxiall cold pressing at subsequent sintering at 1550 ºC for 4 hours. The density and porosity of the sintered samples was measured following the ASTM Standard C20-00 for alcohol immersion method. A pure cubic phase of SCZ sample was identified by X-ray diffraction (XRD) at room temperature. Quantitative compositional analyses for Zr, Sc, Ce, Hf and Ti were carried out on a Cameca SX50 electron microprobe with wavelength-dispersive spectroscopy (WDS) and energy-dispersive spectroscopy (EDS). Scanning Electron Microscopy (SEM) images were acquired using both secondary electron (SE) and back-scattered electron (BSE) detectors. WDS and EDS analysis also revealed that Zr, Sc, Ce, Hf and Ti are relatively homogeneously distributed in the structure. The average grain size of sintered SCZ samples was measured to be 4 μm. Thermal expansion at different temperatures for the SCZ ceramic was determined using Thermal Mechanical Analyzer, and the instantaneous Coefficient of Thermal Expansion (CTE) was found to be 8.726х10⁻⁶ 1/°C in the in 25-400 °C temperature range. CTE increases monotonically with temperature above 400 ºC to 1.16х10⁻⁵ at 890 °C, most likely as a result of thermo-chemical expansion due to an increase in oxygen vacancy concentration. Room temperature Vickers hardens of 12.5 GPa was measured at loads of 1000 g, while indentation fracture toughness was found to vary from 2.25 to 4.29 MPa m¹⁄², depending on the methodology that was used to calculate fracture toughness from the length of the median corner cracks. Elastic moduli, namely Young and shear moduli were determined using Resonance Ultrasound Spectroscopy (RUS). It was found that elastic moduli decreases with temperature in non-linear manner, with significant drop in the 300-600 °C temperature range, the same temperature range in which loss modulus determined by Dynamic Mechanical Analyzer exhibits frequency dependant peaks. The high loss modulus and significant drop in elastic moduli in that temperature regime is attributed to the relaxation of doping cation-oxygen vacancies clusters. The flexural strength in 4-point bending was measured at room temperature, 400 °C, 600 °C and 800 °C. and the results were analyzed using Weibull statistics. It was found that flexural strength changes with temperature in a sigmoidal way, with the minimum strength at around 600 °C. Non-linear decrease in strength with temperature can be traced back to the changes in elastic moduli that are caused predominately by relaxation of oxygen vacancies.

Une méthode de couplage continu-discret a été développée pour simuler les mécanismes complexes d'endommagement de la silice soumise à un choc laser de haute puissance. Dans un premier temps, une classification des méthodes numériques existantes a été faite pour choisir celles les mieux adaptées à la simulation du comportement sous choc de la silice. Comme résultat de cette classification, deux méthodes ont été retenues: la méthode des éléments discrets (DEM) et la méthode des éléments naturels contraints (CNEM). Ces méthodes sont alors couplées en se basant sur la technique dite "Arlequin". Puis, un modèle numérique permettant de tenir compte des différents phénomènes qui caractérise le comportement de la silice sous haute pression a été développé. Pour bien caractériser les mécanismes de fissuration de la silice à l’échelle microscopique, un nouveau modèle de rupture a été développé dans ce travail. Finalement, ces deux modèles, modèle de comportement et modèle de rupture, ont été intégrés dans la méthode du couplage pour simuler d'un point de vue mécanique le choc laser sur un échantillon en silice. / A discrete-continuum coupling approach has been developed to simulate the laser-induced damage in silica glass. First, a classification of the different numerical methods has been performed to select the ones that best meet the objectives of this work. Acting upon this classification, the Discrete Element Method (DEM) and the Constrained Natural Element Method (CNEM) have been retained. Subsequently, a coupling approach between these methods has been proposed. This approach is based on the Arlequin technique. In the second part, a numerical model of the silica glass mechanical behavior has been developed to better characterize the silica glass response under highly dynamic loadings and particularly loading generated by a laser beam. To correctly characterize the silica glass cracking mechanisms, a new fracture model has been proposed in this work. Finally, all these developments have been used to simulate the laser-induced damage in silica glass.