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The aerospace industry strives to develop new methods of refining gas turbine engines by increasing power and thermal efficiencies while simultaneously reducing cost. Turbine engines operate under high temperatures and therefore thermal barrier coatings (TBCs) composed of yttria-stabilized zirconia (YSZ) play an important role in improving the performance of the components that make up the engine. Failure of the TBC could lead to catastrophic events, thus requiring consistent and accurate characterization for supplier qualification and production quality assurance. However, due to porosity and the anisotropic behavior of the coating and variability in processing of TBCs, consistent characterization has proven to be extremely challenging. One of the reoccurring issues is the inconsistency in measuring percent porosity, which stems from the difficulty in distinguishing filled pores from damaged, unfilled voids. Sample preparation of TBCs involves sectioning, mounting, grinding, polishing, and characterization. Eliminating variability in characterization begins with mounting which is a critical step to protect the surface integrity and edge retention of the coating during grinding and polishing. The curing kinetics of a slow cure two part epoxy was investigated and the TBC samples were mounted and cured at heating rates of 2, 5, and 10°C/min to 55°C and 70°C. Grinding and polishing procedures simulated industry practices followed by characterization with optical microscopy. Results showed that heating rates of 2°C/min to 55°C and 70°C have the best impregnation properties while uncontrolled or high heating rates of 10°C/min had an increase in the amount of pullouts and lack of infiltration from the epoxy. The curing kinetics of the epoxy needs to be controlled to eliminate the ambiguity of filled and unfilled pores. / Master of Science

The research work performed in this thesis has been carried out at the Production Tech-nology Centre where the Thermal Spray research group of University West has its work-shop and labs. This research work has been performed in collaboration with the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, India. First of all, I would like to express my sincere thanks and gratitude to my supervisors Dr. Nicolaie Markocsan and Dr. Nicholas Curry for their guidance, great support and valuable suggestions without which this work could not have been possible. I would also like to thanks Prof. Per Nylén for keeping faith in me and providing me an opportunity to work at PTC, which is a great place to perform research. It is my pleasure being their student and I wish I would keep learning from all of them, both on academic and personal grounds. I would also like to thank my colleagues at PTC Mr. Mohit Gupta and Mr. Stefan Björklund, for their help and support during this work. I would like to acknowledge the H.C. Starck Company for its financial support for the pro-ject; Dr. Filofteia-Laura TOMA at Fraunhofer IWS, Dresden to help us in spraying suspen-sion sprayed YSZ top coats, G Shivkumar from ARCI to help us in spraying solution pre-cursor sprayed top coats and Toni Bogdanoff, Jönköping University to help us in conduct-ing the LFA experiment

Atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) are commonly used for thermal protection of components in modern gas turbine application such as power generation, marine and aero engines. TBC is a duplex material system consisting of an insulating ceramic topcoat layer and an intermetallic bondcoat layer. TBC microstructures are highly heterogeneous, consisting of defects such as pores and cracks of different sizes which determine the coating's final thermal and mechanical properties, and the service lives of the coatings. Failure in APS TBCs is mainly associated with the thermo-mechanical stresses developing due to the thermally grown oxide (TGO) layer growth at the topcoat-bondcoat interface and thermal expansion mismatch during thermal cycling. The interface roughness has been shown to play a major role in the development of these induced stresses and lifetime of TBCs.The objective of this thesis work was two-fold for one purpose: to design an optimised TBC to be used for next generation gas turbines. The first objective was to investigate the relationships between coating microstructure and thermal-mechanical properties of topcoats, and to utilise these relationships to design an optimised morphology of the topcoat microstructure. The second objective was to investigate the relationships between topcoat-bondcoat interface roughness, TGO growth and lifetime of TBCs, and to utilise these relationships to design an optimal interface. Simulation technique was used to achieve these objectives. Important microstructural parameters influencing the performance of topcoats were identified and coatings with the feasible identified microstructural parameters were designed, modelled and experimentally verified. It was shown that large globular pores with connected cracks inherited within the topcoat microstructure significantly enhanced TBC performance. Real topcoat-bondcoat interface topographies were used to calculate the induced stresses and a diffusion based TGO growth model was developed to assess the lifetime. The modelling results were compared with existing theories published in previous works and experiments. It was shown that the modelling approach developed in this work could be used as a powerful tool to design new coatings and interfaces as well as to achieve high performance optimised morphologies.

L’ingestion de sables et de cendres volcaniques par les moteurs d’avion conduit à la formation de dépôts silicatés (CMAS) qui s’infiltrent dans la porosité du revêtement barrière thermique (BT) en zircone yttriée des aubes de turbine. De nouvelles compositions de BT issues du système ZrO2-RE2O3 (RE = La-Lu) sont donc envisagées. En effet, leur réactivité chimique au contact des CMAS peut conduire à la formation de phases cristallisées, notamment la phase apatite Ca2RE8(SiO4)6O2, qui bloquent l’infiltration du CMAS. Cependant, divers silicates du système CaO-RE2O3-SiO2 sont susceptibles d’entrer en compétition avec sa formation et de plus, la composition du CMAS varie selon les régions survolées. L’objectif de ces travaux de thèse est de déterminer l’influence de la composition du CMAS et de la terre rare sur les mécanismes réactionnels de dissolution et de précipitation. Divers oxydes RE2O3 à basicité croissante (RE = Yb, Dy, Gd, Sm et Nd) et un CMAS de composition simplifiée du système CaO-Al2O3-SiO2 (CAS) ont été choisis. Des teneurs fixes en MgO et Fe2O3 ont été ensuite ajoutées au CAS en faisant varier le rapport CaO/SiO2 entre 0,4 et 1,6. Les phases apatite et cyclosilicate Ca3RE2(Si3O9)2 ont également été synthétisées afin d’étudier leur dissolution. Il est montré que le mécanisme de dissolution des RE2O3 est indirect, les équilibres locaux établis avec cet oxyde imposant systématiquement la formation de la phase apatite. Sa cristallisation est favorisée par un rayon cationique RE3+ proche de celui de Ca2+. Lorsque l’écart est important, la nucléation de la phase cyclosilicate est rapidement observée dans le CAS avec une répartition préférentielle de ces cations RE3+ dans les sites de coordinence 6. La solubilité en RE dans le liquide silicaté augmente avec la basicité de l’oxyde RE2O3 et en présence de MgO et Fe2O3. La variation de composition du CMAS modifie la nature des phases à l’équilibre. Leurs limites de solubilité en RE sont inférieures à celles de la phase apatite, ce qui réduit d’autant leur vitesse de redissolution dans le liquide silicaté / Fine particles of sand, dust or volcanic ashes ingested by aircraft engines are well-known to damage Thermal Barrier Coatings (TBC) when they infiltrate their porous microstructure as molten silicate (CMAS). They are mainly constituted of CaO-MgO-Al2O3-SiO2 in variable proportions and also contain metallic oxides. RE2Zr2O7 compositions are TBC candidate materials as they have shown efficiency to mitigate CMAS infiltration due to their reactivity with synthetic CMAS. Indeed, the dissolution reaction leads to rapid sealing of the topcoat porosity mainly due to the formation of crystalline Ca2Gd8(SiO4)6O2 apatite. However, many rare-earth silicates are likely to compete with apatite crystallization and little is known on reaction kinetics and thermodynamics involving RE2O3 and multi-component CMAS system. This work aims to determine the influence of CMAS and rare earth composition on dissolution and precipitation mechanisms. A simplified CAS was first selected with eutectic (1170°C), 65SiO2-26CaO-9Al2O3 (mol. %) composition. Dissolution of various RE2O3 with increasing basicity (RE = Yb, Dy, Gd, Sm and Nd) as well as synthetic apatite and cyclosilicate Ca3RE2(Si3O9)2 phases was then performed at 1200°C in CAS-melt. Finally, fixed MgO and Fe2O3 contents were added to CAS melt with an increasing CaO/SiO2 ratio. The results showed that RE2O3 dissolution mechanism is indirect. Apatite formation results from local equilibrium at the interface with solid RE2O3 whatever the rare earth and CMAS composition. Its crystallization is favored when Ca2+ and RE3+ ionic radii are close as they are both distributed within 9-fold coordination sites. Conversely, Ca and RE mismatch leads to rapid nucleation of cyclosilicate phase in CAS as they are preferentially distributed within a 6-fold coordination site. MgO and Fe2O3 addition in CAS as well as RE2O3 basicity tend to increase RE solubility in silicate melt. Phases in thermodynamic equilibrium strongly depend on CMAS composition but generally exhibits lower RE solubility and dissolution rate in melt than apatite

Dans le secteur aéronautique en pleine expansion, les préoccupations environnementales prennent une place de plus en plus importante. Les motoristes recherchent des solutions innovantes pour augmenter les rendements tout en diminuant les coûts. Dans cette perspective, de nouveaux systèmes de barrières thermiques synthétisés par la voie sol-gel à partir de poudres commerciales, de céramiques avec différents facteurs de forme et d'agents porogènes ont été mis en œuvre et évalués. Certains systèmes présentent une durée de vie de plus de 1000 cycles en oxydation cyclique. Malgré tout, cet accroissement des températures de fonctionnement des moteurs, induit une élévation des températures de surfaces des barrières thermiques et peut générer de nouvelles dégradations du système complet : la corrosion à hautes températures par les CMAS. Pour pallier ces inconvénients, il est possible de développer des revêtements anti-CMAS, susceptibles de réagir avec les composés CMAS avant qu'ils n'aient un effet néfaste sur l'intégrité de la barrière thermique. Dans cette étude, nous nous sommes intéressés particulièrement aux revêtements sacrificiels anti-CMAS à base d'yttrine et de systèmes pyrochlore, qui ont été testés sur des barrières thermiques industrielles de type EBPVD. Par ailleurs, les procédés que nous avons développés, basés sur la voie sol-gel, nous permettent, de par leur facilité de mise en œuvre, d'envisager des perspectives prometteuses en termes de réparabilité des barrières thermiques endommagées. En effet, compte tenu du coût élevé de fabrication des pièces, les aubes devraient être réparées plusieurs fois avant d'être mises au rebut. Dans ce travail, un procédé de mise en forme a été évalué dans ce sens. Il s'agit de l'électrophorèse qui est une technique bien adaptée au dépôt sur pièces complexes. L'objectif de ces investigations a donc été double : tout d'abord créer de nouveaux systèmes de barrières thermiques avec des propriétés anti-CMAS par électrophorèse puis réparer les barrières thermiques EBPVD endommagées et leur déposer une couche protectrice anti-CMAS par ce même procédé. Cet aspect " procédé " sera abordé en dernière partie de ces travaux. / In the aeronautics sector, environmental concerns are becoming increasingly important. Engine manufacturers are looking for innovative solutions to increase efficiency while lowering costs. The objective is to optimize thermal conductivity and durability with the cyclic oxidation resistance. In this perspective, new thermal barrier systems synthesized by the sol-gel route from commercial powders, ceramics with various form factors and pore-forming agents have been implemented and evaluated. Some systems are a lifetime higher than 1000 cycles in cyclic oxidation. However, this increase in the operating temperatures of the engines induces an increase in the temperature of the surfaces of the thermal barriers and can generate further degradations of the complete system: the corrosion by CMAS. To overcome these disadvantages, it is possible to develop anti-CMAS coatings capable of reacting with CMAS compounds before they have a detrimental effect on the integrity of the thermal barrier. In this study, we were particularly interested in anti-CMAS protective coatings based on yttria and pyrochlore systems, which were tested on industrial thermal barriers realized by EBPVD. Moreover, the processes we have developed, based on the sol-gel path, allow us, because of their ease of implementation, to envisage promising prospects in terms of repair of damaged thermal barriers. Indeed, given the high cost of manufacturing parts, the blades should be repaired several times before being discarded. In this work, a shaping process has been evaluated in this direction. This is electrophoretic deposition which is a technique allowing to deposit on complex parts. The objective of these investigations was therefore twofold: firstly to create new thermal barrier systems with anti-CMAS properties by electrophoretic deposition and then to repair the damaged EBPVD thermal barriers and to deposit an anti-CMAS protective layer by this same process. This "process" aspect will be discussed at the end of this work.

Thermal barrier coatings (TBC) are used in the hot sections of gas turbine engine in order to insulate the substrate at high temperature. Molten salt infiltration retards the durability of TBCs. The current standard material, i.e. 8YSZ is susceptible to molten salt infiltration. Therefore, alternate TBC materials are desirable. In addition to material composition, the TBC microstructure plays an important role in mitigating molten salt infiltration. Therefore, in this work, three different TBC variations were investigated. The first variation was a columnar microstructured 48YSZ TBC processed by SPS (48YSZ-SPS). The second variation was a columnar microstructured 8YSZ TBC processed by SPS (8YSZ-SPS), and the third variation was a lamellar microstructured 8YSZ TBC deposited by APS (8YSZ-APS). The as-sprayed TBC specimens were characterized by SEM/EDS, porosity analysis and XRD measurements. Later, the TBC specimens were exposed to hot corrosion test and their interaction with the molten salts were investigated using SEM (EDS and XRD). It was shown that an increase in stabilizer content (yttria content) in zirconia (in the case of 48YSZ) leads to an improved hot corrosion resistance due to the adequate amount of yttria content, which restricts the molten salt infiltration by forming needle like YVO4 phase. In terms of microstructure comparison, the infiltration behavior was similar for columnar microstructured 8YSZ and lamellar microstructured 8YSZ-APS as the molten salts infiltrated the coatings completely compared to the 48YSZ TBC. Furthermore, it seems that the molten salt infiltrates the TBC through globular pores, delamination cracks and splat boundaries in the case of APS-TBCs whereas the column gaps favor easier infiltration of molten salts in the case of columnar microstructured SPS processed TBCs.

<p>Thermal barrier coatings (TBC) are used for reduction of component temperatures in gas turbines. The service temperature for turbines can be as high as 1100ºC and the components are exposed to thermal cycling and gases that will cause the component to oxidize and corrode. The coatings are designed to protect the substrate material from this, but eventually it will lead to failure of the TBC. It is important to have knowledge about when this failure is expected, since it is detrimental for the gas turbine.</p><p>The scope of this thesis has been to see if an existing life model for thin TBC also is valid for thick TBC. In order to do so, a thermal cycling fatigue test, a tensile test and finite element calculation have been performed. The thermal cycling fatigue test and finite element calculation were done to find correlations between the damage due to thermal cycling, the number of thermal cycles and the energy release rate. The tensile test was preformed to find the amount accumulated strain until damage.</p><p>The thermal cycling lead to failure of the TBC at the bond coat/top coat interface. The measurment of damage, porosity and thickness of thermally grown oxide were unsatisfying due to problems with the specimen preparation. However, a tendency for the damage development were seen. The finite element calculations gave values for the energy release rate the stress intensity factors in mode~I and mode~II that can be used in the life model. The tensile test showed that the failure mechanism is dependent of the coating thickness and it gave a rough value of the maximum strain acceptable.</p>

From many years YSZ is used as the top coat material for TBC's, as it has good phase stability up to 1200°C, higher fracture toughness, lower thermal conductivity, erosion resistance &amp; higher coefficient of thermal expansion. But, it has a drawbacks at high temperature such as sintering and transformation of phases. For this reason new ceramic materials with pyrochlores crystal structure such as Gd2Zr2O7 are being considered as it has high melting points, phase stability, lower thermal conductivity and CMAS resistance. But it has low fracture toughness when compared to YSZ. In order to take advantage of low thermal conductivity and high thermal stability of gadolinium zirconate and avoiding the drawbacks of low coefficient of thermal expansion and low toughness using YSZ, a double/multi-layer coatings approach is being used. Therefore, multi-layer TBCs are sprayed and compared with single layer coating in this work. These coatings are processed by suspension plasma spraying. For single layer coating YSZ is used, for double layer coating YSZ as the intermediate coating and Gd2Zr2O7 as the top coat is used. Additionally, a triple layer coating system comprising YSZ, Gd2Zr2O7 and dense Gd2Zr2O7 as top coat is also sprayed. The as sprayed coatings are characterized for microstructure analysis using optical microscope and scanning electron microscope (SEM), elemental analysis of TGO using Energy-Dispersive Spectrometer (EDS). XRD analysis was done to identify various phases in the coating. Porosity analysis using Archimedes principle was carried out. Thermal cyclic fatigue (TCF) test of the sprayed coatings was carried out at 1100°C. Failure analysis of the TCF specimens was carried out using SEM/EDS. TCF results showed that the triple layer coatings (dense Gd2Zr2O7/Gd2Zr2O7/YSZ) had higher thermal cyclic fatigue life and lower TGO thickness when compared to single layer (YSZ) and double layer (Gd2Zr2O7/YSZ) TBCs.

Thermal barrier coatings (TBC) are used for reduction of component temperatures in gas turbines. The service temperature for turbines can be as high as 1100ºC and the components are exposed to thermal cycling and gases that will cause the component to oxidize and corrode. The coatings are designed to protect the substrate material from this, but eventually it will lead to failure of the TBC. It is important to have knowledge about when this failure is expected, since it is detrimental for the gas turbine. The scope of this thesis has been to see if an existing life model for thin TBC also is valid for thick TBC. In order to do so, a thermal cycling fatigue test, a tensile test and finite element calculation have been performed. The thermal cycling fatigue test and finite element calculation were done to find correlations between the damage due to thermal cycling, the number of thermal cycles and the energy release rate. The tensile test was preformed to find the amount accumulated strain until damage. The thermal cycling lead to failure of the TBC at the bond coat/top coat interface. The measurment of damage, porosity and thickness of thermally grown oxide were unsatisfying due to problems with the specimen preparation. However, a tendency for the damage development were seen. The finite element calculations gave values for the energy release rate the stress intensity factors in mode~I and mode~II that can be used in the life model. The tensile test showed that the failure mechanism is dependent of the coating thickness and it gave a rough value of the maximum strain acceptable.