Superior performance of plasma sprayed YSZ thermal barrier coatings with oxide dispersion strengthened bond coats

Advanced thermal barrier coatings are essential to increase the efficiency of next-generation gas turbine engines. Different materials and process technologies give the possibility to extend the lifetime of TBCs. One limiting factor of the TBC lifetime is the growth of the TGO during thermal exposure resulting in a accelerated crack growth at the top coat- bond coat interface. The oxidation resistance and the temperature of the bond coat are key factors influencing the TGO growth rate. Oxide dispersion strengthened (ODS) bond coats have a slower oxygen scale growth during thermal exposure in comparison to standard bond coats. In previous studies TBC systems with an additional thin ODS bond coat on top of a standard bond coat showed a higher thermal cycling performance. These studies used Inconel 738 and Amdry 386 as substrate and bond coat material, respectively. This study investigates in the thermal cycling performance of the ODS bond coat TBC systems combined with a different substrate ERBO 1 and bond coat material Amdry 995. TBC systems with the new material combination show high cycling lifetimes and superior performance in comparison to previous samples. Samples were tested by a cyclic burner rig facility. Surface was heated by a gas burner to 1400°C while the backside is cooled by pressurized air to 1050°C. One cycle consists of 5 min heating followed by 2 min cooling. Cross sections of the samples were analyzed by SEM and laser microscope. The lifetime of the samples was evaluated especially with respect to diffusion processes, material properties, and bond coat temperature. ODS powders with higher aluminum oxide additions were produced by high energy milling to fit the CTE of the ODS bond coat to the one of the top coat. This will reduce the initial crack formation on the top a wavy top coat - bond coat interface and increase lifetime. The advanced bond coats were applied by low pressure plasma spraying, the standard YSZ top coat by atmospheric plasma spraying. The performance was evaluated by a gas burner rig test

Factors influencing the performance of zirconia based thermal barrier coatings

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Development of thermally sprayed environmental barrier coatings

Ceramic matrix composites (CMCs) are candidate materials for high-temperature applications such as gas turbines. As corrosion by water vapor and deposits is a limiting issue, the application of suitable environmental barrier coatings (EBCs) is inevitable. Gas tightness, chemical stability and a good adhesion of the EBC are crucial aspects for providing an effective barrier against the combustion atmosphere. Thermal spray technologies offer a variety of promising process routes to the manufacturing of ceramic coating systems complying with these demands. This contribution covers coatings for both silicon and oxide based CMCs. Different EBC materials (e.g. silicates, aluminates, rare-earth oxides) were examined and optimized for use as EBC by air plasma spraying (APS). Different thermal spray techniques including high-velocity oxygen-fuel spraying (HVOF), suspension plasma spraying (SPS) and plasma-spray physical vapor deposition (PS-PVD) techniques were assessed for the manufacture of coatings with low amorphous phase content. The application of bond coat as well as alternative surface modification technologies was tested to increase the adhesion of the EBCs. Microstructures and chemical stability of the coatings were analyzed and the performance was tested in terms of adhesion strength or degradation under high-temperature exposure including cyclic oxidation

Development of environmental barrier coatings for Al2O3/Al2O3 CMCs with improved Adhesion by texturing with laser ablation

Al2O3 /Al2O3 ceramic matrix composites (CMC) are candidate materials for high-temperature applications such as gas turbines. As water vapor corrosion of oxide/oxide CMC is a major issue, the application of suitable environmental barrier coatings (EBC) is inevitable. Besides the gas tightness a good adhesion of the EBC is a crucial aspect for providing an effective barrier against the combustion atmosphere. Due to the brittleness of the ceramic matrix conventional surface treatments like grinding and sandblasting fail to increase roughness without causing damage to the substrate. Therefore there is a need for new methods of surface preparation of CMCs. This work examines the suitability of surface preparation with laser ablation for use prior to air plasma spraying (APS) on an oxide/oxide-CMC. Laser ablation allows controlling of the surface’s structure and roughness. The effects of different laser parameters on the alumina surface were examined and a variety of different structures, for example a honeycomb or a cauliflower like structure, were prepared. The laser treated surfaces were coated with potential EBC-candidates, such as Y2O3 and Gd2Zr2O7 and the impact of laser textures on the coating adhesion was examined. Evaluation of the coated samples was done by pull-off adhesion testing and thermal cycling. Results indicate that laser pretreatment helps to increase the adhesion strength of the EBC-system

Thermally sprayed protective coatings under demanding load conditions

Materials in turbines are facing increasingly demanding conditions under operation. This is due to their diversifying field of application as e.g. in interplay with renewable energy sources. Each set of loading conditions, in terms of e.g. operation temperature, start/stop-frequency or contaminants present in the combustion atmosphere, shows a specific footprint of degradation pathways. Understanding and performance data are available for many individual degradation footprints as to date materials and coatings are developed in respect to each one of that kind of loading scenarios. Less often, materials are assessed at a wider range of conditions where changes and interplay of degradation modes can be observed. Today’s demand for design of material systems for flexible or volatile conditions of operation requires to consider the wider range of operation regimes including complex sequences of loading phases adding up to the overall degradation. Performance and degradation modes of APS TBC systems (both single layer YSZ as well as a double layer of YSZ plus Gd2Zr2O7) were studied under various conditions in cyclic testing. This includes scenarios with isothermal and gradient testing as well as sequential vs simultaneous loading with CMAS. Results are evaluated with respect to changes of (coexisting) degradation modes and spallation lifetime. Applicability of some modeling tools is discussed for lifetime prediction. Complete affiliations: 1) Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Materials Synthesis and Processing (IEK-1), 52425 Jülich, Germany 2) Jülich Aachen Research Alliance: JARA-Energ

In-situ studies on defect structure and electronic conductivity in iron oxides with perovskite structure

Übergangsmetalle und ihre Verbindungen zeigen ein besonders reichhaltiges Spektrum physikalischer und chemischer Eigenschaften. Ihre Oxide und ihre oxidischen Verbindungen, die weitere Elemente aus fast allen Gruppen des Periodensystems enthalten können, zeigen einen sehr breiten Variationsbereich der elektrischen und magnetischen Eigenschaften. Für die Grundlagenforschung sind viele 3d- und 4d-Übergangsmetalloxide ein fruchtbares Feld zum Studium schmalbandiger, korrelierter Systeme, in denen sich Elektronen nicht wie in guten Metallen nahezu frei bewegen können. In korrelierten Systemen wird die kinetische Energie der Ladungsträger (in d-Zuständen) vergleichbar mit der Coulomb-Abstoßung oder sogar kleiner, so daß die Ladungsträger sich aus dem Weg gehen oder in Grüppchen beisammen bleiben und auf diese Weise ihre Mobilität eingeschränkt wird. Viele der oben erwähnten elektronischen Eigenschaften werden durch solche Korrelationseffekte hervorgerufen. Während die Elektronen der 4d- und 5d-Orbitale durch die eigenen, besetzten s- und p-Orbitale relativ gut abgeschirmt sind und ähnlich wie die Valenzelektronen der f-Systeme aufgrund der schwachen Hybridisierung mit den Ligandenorbitalen als lokalisiert betrachtet werden können, gilt diese Vereinfachung in den 3d-Systemen nicht allgemein. Durch den erhöhten Überlapp zwischen den Übergangsmetall-(3d)- und Sauerstoff-(2p)-Orbitalen steigt der kovalente Charakter der Bindung, und man findet die Tendenz, ausgeprägt delokalisierte Elektronenzustände auszubilden. Diese Tendenz ist vermittels der Atomabstände und Bindungswinkel wiederum äußerst empfindlich von der Kristallstruktur abhängig. Ein Übergang zwischen (überwiegend) lokalisiertem und delokalisiertem Verhalten kann hier schon durch kleine Änderungen der thermodynamischen Bedingungen verursacht und im Experiment beobachtet werden. Eine der häufigsten und variabelsten Strukturen, in der Übergangsmetalloxide kristallisieren, ist die Kristallstruktur der Perowskite und ihrer verwandten Schichtstrukturen. Zwei Eisenoxide mit Perowskitstruktur stehen im Mittelpunkt dieser Arbeit: Das Strontiumferrat, SrFeO3, und das Yttriumorthoferrit, YFeO3. Das stöchiometrische Strontiumferrat besitzt die ideale kubische Perowskitstruktur. Bei der Reduktion des Strontiumferrats bleibt die Perowskitstruktur im sehr großen Stabilitätsbereich (Stöchiometrieabweichung für Sauerstoff zwischen 0 und 0.5 pro Formeleinheit), trotz der enormen Defizite im Sauerstoffteilgitter, erhalten. Oberhalb einer Ordnungstemperatur von ca. 300C findet man eine Hochtemperaturphase, in der die Sauerstoffleerstellen ohne nachweisbare Fernordnung statistisch im Sauerstoffteilgitter verteilt vorliegen. Strontiumferrat zeigt beim Ausbau von Sauerstoff aus dem vollständig oxidierten SrFeO3 einen Übergang von metallischen Leitfähigkeitsverhalten zu halbleitendem Verhalten im nichtstöchiometrischen Strontiumferrat. Ziel dieser Arbeit war es, Informationen über den Zusammenhang zwischen Änderungen der makroskopischen Stöchiometrie sowie der lokalen Eisen-Sauerstoffkoordination einerseits und Änderungen der Eisenvalenz sowie die Lokalisierung der polaronischen Defekte andererseits zu gewinnen. Dazu wurden die lokalen Sondenmethoden Mössbauerspektroskopie und Röntgenabsorptionsspektrsokopie am Eisen in Kombination mit der Messung der makroskopischen elektronischen Transportgrößen eingesetzt.Transition metals and their compounds show a rich spectrum of physical and chemical properties. Especially their oxides which may include elements of nearly all groups of the periodic table possess a wide range of electrical and magnetical properties: They may be isolators, semiconductors, metallic conductors or even superconductors and so on. Often they show a metal-insulator transition, which is accompanied by complex magnetic ordering effects. Due to the strong correlation with the crystal structure those properties change with the variation of parameters such as temperature, pressure or chemical composition. The diversity of properties which results from their electronic structure generates various technical applications. From the point of view of fundamental research the 3d transition metal oxides are favorable compounds to study correlated electronic systems with small band widths. Whereas the outer d electrons of higher period transition metals can be treated as localized due to the shielding effects from their own s and p orbitals suchlike simplification doesn't hold for the 3d systems in general. By means of the increased overlap between 3d orbitals of the transition metal and the 2p orbitals of oxygen the covalent character of the bonding is enforced as well as the tendency to form delocalized electronic states. Due to the strong dependency on atomic distances and bonding angles those tendencies are closely coupled with the local crystal symmetry which is one subject of this theses. One of the most common and most variable structures of transmission metal oxides is the perovskite structure and related layered structures. Two iron oxides of the perovskite type have been in the focus of the presented work: strontium ferrate, SrFeO3, and yttrium orthoferrite, YFeO3. Stoichiometric strontium ferrate shows the ideal cubic perovskite structure. On reduction a distorted perovskite structure is conserved up to a limiting oxygen deficiency of one sixth. Above a transition temperature of approximately 300C a so called cubic high temperature phase is found where no long range vacancy ordering or static relaxation of the local symmetry occurs. The removal of oxygen from the stoichiometric strontium ferrate is accompanied by a change of the electrical conductivity from metallic to semiconducting behaviour. It has been the aim of the current work to gather information on the interrelationship between changes of the overall stoichiometry as well as of the local iron coordination on the one hand and of changes of the valence of iron as well as of the localization of the polaronic defects on the other hand. Therefor the local probe techniques Mössbauer spectroscopy and X-ray absorption fine structure spectroscopy have been combined with the measurement of the macroscopic electronic transport properties. Yttrium orthoferrite is a mixed ionic and electronic conductor which is a promising canditate for the application as cathode material in solid oxide fuel cells. The origin of the electronic majority charge carriers as well as the mechanism of conductivity enhancement in the doped material, (YCa)(FeMn)O3, is not yet fully understood. Based on measurements of the electrical transport properties Yoo et al. elaborated a quantitative defect model involving a charge disproportionation of trivalent iron for the calcium doped material, (YCa)FeO3. This study aimed to prove this defect model by Mössbauer spectroscopy on the local iron probe