State of the art thermal barrier coatings (TBCs) for gas turbine applications comprise (7 wt.%) yttria partially stabilized zirconia (7YSZ). 7YSZ offers a range of attractive functional properties – low thermal conductivity, high thermal expansion coefficient and high in-plane strain tolerance. However, as turbine entry temperatures are raised, the performance of 7YSZ coatings will be increasingly affected by sintering and environmental contamination, by calcia-magnesia-alumina-silica (CMAS) deposits. The effect of sintering-induced stiffening on the driving force for spallation of plasma-sprayed (PS) TBCs was investigated. Spallation lifetimes of TBC specimens sprayed onto alumina substrates were measured. A simple fracture mechanics approach was employed in order to deduce a value for the strain energy release rate. The critical strain energy release rate was found to be constant, and if this value had been known beforehand, then the rationale presented here could be used for prediction of coating lifetime. The effect of vermiculite (VM) and volcanic ash (VA) contamination on the sintering-induced spallation lifetime of PS TBCs was also investigated. The presence of both VM and VA was found to accelerate the rise in their Young’s modulus with sintering. Spallation results show that coating lifetime may be significantly reduced, even at relative low addition levels, due to the loss of strain tolerance caused by the penetration of glassy deposits. This result gives a clear insight into the role CMAS plays in destabilizing TBCs. Finally, the adhesion characteristics of ingested volcanic ash were studied using a small jet engine. The effects of engine speed and particle size were investigated. Deposition on turbine surfaces was assessed using a borescope. Deposition mainly occurred on the nozzle guide vane and blade platform. A numerical model was used to predict particle acceleration and heating in flight. It was observed that larger particles are more likely to adhere because they have greater inertia, and thus are more likely to impact surfaces. The temperature of the larger particles at the end of its flight was predicted to be below its softening point. However, since the component surface temperatures are expected to be hotter, adhesion of such particles is probable, by softening/melting straight after impact.

Thermal barrier coatings (TBCs) have been widely used in gas turbine engines to protect the underlying metal from high operating temperature so as to improve the durability of the components and enhance the engine efficiency. However, since the TBCs always operate in a demanding high-temperature environment of aircraft and industrial gas-turbine engines, a better understanding of this complex system is required to improve the durability and reliability. The objective of this study is to investigate the effects of surface modification for the NiCoCrAlY bond coats on the thermal cycling lifetime and failure characteristics of TBCs. Parameters of modification for the bond coats included as-sprayed, barrel-finished, hand-polished and pre-oxidation heat treatment at 1100[degrees]C in P=10O2-8 atm up to 4 hours, carried out prior to the electron beam physical vapor deposition (EB-PVD) of ZrO2-7wt% Y2O3 (7YSZ) ceramic topcoat. The resulting characteristics of the bond coat and the thermally grown oxide (TGO) scale were initially documented by surface roughness, phase constituents of the TGO scale, and residual stress of the TGO scale. The thermal cycling test consisted of 10-minute heat-up to 1121°C, 40-minute hold at 1121°C, and 10-minute forced air-quench. As-coated and thermally-cycled TBCs were characterized by optical profilometry (OPM), photo-stimulated luminescence spectroscopy (PSLS), optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and scanning/transmission electron microscopy (TEM/STEM) equipped with high angle annular dark field (HAADF) and X-ray energy dispersive spectroscopy (XEDS). TBC specimens for TEM/STEM analysis were prepared by focused ion beam (FIB) in-situ lift-out (INLO) technique. Superior thermal cycling lifetime was observed for TBCs with as-sprayed bond coats regardless of pre-oxidation heat treatment, and TBCs with hand-polished bond coats only after pre-oxidation heat treatment. With pre-oxidation heat treatment, relative photostimulated luminescence intensity of the equilibrium α-Al2O3 increased. Thus, the improvement in TBC lifetime can be correlated with an increase in the amount of α-Al2O3 in the TGO scale, given a specific surface modification/roughness. The lifetime improvement due to pre-oxidation was particularly significant to TBCs with smooth hand-polished bond coats and negligible for TBCs with rough as-sprayed bond coats. Spallation-fracture paths depended on the lifetime of TBCs. Premature spallation of TBCs occurred at the interface between the YSZ and TGO. Longer durability can be achieved by restricting the fracture paths to the TGO/bond coat interface. Small particulate phase observed through the TGO scale was identified as Y2O3 (cubic) by diffraction analysis on TEM. While small addition of Y in the NiCoCrAlY bond coat helps the adhesion of the TGO scale, excessive alloying can lead to deleterious effects.

No description available.

The spallation of aluminum, chromium, and iron oxide scales is a chronic problem that critically impacts technological applications like aerospace, power plant operation, catalysis, petrochemical industry, and the fabrication of composite materials. The presence of interfacial impurities, mainly sulfur, has been reported to accelerate spallation, thereby promoting the high-temperature corrosion of metals and alloys. The precise mechanism for sulfur-induced destruction of oxides, however, is ambiguous. The objective of the present research is to elucidate the microscopic mechanism for the high-temperature corrosion of aluminum alloys in the presence of sulfur. Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM) studies were conducted under ultrahigh vacuum (UHV) conditions on oxidized sulfur-free and sulfur-modified Al/Fe and Ni3Al(111). Evaporative deposition of aluminum onto a sulfur-covered iron surface results in the insertion of aluminum between the sulfur adlayer and the substrate, producing an Fe-Al-S interface. Aluminum oxidation at 300 K is retarded in the presence of sulfur. Oxide destabilization, and the formation of metallic aluminum are observed at temperatures > 600 K when sulfur is located at the Al2O3-Fe interface, while the sulfur-free interface is stable up to 900 K. In contrast, the thermal stability (up to at least 1100 K) of the Al2O3 formed on an Ni3Al(111) surface is unaffected by sulfur. Sulfur remains at the oxide-Ni3Al(111) interface after oxidation at 300 K. During annealing, aluminum segregation to the g ¢ -Al2O3-Ni3Al(111) interface occurs, coincident with the removal of sulfur from the interfacial region. A comparison of the results observed for the Al2O3/Fe and Al2O3/Ni3Al systems indicates that the high-temperature stability of Al2O3 films on aluminum alloys is connected with the concentration of aluminum in the alloy.

Space vehicles, while entering the planetary atmosphere, experience high loads of heat. Ablative materials are commonly used for a thermal protection system, which undergo mass removal mechanisms to counter the heat rates. Spallation is one of the ablative processes, which is characterized by the ejection of solid particles from the material into the flow. Numerical codes that are used in designing the heat shields ignore this phenomenon. Hence, to evaluate the effectiveness of spallation phenomenon, a numerical model is developed to compute the dynamics and chemistry of the particles. The code is one-way coupled to a CFD code that models high enthalpy flow field around a lightweight ablative material. A parametric study is carried out to examine the variations in trajectories with respect to ejection parameters. Numerical results are presented for argon and air flow fields, and their effect on the particle behavior is studied. The spallation code is loosely coupled with the CFD code to evaluate the impact of a particle on the flow field, and a numerical study is conducted.

Les mécanismes de la réaction de spallation ne sont pas encore parfaitement compris, parce qu'il est difficile de séparer expérimentalement les effets respectifs des différentes étapes de la réaction. Afin de comprendre ces mécanismes, nous avons mis au point un dispositif expérimental permettant d'effectuer des mesures exclusives. Le système de détection des particules légères chargées est plus spécialement détaillé. <br /> Afin de valider notre dispositif expérimental, nous avons comparé nos résultats préliminaires sur le système 56Fe+p à 1 GeV/u avec des mesures inclusives obtenues auparavant sur le spectromètre FRS du GSI. La comparaison des sections efficaces différentielles en charge montre un accord raisonnable. Cependant, notre dispositif a permis d'étendre les mesures précédentes aux charges Z=1 et Z=2, importantes pour les études relatives aux dommages des matériaux. Compte tenu des barres d'erreurs que nous avons obtenues, l'évolution des vitesses moyennes longitudinales avec la masse du résidu de spallation est comparable à celle observée au FRS. Ces premiers résultats, même s'ils restent préliminaires, permettent de valider notre dispositif expérimental. Il est maintenant possible d'envisager d'exploiter les points forts des mesures exclusives réalisées, à savoir les corrélations entre les différents observables mesurées. Enfin, les problèmes expérimentaux soulevés lors de l'analyse effectuée pourront être pris en compte lors des expériences, afin notamment de mieux définir les programmes de mesure nécessaires aux étalonnages des détecteurs.

Le travail de thèse s'articule autour de deux études : étude de la réaction de spallation p+Au à 2.5 GeV et étude de la production neutronique en cibles épaisses. Ces deux études s'inscrivent dans un programme de recherche initié dans le cadre du projet de Source Européenne de Spallation (ESS) et du projet français de traitement des déchets radioactifs (GEDEON). A l'aide d'un dispositif expérimental très complet alliant un détecteur de neutrons à des détecteurs de particules chargées, tous deux d'efficacité supérieure à 80\%, nous avons pu étudier l'évolution du processus élémentaire de spallation en fonction de l'énergie déposée par le proton incident dans le noyau (énergie d'excitation). Nous avons ainsi montré le réalisme d'une description du processus en deux étapes et validé l'utilisation du code de cascade intra-nucléaire de J. Cugnon (INCL2.0) couplé à un code de désexcitation statistique (GEMINI). Les sections efficaces de production des particules chargées légères ont été mesurées et la contribution de l'émission de prééquilibre évaluée. La formation des particules composites ``directes'' a été ajoutée au code INCL2.0 à l'aide d'un modèle de coalescence. La production neutronique (multiplicités moyennes et distributions) a été étudiée en fonction de l'énergie du faisceau, de la géométrie de la cible et de la nature de la cible. Nous avons montré que le gain en neutrons, lorsque l'épaisseur de cible augmente, est dominé par la probabilité de réaction (pour des épaisseurs de cibles inférieures à 2 ou 3 longueurs d'interaction) et d'une façon moindre par le développement des réactions secondaires. Ces deux paramètres ont été étudiés séparément. Les données expérimentales ont été comparées aux résultats du code de transport HERMES, validant celui-ci pour l'émission de neutrons.

This document describes work performed related to the determination of how elemental additions affect the interfacial fracture toughness of thermal barrier coatings at the bond coat/thermally grown oxide interface in gas turbines. These turbines are exposed to cyclical thermal loading, therefore a simulation was designed to model this interface in a temperature cycle between 200 K and 1000 K that included oxide growth between 2 μm and 27 μm. The fracture toughness of this interface was then determined to elucidate the function of elemental additions. It was shown that minimal concentrations of atomic species, such as hafnium and yttrium cause notable increases in the toughness of the bond coat/thermally grown oxide interface, while other species, such as sulphur, can dramatically reduce the toughness. Furthermore, it was shown that, contrary to some empirical results, the addition of platinum has a negligible effect on the fracture toughness of this interface.

Les réacteurs hybrides, bases sur le couplage entre un accélérateur de particules et un coeur sous-critique via une cible<br />de spallation, présentent des possibilités de réduction de la radiotoxicité des déchets de haute activité et a vie<br />longue promis au stockage. Les différents concepts proposes ces dernières années dans la communauté scientifique montrent<br />la nécessite de réaliser un démonstrateur.<br />Ce travail de thèse a porte sur l'optimisation par simulation Monte Carlo a l'aide du code MCNPX, de la neutronique d'un tel<br />système dans le but de réaliser un réacteur pilote.<br />Tout d'abord, nous avons indique les principales caractéristiques neutroniques d'un réacteur hybride avant de présenter le<br />concept de démonstrateur refroidi au gaz base sur le remontage effectue par la société Framatome ANP. Nous avons<br />caractérise puis optimise la neutronique a travers la géométrie et les matériaux utilises pour ce démonstrateur.<br />Dans le cadre de l'incinération des actinides mineurs, nous avons calcule l'évolution des combustibles envisageables suivant<br />les phases de démonstration prévues. Les grandeurs liées a l'incinération des actinides mineurs sont rapportées. En vue<br />de la transmutation du 99Tc et de l'129I, nous avons calcule les temps caractéristiques et les taux de transmutation<br />a l'équilibre.<br />D'autre part, nous avons analyse le passage du démonstrateur vers un réacteur incinérateur de puissance a partir de<br />critères physiques tels que les facteurs de forme et les niveaux de flux. A partir de cette analyse, des solutions innovantes sont<br />proposées pour améliorer les facteurs de forme d'un incinérateur de puissance.<br />Enfin, dans des perspectives a plus long terme, l'utilisation des réacteurs hybrides dans le cadre de la génération<br />d'233U pour accélérer le démarrage d'une filière de réacteurs a sels fondus basée sur le cycle<br />232Th/233U a été explorée et s'avère particulièrement efficace.

This document describes work performed related to the determination of how elemental additions affect the interfacial fracture toughness of thermal barrier coatings at the bond coat/thermally grown oxide interface in gas turbines. These turbines are exposed to cyclical thermal loading, therefore a simulation was designed to model this interface in a temperature cycle between 200 K and 1000 K that included oxide growth between 2 μm and 27 μm. The fracture toughness of this interface was then determined to elucidate the function of elemental additions. It was shown that minimal concentrations of atomic species, such as hafnium and yttrium cause notable increases in the toughness of the bond coat/thermally grown oxide interface, while other species, such as sulphur, can dramatically reduce the toughness. Furthermore, it was shown that, contrary to some empirical results, the addition of platinum has a negligible effect on the fracture toughness of this interface.