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Ab Initio Modeling of Thermal Barrier Coatings: Effects of Dopants and Impurities on Interface Adhesion, Diffusion and Grain Boundary StrengthOzfidan, Asli Isil 09 May 2011 (has links)
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.
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高エネルギー反射光によるEB-PVD遮熱コーティングの残留応力分布の解析鈴木, 賢治, SUZUKI, Kenji, 松本, 一秀, MATSUMOTO, Kazuhide, 久保, 貴博, KUBO, Takahiro, 町屋, 修太郎, MACHIYA, Shutaro, 田中, 啓介, TANAKA, Keisuke, 秋庭, 義明, AKINIWA, Yoshiaki 11 1900 (has links)
No description available.
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Estimation of Spalling Stress in Thermal Barrier Coatings Using Hard Synchrotron X-RaysSUZUKI, Kenji, TANAKA, Keisuke, AKINIWA, Yoshiaki 07 1900 (has links)
No description available.
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Microstructural evolution in coated conventionally cast Ni-based superalloysNewman, Scott January 2013 (has links)
The aim of this research project was to investigate the microstructural changes of MCrAlY coated Ni-based superalloys that are routinely used in industrial gas turbine engines for power generation. One of the main aims of the characterisation was to understand ageing time and temperature effects on the microstructural evolution so that a methodology could be developed where the characterisation of a thermally exposed microstructure can be used to estimate unknown service exposure conditions.
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Isothermal Oxidation Study of Gd2Zr2O7/YSZ Multi-Layered Thermal Barrier CoatingsLi, Ran January 2015 (has links)
Conventional yttria stabilized zirconia (YSZ) are widely used in the gas turbine to protect the substrate material from high temperature. But the common YSZ top coatings have limitations at higher temperature (above 1200 ℃) due to significant phase transformation and intensified sintering effect. Among the list of pyrochlores, gadolinium zirconate offer very attractive properties like low thermal conductivity, high thermal expansion coefficient and CMAS resistance. However, a lower fracture toughness than YSZ and tendency to react with alumina (thermal grown oxide) can lead to lower lifetime. Therefore, multi-layered thermal barrier coating approach was attempted and compared with single layer system. Single layer (YSZ) was processed by suspension plasma spraying (SPS). Double layer coating system comprising of YSZ as the bottom ceramic layer and gadolinium zir-conate as the top ceramic coat was processed by SPS. Also, a triple layer coating system with denser gadolinium zirconate on top of double layer system, was sprayed. Denser gado-linium zirchonate acts as the sealing layer and arrest the CMAS penetration. Isothermal oxidation performance of the sprayed coating systems including bare substrate and sub-strate with bond coat were investigated for a time period of 10hr, 50hr and 100hr at 1150℃ in air environment. Weight gain was observed in all the systems investigated. Microstruc-tural analysis was carried out using optical microscopy, SEM/EDS. Phase analysis was done using X-ray diffraction (XRD). Porosity measurement was made by water impregna-tion method. It was observed that multi-layered thermal barrier coating systems of YSZ/GZ and YSZ/GZ/GZ(dense) showed lower weight gain and TGO thickness than the single layer YSZ for all exposure time (10hr, 50hr & 100hr). The triple layer system had lower weight gain and TGO thickness compared to double layer system due to lower po-rosity content. Also, from the porosity measurement data, it could be seen that sintering effect is more dominant at 10 hr. of oxidation for all the coatings systems.
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Ab Initio Modeling of Thermal Barrier Coatings: Effects of Dopants and Impurities on Interface Adhesion, Diffusion and Grain Boundary StrengthOzfidan, Asli Isil January 2011 (has links)
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.
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SMALL-SCALE MECHANICAL BEHAVIORS OF ZIRCONIA PROCESSED BY DIFFERENT TECHNIQUESJaehun Cho (9167816) 29 July 2020 (has links)
<p><a>Zirconium
oxide (zirconia, ZrO<sub>2</sub>) is one of the essential structural ceramics
for industrial applications due to its superb strength and fracture toughness.
ZrO<sub>2</sub> 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 ZrO<sub>2</sub> 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 ZrO<sub>2</sub>. 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 ZrO<sub>2</sub> at micro and nanoscale is critical in that
superelasticity and shape memory effect of ZrO<sub>2</sub> 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 ZrO<sub>2</sub> 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 ZrO<sub>2</sub> prepared by field-assisted sintering largely improved their
plasticity. Temperature and processing technique-dependent underlying
deformation mechanisms and fracture behavior of ZrO<sub>2</sub> are discussed.</p>
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In-situ synchrotron studies of turbine blade thermal barrier coatings under extreme environmentsKnipe, Kevin 01 January 2014 (has links)
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.
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Effect of Ta, Hf, and Si on the High Humidity Oxidation Resistance of MCrAlY Bond Coat MaterialsKaterina Luiza, Monea 18 January 2024 (has links)
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 1100C 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.
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Development of High Temperature Erosion Tunnel and Tests of Advanced Thermal Barrier CoatingsShin, Dongyun 07 June 2018 (has links)
No description available.
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