Solid oxide fuel cell compositionally graded electrolytes could offer the advantage of improving electrical performance and efficiency compared to single-layered or bi-layered yttria stabilized zirconia and samaria doped ceria electrolytes and improving mechanical performance by reducing thermal expansion mismatch stresses compared to bi-layered electrolytes with sharp interfaces. Manufacturing of these graded structures is difficult if implementing conventional wet ceramic techniques. Suspension plasma spraying is an emerging technology that has the potential to rapidly produce thin, dense ceramic layers with no requirement for post deposition heat treatments. However, SPS requires a careful examination of the stability of the feedstock suspensions in order to produce high quality coatings. Optimum suspension formulations with excellent particle dispersion were designed based on rheological and electrostatic stability measurements. These optimized suspensions were used as feedstocks for the fabrication of suspension plasma sprayed compositionally graded YSZ/SDC layers. The feasibility of fabricating graded electrolyte structures was thus demonstrated.

Solid oxide fuel cell compositionally graded electrolytes could offer the advantage of improving electrical performance and efficiency compared to single-layered or bi-layered yttria stabilized zirconia and samaria doped ceria electrolytes and improving mechanical performance by reducing thermal expansion mismatch stresses compared to bi-layered electrolytes with sharp interfaces. Manufacturing of these graded structures is difficult if implementing conventional wet ceramic techniques. Suspension plasma spraying is an emerging technology that has the potential to rapidly produce thin, dense ceramic layers with no requirement for post deposition heat treatments. However, SPS requires a careful examination of the stability of the feedstock suspensions in order to produce high quality coatings. Optimum suspension formulations with excellent particle dispersion were designed based on rheological and electrostatic stability measurements. These optimized suspensions were used as feedstocks for the fabrication of suspension plasma sprayed compositionally graded YSZ/SDC layers. The feasibility of fabricating graded electrolyte structures was thus demonstrated.

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

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 & 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.

The upward trend in internal combustion engine efficiency is likely driven by the depletion of fossil fuels. Since no replacement in sight can deliver energy comparable to the conventional oil, there is a need to use it more rationally and effectively. Thermal barrier coatings have been seen for a long time as a solutionto increase the thermal efficiency of gas turbine engines but suffer from the lackof strong applicability in internal combustion engines. This is due to the different restrictions when comparing the environment on the gas turbines and in internal combustion engines. To overcome this problem and, at the same time, expand the application field of thermal barrier coatings, more efforts need to be devoted.In this work, different top coat materials using various deposition techniques were evaluated and categorized in three different thermal barrier coating (TBC) architectures. The first was the lamellar yttria-stabilized zirconia (YSZ) top coat deposited by atmospheric plasma spray (APS), used as a reference sample. The second architecture was a columnar suspension plasma spray (SPS) TBC with YSZ and gadolinium zirconate (GZO) top coat. The SPS process can produce avariety of microstructures, and they were, for the first time, tested in an internal combustion engine. The third architecture was an SPS top coat, with an additional layer on the top, called a sealing layer of either metallic or ceramic material, both never investigated in a diesel engine application earlier. For the thermophysical properties investigation, a combination of laser flashanalysis (LFA) and modeling with object-oriented finite element (OOF) was employed to understand the properties in all the applications. The performance of the coatings was evaluated in two different ways, by thermal cyclic tests, basedon the TBCs behavior under cyclic thermal loads and by single-cylinder engine experiment. The characterization of the coatings was done by scanning electron microscope (SEM) before and after the thermal cyclic tests.The performance properties were correlated with coatings microstructure and thermophysical properties. It was shown that a columnar TBC produced by SPS had a superior engine efficiency in the single cylinder engine experiment.

Thermal barrier coatings (TBC’s) are used to provide both thermal insulation and oxidation protection to high temperature components within gas turbines. The development of turbines for power generation and aviation has led to designs where the operation conditions exceed the upper limits of most conventional engineering materials. As a result there has been a drive to improve thermal barrier coatings to allow the turbine to operate at higher temperatures for longer. The focus of this thesis has been to design thermal barrier coatings with lower conductivity and longer lifetime than those coatings used in industry today. The work has been divided between the development of new generation air plasma spray (APS) TBC coatings for industrial gas turbines and the development of suspension plasma spray (SPS) TBC systems. The route taken to achieve these goals with APS TBC’s has been twofold. Firstly an alternative stabiliser has been chosen for the zirconium oxide system in the form of dysprosia. Secondly, control of the powder morphology and spray parameters has been used to generate coating microstructures with favourable levels of porosity. In terms of development of SPS TBC systems, these coatings are relatively new with many of the critical coating parameters not yet known. The focus of the work has therefore been to characterise their lifetime and thermal properties when produced in a complete TBC system. Results demonstrate that dysprosia as an alternative stabiliser gives a reduction in thermal conductivity. While small at room temperature and in the as produced state; the influence becomes more pronounced at high temperatures and with longer thermal exposure time. The trade-off for this lowered thermal conductivity may be in the loss of high temperature stability. Overall, the greatest sustained influence on thermal conductivity has been from creating coatings with high levelsof porosity. In relation to lifetime, double the thermo-cyclic fatigue (TCF) life relative to the industrial standard was achieved using a coating with engineered porosity. Introducing a polymer to the spray powder helps to generate large globular pores within the coating together with a large number of delaminations. Such a structure was shown to be highly resistant to TCF testing. SPS TBC’s were shown to have much greater performance relative to their APS counterparts in thermal shock life, TCF life and thermal conductivity. Columnar SPS coatings are a prospective alternative for strain tolerant coatings in gas turbine engines.

L’augmentation des températures de fonctionnement des turboréacteurs est un axe de développement privilégié dans l’industrie aéronautique. Une solution est l’amélioration des systèmes barrières thermiques. Ce travail de thèse s’intéresse au procédé de projection plasma de suspensions (SPS) qui permet d’envisager une amélioration significative des performances pour les prochaines générations de barrières thermiques, comparé au procédé d’évaporation sous faisceau d’électrons (EB-PVD). Le procédé SPS a en effet démontré une capacité à générer des microstructures colonnaires qui présentent un intérêt pour l’accommodation des contraintes thermo-mécaniques. Une étude microstructurale a conduit à l’identification des paramètres influant sur les variations de morphologies des revêtements (taille de colonnes, distribution de taille, compacité). Deux nuances optimisées en zircone yttriée (YSZ), nommées colonnaire et colonnaire compacte, ont été caractérisées de façon approfondie afin de déterminer les bénéfices du procédé SPS. Ces nuances se caractérisent par une conductivité thermique inférieure à 1 W.m-1.K-1, sur une plage de température allant de 25 à 1100 °C, soit des valeurs avantageuses par rapport à celles des revêtements EB-PVD (1,3 – 1,5 W.m-1.K-1). La durée de vie des dépôts SPS, estimée par cyclage thermique, est au moins équivalente à un dépôt YSZ réalisé par EB-PVD et cyclé en même temps. Le résultat le plus élevé obtenu, supérieur à 2000 cycles, est particulièrement prometteur. La capacité de fonctionnalisation du procédé SPS a par ailleurs permis la réalisation de systèmes multifonctionnels comprenant un dépôt colonnaire YSZ et un dépôt homogène Gd2Zr2O7 en surface. Cette architecture bicouche a pour objectif de pallier les infiltrations chimiques de type CMAS (CaO–MgO–Al2O3–SiO2) qui constituent un frein pour l’augmentation de la température de fonctionnement. Le caractère anti-CMAS du matériau Gd2Zr2O7 mis en forme par SPS a été évalué jusqu’à 1300 °C. / The increase of operating temperature of gas turbine engines is an issue of interest for the aeronautic industry. A solution is the enhancement of thermal insulation properties of thermal barrier coatings (TBCs). The present work is related to suspension plasma spraying process (SPS) that allows to consider significant improvements for the next generation of TBC systems, compared to the currently used process, namely electron beam physical vapor deposition (EB-PVD). Indeed, SPS process can produce columnar microstructures able to provide high thermo-mechanical compliance. A microstructural study led to identify parameters which impacted the coating morphology (column size, distribution, and compaction). Two optimized yttria-stabilized zirconia (YSZ) microstructures were carefully characterized to highlight SPS process advantages. Low thermal conductivities (< 1 W.m-1.K-1) were obtained within a large temperature range (25 °C – 1100 °C), compared to EB-PVD YSZ coatings (1,3 – 1,5 W.m-1.K-1). Thermal lifetime was estimated thanks to thermal cyclic fatigue tests. A similar level of thermal lifetime was reached with SPS coatings compared to EB-PVD one. Some SPS columnar coatings even showed more than 2000 cycles to failure. The ability of SPS to perform multifunctional systems, including a YSZ columnar structure with a homogeneous Gd2Zr2O7 coating on the top, was investigated. This architecture must provide a chemical protection to CMAS (CaO–MgO–Al2O3–SiO2) aggressions. These contaminants would impede the increase of temperature in next generation of gas turbine engines. The anti-CMAS behavior was assessed for SPS Gd2Zr2O7 coatings until 1300 °C.

La projection par plasma d'arc de suspension permet d'obtenir des revêtements finement structurés à gradients de propriétés qui répondent aux besoins, par exemple, des applications photocatalytiques, les piles à combustible à oxyde solide ou les revêtements de barrière thermique. Cependant, les torches à plasma, même alimentées par dessources de courant continu régulé, génèrent des jets de plasma fortement fluctuants. Ces instabilités causent des variations importantes dans les transferts thermiques et dynamiques des particules, ce qui diminue la fiabilité et la reproductibilité de la méthode. Par conséquent, des efforts particuliers doivent être faits pour améliorer la projectionpar plasma d'arc de suspension et, ainsi, les propriétés des revêtements. Depuis de nombreuses années, la recherche s'est concentrée sur l'amélioration des transferts de chaleur et de quantité de mouvement entre la matière et le plasma au moyen de la mise au point de nouvelles torches et la réduction des instabilités de l'arc. Cette thèse présenteune nouvelle approche pour la projection par plasma d'arc de suspension. L'étude approfondie des instabilités du plasma sont réalisées ce qui conduit à la production du jet laminaire de plasma pulsé caractérisé par une forte modulation de l'enthalpie spécifique. Ces oscillations régulières de plasma sont associées à l'injection de la suspensionsynchronisée, ce qui est réalisé à l'aide de l'impression à jet d'encre déclenchée par le signal de tension d'arc. Les résultats sont évalués par le système d'imagerie résolue en temps et la spectroscopie d'émission optique résolue en temps. Cette nouvelle méthode offre la possibilité de contrôler les transferts de chaleur et de quantité de mouvemententre les particules et le plasma. / Suspension plasma spraying permits to elaborate finely structured coatings with graded properties which address the needs, for example, in the photocatalytic applications, the solid oxide fuels or the thermal barrier coatings. However, the plasma torches, even powered by dc regulated sources, generate highly fluctuating plasma jets. These instabilities result in large variations in dynamic and heat transfers to particles, what decreases the reproducibility and reliability of the method. Consequently, the special efforts have to be devoted to ameliorate the suspension plasma spraying method and, thus, the properties of the coatings. In recent years, the research has been focused on the improvement of heat and momentum transfers between material and plasma by means of the development of new non-conventional torches and the reduction of arc instabilities. The following dissertation presents a new approach to the suspension plasma spraying. The profound studies of the plasma instabilities are performed, what leads to the production of the pulsed laminar plasma jet characterized by high modulation of the specific enthalpy. These regular plasma oscillations are combined with phased injection of suspension, what is achieved by using the ink-jet printer triggered by the arc voltage signal. The results are evaluated by time-resolved imaging system and the time-resolved emission optical spectroscopy. This new method presents the possibility to control heat and momentum transfers between the particles and the plasma.

La technologie de projection plasma sous basse pression a attiré l’attention de nombreux chercheurs comme une nouvelle technique qui permet d’établir un pont entre la projection thermique conventionnelle et le dépôt physique en phase vapeur. Ainsi, cette technologie étend les limites de projection thermique classique et augmente également la vitesse de croissance des dépôts par rapport aux procédés PVD ou CVD classiques. Cette technique peut évaporer totalement ou partiellement les poudres injectées et mener à la réalisation de revêtements à microstructure colonnaire et/ou plus denses, difficiles à réaliser avec des procédés de projection thermique conventionnels. La projection plasma de suspension a été effectuée pour la première fois sous basse pression. L’injection axiale de suspension avec une torche tri-cathodes permet d’augmenter l’échange enthalpique entre le jet de plasma et les poudres après l’évaporation du solvant. La spectroscopie à l’émission optique (OES) a été utilisée pour estimer la température électronique et vérifier l’existence de phase vapeur d’YSZ dans le jet de plasma. Finalement, des revêtements plus denses furent réalisés (comparés à ceux préparés par projection plasma de suspension à pression atmosphérique présentant des particules fondues, agglomérées et de la condensation de vapeur. Des tests de nano-indentation instrumentée ont été effectués sur la surface polie des dépôts réalisés. Les résultats montrent des valeurs de 5,8 GPa pour la dureté et 114,5 GPa pour le module d’élasticité, augmentant de 61% et 31%, respectivement, en comparaison avec les valeurs obtenues par SPS sous atmosphère ambiante. Les essais de projection de poudre YSZ agglomérée ont été réalisés avec une torche F4-VB dans le but de synthétiser une phase vapeur d’YSZ. On observe que les dépôts peuvent se former derrière les échantillons en céramique, sans vis-à-vis du plasma, par condensation de vapeur. En face de cette torche, des revêtements composites ont été obtenus par un mélange de poudres fondues et condensation de vapeur, simultanément. Cependant la quantité de phase vapeur est très faible dans le jet de plasma. Pour comprimer ce jet sous basse pression et afin d’améliorer l’échange d’enthalpie entre le jet de plasma et les poudres injectées, une buse rallongée a été mise en place sur la torche F4-VB. Les revêtements présentent ainsi une microstructure plus dense. Ceci est attribué à la haute vitesse des particules fondues vers le substrat suite à l’utilisation de la buse modifiée. Ce type de revêtement montre une valeur maximale de microdureté Vickers de 1273 Hv100 g. Par ailleurs, la réalisation de dépôts de carbures a été effectuée. Les résultats montrent la possibilité de former des carbures par projection plasma sous basse pression. Les revêtements composites (TiC/Ti) sont déposés par projection plasma réactif sous basse pression en utilisant le méthane comme gaz porteur. La température électronique Te calculée est d’environ 6200 K selon les résultats d’OES, ce qui est supérieur à la température d’ébullition du Ti et de TiC. Le revêtement de Ti pur présente une microstructure dense alors que TiC/Ti présente une microstructure lamellaire. Cependant, la quantité de TiC dans les revêtements est d’environ 20 vol.%. La microdureté Vickers, effectuée sur surface polie, a tendance à diminuer de 846±152 à 773±86 Hv100 g avec l’augmentation de la distance de projection. / As a new technology, the very low pressure plasma spraying has attracted attentions of many researchers, making it possible to establish a bridge between the conventional atmospheric plasma spraying (APS) and the vapor deposition (PVD or CVD). As a result, this new technology enlarges the limitation of APS and increases the deposition rate in comparison with the PVD or CVD. It is possible to evaporate partially the injected material and even evaporate completely and finally realize the columnar or dense coatings from the vapor or the mixture of vapor and liquid. The suspension plasma spraying is performed for the first time at low pressure. Taking consideration of the configuration of the three-cathode torch, the axial injection of the suspension is conducted which can increase the enthalpy change between the plasma jet and the sprayed material. The data of optical emission spectroscopy (OES) could be used to calculate the electron temperature and verify the existence of vapor of YSZ in the plasma jet. Finally, the dense coating was prepared by suspension plasma spraying at low pressure, which is composed of the melted particles, the agglomerated particles and the vapor deposition. The test of nano-indentation is conducted on the polished surface. It shows a value of 5.8 GPa for the microhardness and 114.5 GPa for the elastic modulus, increasing 61% and 31%, respectively, compared with the values obtained by SPS in the ambient atmosphere. In this study, another torch F4-VB is also conducted even of it has a low power in compared with that of O3CP torch. The powder feed rate is reduced to about 1.5 g·min-1 to achieve the vapor of YSZ taking the low power input of the torch into consideration. The columnar structure coating is realized from vapor deposition out of line of sight of projection upon the ceramic tubes. The composite structure coating is deposited by the mixture of melted particles and the vapor deposition simultaneously in front of this torch. But the quantity of vapor of YSZ is low in plasma jet. In order to compress the larger plasma jet and then improve the enthalpy change between the plasma jet and sprayed particles, an extended nozzle is prepared. It shows that the coating has a dense structure, which can be attributed to higher velocity of the melted powders. The coating shows a maximum value of microhardness Vickers up to 1273 Hv100 g. The composite coating of TiC/ Ti is realized by reactive plasma spraying using the methane as the carrier gas. The electron temperature Te is calculated to be 6200 K, which is over the boiling point of TiC and Ti. The coating Ti shows a dense structure and the composite coating TiC/Ti shows a lamellar structure. But the quantity of TiC in the composite coating is very low, about 20 vol.%. The Vickers microhardness is performed on the polished surface. It shows a decreasing tendency from 846 ± 152 to 773 ± 86 Hv100g with the increase of spraying distance. The tribological test is also implemented showing a high value of the coefficient of friction of 0.78 to 0.85, which can lead to a high abrasion. In order to synthesize a larger quantity of TiC in the composite coating, a higher power input torch should be put into action in the future.

Thermal barrier coating (TBC) systems are widely used on gas turbine components to provide thermal insulation and oxidation protection. TBCs, incombination with advanced cooling, can enable the gas turbine to operate at significantly higher temperatures even above the melting temperature of the metallic materials. There is a permanent need mainly of environmental reasons to increase the combustion turbine temperature, hence new TBC solutions are needed.By using a liquid feedstock in thermal spraying, new types of TBCs can be produced. Suspension plasma/flame or solution precursor plasma spraying are examples of techniques that can be utilized for liquid feedstock thermal spraying.This approach of using suspension and solution feedstock, which is an alternative to the conventional solid powder feed stock spraying, is gaining increasing research interest, since it has been shown to be capable of producing coatings with superior coating performance.The objective of this research work was to explore relationships between process parameters, coating microstructure, thermal diffusivity and thermal conductivity in liquid feedstock thermal sprayed TBCs. A further aim was to utilize this knowledge to produce a TBC with lower thermal diffusivity and lower thermal conductivity compared to state-of-the-art in industry today, i.e. solid feed stock plasma spraying. Different spraying techniques, suspension high velocity oxy fuel,solution precursor plasma and suspension plasma spraying (with axial and radialfeeding) were explored and compared with solid feedstock plasma spraying.A variety of microstructures, such as highly porous, vertically cracked and columnar, were obtained. It was shown that there are strong relationships between the microstructures and the thermal properties of the coatings.Specifically axial suspension plasma spraying was shown as a very promising technique to produce various microstructures as well as low thermal diffusivity and low thermal conductivity coatings.