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.

The aim of this thesis is to investigate the effects of additives, reactive elements and impurities, on the lifetime of thermal barrier coatings. The thesis consists of a number of studies on interface adhesion, impurity diffusion, grain boundary sliding and cleavage processes and their impact on the mechanical behaviour of grain boundaries. The effects of additives and impurity on interface adhesion were elaborated by using total energy calculations, electron localization and density of states, and by looking into the atomic separations. The results of these calculations allow the assessment of atomic level contributions to changes in the adhesive trend. Formation of new bonds across the interface is determined to improve the adhesion in reactive element(RE)-doped structures. Breaking of the cross interface bonds and sulfur(S)-oxygen(O) repulsion is found responsible for the decreased adhesion after S segregation. Interstitial and vacancy mediated S diffusion and the effects of Hf and Pt on the diffusion rate of S in bulk NiAl are studied. Hf is shown to reduce the diffusion rate, and the preferred diffusion mechanism of S and the influence of Pt are revealed to be temperature dependent. Finally, the effects of reactive elements on alumina grain boundary strength are studied. Reactive elements are shown to improve both the sliding and cleavage resistance, and the analysis of atomic separations suggest an increased ductility after the addition of quadrivalent Hf and Zr to the alumina grain boundaries.

Les barrières thermiques (BT) élaborées par projection plasma sous air (APS) sont utilisées par l’industrie aéronautique pour protéger les pièces fixes des parties chaudes des turbines à gaz. Les BT consistent en un système bicouche composé d'une couche de liaison NiCrAlY de 200 µm d'épaisseur et d’un revêtement céramique de ZrO2-8%Y2O3 (YSZ) de 1 mm d'épaisseur, déposés sur le substrat métallique à protéger. Les principaux objectifs de cette thèse sont d’une part de comparer la tenue en oxydation cyclique de deux microstructures de barrières thermiques élaborées par APS et d’autre part de caractériser le comportement mécanique de celles-ci, afin de comprendre et de modéliser les mécanismes d’endommagement de ces dépôts afin d’en améliorer la conception. Tout d’abord, l’élaboration par projection plasma des différentes BT a été réalisée. Pour évaluer les performances de ces BT en termes de durée de vie et identifier les mécanismes d’endommagement, elles ont ensuite été testées en oxydation cyclique sous gradient, pour reproduire les conditions réelles d’utilisation en service. Ensuite une caractérisation exhaustive des propriétés physiques et mécaniques des différents matériaux a été menée. Ainsi, les - substrat seul, sous-couche NiCrAlY, les deux revêtements de YSZ à microstructures différents et les deux systèmes BT complets - ont été testés en flexion 3 points (F3P) et en flexion biaxiale Small Punch Test (SPT). A partir des propriétés obtenues et de ces résultats, des modélisations éléments finis ont été proposées : les modes d’endommagement observés pendant les essais de F3P et SPT ont été reproduits. La compréhension de des phénomènes d'endommagement et la prédiction de la durée de vie des BT sont des enjeux majeurs pour les motoristes qui souhaitent élaborer un modèle pertinent de durée de vie. / Thermal barriers coatings (TBC) developed by air plasma spraying (APS) are used by the aviation industry to protect the fixed parts of hot sections of gas turbines. The TBC system consists of a bilayer system composed of a 200 m thick NiCrAlY bondcoat and a 1 mm thick ZrO2-8% Y2O3 (YSZ) ceramic coating, deposited on the metal substrate to be protected. The main objectives of this PhD thesis are, on the one hand, to compare the cyclic oxidation behavior of two microstructures of TBC developed by APS and, on the other hand, to characterize the mechanical behavior of these TBC, in order to understand, model the damage mechanisms. and improve their design. Firstly, the plasma projection of the different TBCs was carried out. To evaluate their performance in terms of lifetime and identify the mechanisms of damage, they were then tested in cyclic oxidation with gradient to reproduce the actual conditions of use in service. Then, an exhaustive characterization of the physical and mechanical properties of the different materials was conducted. Thus, the - substrate alone, NiCrAlY sublayer, the two YSZ coatings with different microstructures, and the two complete TBC systems - were tested in 3-point bending (F3P) and biaxial flexion Small Punch Test (SPT). From the properties obtained and these results, finite element modelisations were proposed: the modes of damage observed during the F3P and SPT tests were reproduced. The understanding of damage phenomenon and the prediction of life of TBCs are major issues for engine makers who wish to develop a relevant model of service life.

<p><a>Zirconium oxide (zirconia, ZrO₂) is one of the essential structural ceramics for industrial applications due to its superb strength and fracture toughness. ZrO₂ has three main polymorphs: cubic, tetragonal, and monoclinic phase, depending on temperature, type, and concentration of dopants. Stabilized zirconia with metastable tetragonal phase can transform into monoclinic phase with ~ 4% volume expansion under an applied external stress. The tetragonal-to-monoclinic transformation can hinder crack propagations by generating a compressive stress field near crack field, thereby enhancing fracture toughness. In addition, other deformation mechanisms such as dislocation activities, crack deflection, and ferroelastic domain switching can further enhance its deformability. Bulk ZrO₂ is typically prepared by sintering at high temperatures over a long period of time. Recently, field-assisted sintering techniques such as flash sintering and spark plasma sintering have been applied to effectively sinter ZrO₂. These techniques can significantly decrease sintering temperature and time, and more importantly introduce a large number of defects in the sintered fine grains.</a></p> <p>The miniaturization of sample dimension can alter the mechanical properties of materials by increasing the surface-to-volume ratio and decreasing the likelihood of retaining process-induced flaws. The knowledge of mechanical properties of ZrO₂ at micro and nanoscale is critical in that superelasticity and shape memory effect of ZrO₂ can be utilized for applications of actuation, energy-damping, and energy-harvesting at small scale. Here, we performed <i>in-situ</i> microcompression tests at various temperatures inside a scanning electron microscope to examine and compare the mechanical properties of ZrO₂ prepared by flash sintering, spark plasma sintering, plasma spray, and thermal spray. Detailed microstructural analyses were conducted by transmission electron microscopy. The unique microstructures in ZrO₂ prepared by field-assisted sintering largely improved their plasticity. Temperature and processing technique-dependent underlying deformation mechanisms and fracture behavior of ZrO₂ are discussed.</p>

The thermo-mechanical effects on the strain evolution within an EB-PVD thermal barrier coating (TBC) is presented in this work using in-situ characterization. Synchrotron X-ray diffraction at sector 1-ID at the Argonne National Laboratory provided both qualitative and quantitative in-situ data on the strain evolution under a thermal cycle with mechanical loading. The results show that at a critical combination of temperature and load, the stress in the thermally grown oxide (TGO) layer in the TBC reaches a tensile region. These significant findings enhance existing literature showing purely compressive strains within the TGO where mechanical loads have been neglected. The results have important implications on the effects on the overall life of the coating. Depth resolved quantitative strain is presented as contour plots over a thermal cycle highlighting the complementary strains in the adjacent layers including the bond coat and the TBC with time and temperature. Systematic identification of the appropriate peaks within the multi-layer TBC system provides guidelines for future strain studies using high energy X-rays. Piezospectroscopic studies with applied mechanical loading are further presented as verification of the room temperature XRD data for future development of the method as an operational technique to be used outside the laboratory environment.

There has been an urge for increasing the efficiency in advanced gas turbine engines. To fulfill these needs the inlet gas temperatures should be increased in the gas turbine engines, thermal barrier coatings (TBCs) have gained significant applications in increasing the gas inlet temperatures. Insulating characteristics of ceramic TBCs allow the operation at up to 150~250 ˚C higher gas temperatures. Because of the severe turbine engine operating conditions that include high temperature, steep temperature gradient, thermal cycling, oxidation and hot-corrosion, TBCs can fail by spallation at the interface between the metal and ceramic. The lack of understanding in failure mechanisms and their prediction warrant a development of non-destructive evaluation technique that can monitor the quality and degradation of TBCs. In addition, the development of NDE technique must be based on a robust correlation to the characteristics of TBC failure. The objective of this study is to develop electrochemical impedance spectroscopy (EIS) as a Non-destructive evaluation (NDE) technology for application to TBCs. To have a better understanding of the multilayer TBCs using EIS they were divided into individual layers and EIS were performed on them. The individual layers included polycrystalline ZrO2-7~8 wt.%Y2O3 (YSZ) (topcoat) of two different densities were subjected to sintering by varying the sintering temperature and holding time for three different thickness and hot extruded NiAl alloy buttons which were subjected to isothermal oxidation with varying temperature and time. NiAl is as similar to the available commercial bondcoats used in TBCs. Then degradation monitoring with electrolyte penetration was carried out on electron beam physical vapor deposited (EB PVD) TBCs as a function of isothermal exposure. Quality control for air plasma sprayed TBCs were carried out as a function of density, thickness and microstructure. Dense vertically cracked TBCs were tested as a function of vertical crack density and thickness. Electrochemical impedance response was acquired from all specimens at room temperature and analyzed with an AC equivalent circuit based on the impedance response as well as multi-layered structure and micro-constituents of specimens. Physical and microstructural features of these specimens were also examined by optical and electron microscopy. The EIS measurement was carried out in a three-electrode system using a standard Flat Cell (K0235) from Princeton Applied Research™ and IM6e BAS ZAHNER™ frequency response analyzer. The electrolyte employed in this investigation was 0.01M (molar) potassium Ferri/Ferro Cyanide {(K3Fe(CN)6/K4Fe(CN)6·3H2O)}. The thickness and density were directly related to the resistance and capacitance of the polycrystalline YSZ with varying thickness and open pores. As the effective thickness of the YSZ increased with sintering time and temperature, the resistance of the YSZ (RYSZ) increased proportionally. The variation in capacitance of YSZ (CYSZ) with respect to the change in porosity/density and thickness was clearly detected by EIS. The samples with high porosity (less dense) exhibited large capacitance, CYSZ, compared to those with less porosity (high density), given similar thickness. Cracking in the YSZ monoliths resulted in decrease of resistance and increase in capacitance and this was related to the electrolyte penetration. Growth and spallation of TGO scale on NiAl alloys during isothermal oxidation at various temperatures and holding time was also correlated with resistance and capacitance of the TGO scale. With an increase in the TGO thickness, the resistance of the TGO (RTGO) increased and capacitance of the TGO (CTGO) decreased. This trend in the resistance and capacitance of the TGO changed after prolonged heat treatment. This is because of the spallation of the TGO scale from the metal surface. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO, excluding the abrupt changes associated with the failure. As a function of isothermal exposure for EB-PVD TBCs, initial increase in the resistance of YSZ with thermal exposure was observed perhaps due to the high temperature sintering of YSZ. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO. An explanation based on electrolyte penetration into sub-critical damage is proposed for the gradual decrease in the resistances of YSZ and TGO with prolonged thermal exposure. Observation of exposed metallic bond coat surface on the fracture surface, which readily provides conduction, was related to the abrupt and large increase in the capacitance of YSZ and TGO. A direct relation between the resistance of the YSZ (RYSZ) and density of the YSZ was observed for APS TBCs with varying topcoat density. APS TBCs with varying topcoat chemistry and thickness were tested and directly related to resistance of topcoat. With the increase in the topcoat thickness, the capacitance decreased and the resistance increased. The higher values of CCAT and RCAT compared to that of CYSZ and RYSZ were related to the higher dielectric constant and resistivity of CaTiO3. Dense vertically cracked TBCs were tested with varying crack density were tested and the variation in the resistance was related indirectly to the cracks and directly to the difference in the thickness of the topcoat. EB-PVD TBCs with varying density (dense and columnar) were tested and the variation in resistance was attributed to the dense structure and columnar structure of the topcoat with columnar structure having lower resistance because of more electrolyte penetration through the columnar structure. From this study, EIS showed a potential as a NDE technique for quality assurance and lifetime remain assessment of TBCs. Future work should continue on developing a mathematical model to study the impedance curves and come up with a model for individual layers of TBC and then sum them up to get the multilayered TBC response. The flexible instrument probe of EIS needs to be designed and tested for field evaluation of TBCs.

Gas Turbine manufacturers strive for increased operating temperatures of gas turbine engines to improve efficiency and performance. One method of increasing the temperature beyond material limits is by applying thermal barrier coatings (TBCs) to hot section components. TBCs provide a thermal gradient between the hot gases and metallic substrate, and allow an increase in turbine inlet temperatures of 100-150ºC. However, spallation of TBCs can cause catastrophic failure of turbine engines by incipient melting of the substrate. To prevent such an occurrence, non-destructive evaluation (NDE) techniques are critical for quality control, health monitoring, and life assessment of TBCs. Two techniques in development for this purpose are thermal wave imaging (TWI) and photostimulated luminescence (PL) spectroscopy. TWI is a promising NDE technique with the ability to detect integrity and thickness of TBCs. In this study, TWI was employed as an NDE technique to examine as-coated TBCs with varying thicknesses, and thermally-cycled TBCs for initiation and progression of subcritical-subsurface damage as a function of thermal cycling. TWI and thermal response amplitude were correlated to the microstructural characteristics and damage progression of TBCs based on phenomenological expressions of thermal diffusion. The TBC specimens examined consisted of air plasma sprayed ZrO2 - 7wt.% Y2O3 on NiCoCrAlY bond coats with Haynes 230 superalloy substrate. As-coated specimens of varying thicknesses were evaluated by TWI to examine its applicability as a thickness measurement tool. It was found that heat dissipation through the TBC following pulsed excitation by xenon flash lamps initially followed the 1-D law of conduction and deviated from it as a function of thickness and time. The deviation resulted from quick dissipation of heat into the conductive metallic substrate. Therefore, with calibration, TWI can be used as a tool for YSZ thickness measurements of APS TBCs in the as-coated condition for quality control measures. Specimens of uniform thickness were evaluated as a function of thermal cyclic oxidation for subcritical-subsurface damage detection. Thermal cycling was carried out in air with 30-minute heat-up, 10-hour dwell at 1150°C, 30-minute air-quench and 1-hour hold at room temperature. During thermal cycling, TBC specimens were evaluated non-destructively by TWI at room temperature every 10 to 20 thermal cycles, and selected specimens were removed from thermal cycling for microstructural analysis by scanning electron microscopy (SEM). Higher thermal response amplitude associated with disrupted heat transfer was observed where localized spallation at or near the YSZ/TGO interface occurred. The health of the TBC was monitored by a rise in thermal response amplitude which may indicate a coalescence of microcracks to a detectable level. PL has been developed to measure stress, and detect subsurface damage and polymorphic transformation within the thermally grown oxide (TGO) of TBCs. PL was employed in this study as an NDE technique for TBCs to correlate subsurface damage as a function of thermal cyclic oxidation. The TBCs consisted of ZrO2 – 7 wt.% Y2O3 applied by electron beam physical vapor deposition with an as-coated (Ni,Pt)Al bond coat on a CMSX-4 superalloy substrate. Specimens were thermally cycled with a 10 minute ramp to a peak temperature of 1121°C, 40 minute hold at peak temperature, and 10 minute forced air quench. The TBCs were periodically removed from thermal cycling for NDE using PL until failure. Two specimens were removed from thermal oxidation after 10% and 70% of the average lifetime for microstructural analysis by SEM. During initial thermal cycling, metastable phases and polymorphic transformations of the Al2O3 scale were examined by PL. The polymorphic transformation from a metastable phase to equilibrium a-Al2O3 was detected. Since metastable phases are thought to be detrimental to coating lifetime, detection of these phases by PL can be used as a quality control tool. Nearing end-of-life, relief of the TGO from the compressive residual stress arising from thermal expansion mismatch was detected with PL and confirmed with microstructural analysis that revealed damage initiation (e.g. microcracking within the TGO scale parallel to the interfaces.) Rise in luminescence near the R-line frequency for polycrystalline a-Al2O3 without any residual stress (i.e. n = 14402 cm-1 and n = 14432 cm-1) corresponded to regions where cracked TGO was adhered to YSZ and not exposed to compressive stresses from thermal expansion mismatch upon cooling.

Thermal Barrier Coatings have been used for decades to impose a thermal gradient between the hot combustion gases and the underlying superalloy substrate in engine turbine blades. Yttria Stabilized Zirconia (YSZ) is an industry standard high temperature ceramic for turbine applications. The protective coating is adhered to the substrate using a nickel based alloy bond coat. Through exposure to high temperature, a Thermally Grown Oxide (TGO) layer develops at the bond coat-YSZ interface. Large residual stresses develop in these layers due to thermal expansion mismatch that occurs during cool down from high temperature spraying and cyclic operating conditions. Despite their standard use, much is to be determined as to how these residual stresses are linked to the various failure modes. This study developed techniques to monitor the strain and stress in these internal layers during thermal gradient and mechanical conditions representing operating conditions. The thermal gradient is applied across the coating thickness of the tubular samples from infrared heating of the outer coating and forced air internal cooling of the substrate. While thermal and mechanical loading conditions are applied, 2-dimensional diffraction measurements are taken using the high-energy Synchrotron X-Rays and analyzed to provide high-resolution depth-resolved strain. This study will include fatigue comparisons through use of samples, which are both 'as-coated' as well as aged to various stages in a TBC lifespan. Studies reveal that variations in thermal gradients and mechanical loads create corresponding trends in depth resolved strains with the largest effects displayed at or near the bond coat/TBC interface. Single cycles as well as experiments targeting thermal gradient and mechanical effects were conducted to capture these trends. Inelastic behavior such as creep was observed and quantified for the different layers at high temperatures. From these studies more accurate lifespan predictions, material behaviors, and causes of failure modes can be determined. The work further develops measurement and analysis techniques for diffraction measurements in internal layers on a coated tubular sample which can be used by various industries to analyze similar geometries with different applications.

The durability of thermal barrier coatings (TBCs) affects the life of the hot section engine components on which they are applied. Fatigue is the general failure mechanism for such components and is responsible for most unexpected failures; therefore it is desirable to develop lifetime approximation models to ensure reliability and durability. In this study, we first examined the microstructural degradation of air plasma sprayed ZrO2-8wt.%Y2O3 TBCs with a low-pressure plasma sprayed CoNiCrAlY bond coat on an IN 738LC superalloy substrate. The durability of TBCs were assessed through furnace thermal cyclic tests carried out in air at 1100°C with a 1-, 10-, and 50-hour dwell period, preceded by a 10-minute heat-up and followed by a 10-minute forced-air-quench. Failure mechanisms of the TBCs were thoroughly investigated through materials characterization techniques including: X-Ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-Ray Spectroscopy. Quantitative microstructural analyses were then carried out to document the growth of the thermally grown oxide (TGO) scale, the depletion of the Al-rich β-NiAl phase in the bond coat, and the population and growth of micro-cracks near the YSZ/bond coat interface. Trends in the TGO growth and the β-phase depletion in the bond coat followed those of diffusion-controlled processes—parabolic growth of the TGO and exponential depletion of the β-phase. Formation and propagation of cracks within the YSZ resulted in complete spallation of the YSZ topcoat from the bond-coated superalloy substrate. Evolution in these microstructural features was correlated to the lifetime of TBCs, which showed cracking within the YSZ to be the cause of failure; thus a lifetime iv approximation model was developed, via modification of Paris Law, based on the experimental data. The model predicted the TBC lifetime within 10% of the experimental lifetime.

The continued focus to include high hydrogen fuels such as Syngas in aircraft operation to reduce emissions and increase engine efficiency has led to an ongoing investigation into bond coat materials capable of withstanding unfavourable oxidation in high temperature humid environments. The increased presence of water in the engine exhaust leads to increased oxygen activity in the hot section of the engine. In this work, four commercially available MCrAlY bond coat materials were oxidized in high temperature environments with various humidities to understand the behaviours of different reactive element inclusions in resisting high temperature oxidation. Oxidation tests were done at 0%, 18%, and 33% water by volume at 1100C in a 1atm environment to simulate conditions expected in engines using high hydrogen fuels. Oxidation was done for 2h and 20h to observe transient oxide formation behaviour. The surfaces and cross sections of the specimens were examined using SEM and EDS analysis, along with XRD analysis. The progression of surface oxides, TGO thickness, and element depletion zones were observed. Two opposing mechanisms are observed: the upward diffusion of metal cations to the free surface and the inward diffusion of oxygen to the alloy. The presence of water is shown to increase internal oxidation of the bond coat alloy and delay the formation of a protective alumina TGO. Tantalum inclusion in the alloy composition is shown to produce the most stable alumina TGO with the least internal oxidation after 20h exposure in 33% H2O (%vol); the most hostile oxidation environment tested.