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Multiscale (analytical and computational) models have been developed based on a thermodynamic variational principle (TVP) to model sintering and eventual mudcracking in thermal barrier coatings (TBCs) made using the electron beam physical vapour deposition (EB-PVD) process. It is assumed that the sintering occurs by interfacial diffusion at local contacts between columns and driven by changes in interface free energy and elastic stored energy of the coating. The models link diffusional processes at the scale of contacting feathery columns with the macroscopic deformation and sintering response. In service, the columns can come into contact and sinter together. As sintering progresses there is a build up of strain energy in the system which reduces the driving force for sintering and leads to either complete or incomplete sintering of the TBC depending on the magnitude of effective modulus (E) of the coating. By seeding the coating with initial imperfections, different types of behaviour are observed depending on the value of E and the spacing between imperfections. For compliant coatings, the response is insensitive to the presence of imperfections and the coating fully sinters. At higher values of E, strain energy is released by the development of intercolumnar cracks in the coating, which can propagate to the interface with the TGO (thermally grown oxide), deflect into the interface and propagate, leading to spallation of regions of the coating and loss of thermal protection. It is observed that cracks develop at initial imperfections in the structure. The greater the spacing between imperfections the faster the development of cracks at these locations. If a TBC contains a distribution of imperfections there is progressive formation of cracks, with the average spacing decreasing with time, after an initial incubation period. The crack density eventually saturates to a constant value, which depends on the mechanical properties of the TBC. Initially, a crack spacing, CS, in the range 1.5H ≤ CS ≤ 3H has been predicted based on trapezoidal contact models. Here H is the thickness of the coating. Crack spacing predicted using this model is consistent in the lower range of experimentally observed crack spacing. However, axisymmetric contact models predict a crack spacing, CS, in the range 4H ≤ CS ≤ 8H, which is in good agreement with experimentally observed crack spacing range 3H ≤ CS ≤ 10H reported in the literature. Compared to the trapezoidal contact models, axisymmetric contact models more accurately predict the sintering response.

Thermal barrier coatings have been widely used to provide thermal protection to components in the hot section of gas turbines. This research focuses on two influencing factors on coating behavior: thermal gradient and cyclic oxidation. The delamination mechanics under thermal gradient is analyzed, taking thermally grown oxide into consideration. Coatings experience thermal gradients at different stages during actual service flight. One is due to engine power shut down when landing and the other due to internal cooling of the substrate. Thermally grown oxide (TGO) also acts as a critical factor in delamination mechanics. The induced stress gradient and corresponding energy release rate for interface delamination and shallower delamination are presented. Mechanism maps that explain the criteria for preventing delamination from developing and propagating are established. Three cooling trajectories are envisaged to analyze the variation in the possibility of delamination. Multilayer coatings used in components of the hot section of aero turbine engines also experience cyclic temperature variation during flight cycles. As experiment conditions vary and coating performance is improved, the time required to run through the test of coating failure can be both time-consuming and prohibitive. Therefore, protocols providing prediction of quantified coating behavior are in demand to shorten life-time tests. Curves of mass change are obtained from quantifying scale growth and loss by different models such as Cyclic Oxidation Spall Program (COSP). A modification is made by combining COSP and a mechanic based model to obtain critical parameters for lifetime prediction from short time experiment. The time for coatings to reach peak temperature during cycling is discovered to influence prominently on modeling results. Predictions for several coating compositions and cycling conditions are consistent with the data from the existing experiments of the coating system.

To fully exploit the strengths of La2Zr2O7 pyroclores and promote them as a next-generation thermal barrier coating (TBC), the improvements of their thermally insulating property and fracture toughness are studied in this thesis. A strong phonon scattering source, rattlers, is found in Y3+-doped La2Zr2O7 pyrochlores. Rattlers dramatically flatten k (thermal conductivity)-T curves, or even make k approach the amorphous limit. The presence of rattlers is strongly dependent on (1) oversized atomic cages that are formed in pyrochlores; and (2) the occupation of smaller guest ions in those oversized cages. To maximize the rattling effect, In3+/Sc3+ ions that are much smaller than Y3+ are introduced to the La2Zr2O7 lattice. As envisaged, the smaller ions in the oversized lattice voids make k glass-like at a much lower doping content. Nevertheless, they are still not effective in reducing the high temperature plateau kmin. Instead, oxygen vacancies are very effective in reducing kmin, because they generate an electrostatic repulsion force among cations surrounding them, resulting in stronger lattice anharmonicity and weaker bonds. The plateau kmin is reduced dramatically by the filling of the B-sites in La2Zr2O7 with a 21% larger (and 50% heavier) Ce4+ guest ion rather than a 96% heavier (but similar-sized) Hf4+ ion, suggesting that a large absolute size of substitutional atoms is more effective in reducing kmin than a heavy absolute mass. This is because: (1) kmin is proportional to (E/M)0.5 (where E is the elastic modulus and M is the average atomic mass); (2) a larger size of guest ions tends to produce a weaker ionic bond and consequently, a lower E; and (3) the changing extent of E by introducing larger guest ions is much greater than that of M induced by adding heavier ones. Lastly, the fracture toughness (KIc) has been increased by dispersing the tetragonal 3 mol% Y2O3-stabilized zirconia (t-3YSZ) particulates in the La2Zr2O7 (LZ) matrix. The tendency of the dispersive t-3YSZ second phases transforming to monoclinic (m) phases strongly depends on the volume fraction introduced. For samples made from equilibrium route, they are toughened by phase transformations within the dispersive t-3YSZ second phases and a crack shielding effect arising from the residual compressive stress within the LZ matrix. An anticipated increase of KIc from ferroelastic toughening together with the residual compressive stress toughening highlights a potential to improve coating durability by depositing t’-3YSZ/LZ composite TBCs by the non-equilibrium route.

Ceramics have a wide variety of applications due to their unique properties; however, the low fracture toughness leads the formation and propagation of unpredictable cracks, and reduces their reliability. To solve this problem, self-healing adaptive oxides were developed. The aim of the work is to gain new insights into self-healing mechanisms of ceramics and their application. Binary oxide systems were investigated that are at least partially healed through the extrinsic or intrinsic addition of silver or silver oxide to form ternary oxides (e.g., Nb2O5 + Ag → AgNbO3). Sintered pellets and coatings were tested. For self-healing TBCs, model systems that were studied include YSZ-Al2O3-SiC, YSZ-Al2O3-TiC, YSZ-Al2O3-Nb2O5, and YSZ-Al2O3-Ta2O5. Laser cladded samples and sintered pellets were produced to test. The healing process occurs due to the formation of oxidation products and glassy phases depending on the self-healing mechanism. X-ray diffraction was used to explore phase evolution, chemical compositions, and structural properties of these samples. SEM equipped with EDS was used to investigate the chemical and morphological properties for the cross-sectional area. Pin-on-disc test was applied to test tribology performance for Nb2O5-Ag2O system, and infiltration test was applied to test CMAS-resistance for TBCs at elevated temperature. The improvements in the performance of these materials were demonstrated.

Thermal barrier coatings (TBCs) are widely used for thermal protection of hot section components in turbines for propulsion and power generation. Applications of TBCs based on a clearer understanding of failure mechanisms can help increase the performance and life-cycle cost of advanced gas turbine engines. Development and refinement of robust nondestructive evaluation techniques can also enhance the reliability, availability and maintainability of hot section components in gas turbines engines. In this work, degradation of TBCs was non-destructively examined by photostimulated luminescence spectroscopy (PSLS) and electrochemical impedance spectroscopy (EIS) as a function of furnace thermal cycling carried out in air with 10-minute heat-up, 0.67, 9.6 and 49.6 - hour dwell duration at 1121°C (2050°F), and 10-minute forced-air quench. TBCs examined in this study consisted of either electron beam physical vapor deposited and air plasma sprayed yttria-stabilized zirconia (YSZ) on a variety of bond coat / superalloy substrates including bond coats of NiCoCrAlY and (Ni,Pt)Al, and superalloys of CMSX-4, Rene‟N5, Haynes 230 and MAR-M-509. Detailed microstructural characterization by scanning electron microscopy and energy dispersive spectroscopy was carried out to document the degradation and failure characteristics of TBC failure, and correlate results of PSLS and EIS. Mechanisms of microstructural damage initiation and progression varied as a function of TBC architecture and thermal cycling dwell time, and included undulation of the interface between the thermally grown oxide (TGO) and bond coats, internal oxidation of the bond coats, and formation of Ni/Co-rich TGO. These microstructural observations were correlated to the evolution in compressive residual stress in the TGO scale determined by PSLS shift. Correlations iv include stress-relief and corresponding luminescence shift towards stress-free luminescence (i.e. = 14402 cm-1 and  = 14432 cm-1 ) associated with subcritical cracking of the TGO scale and stress-relaxation associated with gradual shift in the luminescence towards stress-free luminescence (i.e.  = 14402 cm-1 and  =14432 cm-1 ) is related to the undulation of TGO/bondcoat interface (e.g., rumpling and ratcheting). Microstructural changes in TBCs such as YSZ sintering, TGO growth, and subcritical damages within the YSZ and TGO scale were also correlated to the changes in electrochemical resistance and capacitance of the YSZ and TGO, respectively. With thermal exposure the YSZ/TGO resistance and capacitance increased and decreased as result of sintering and TGO growth. With progressive thermal cycling damages in the TGO was related to the TGO capacitance showing a continuous increase and at failure TGO capacitance abruptly increased with the exposure of bondcoat. Further correlations among the microstructural development, PSLS and EIS are documented and discussed, particularly as a function of dwell time used during furnace thermal cycling test, with due respect for changes in failure characteristics and mechanisms for various types of TBCs

No description available.

No description available.

The thermal conductivity of various porous thermal barrier coating (TBC) systems used in elevated temperature for industrial applications has been evaluated using a proposed six-phase model. These porous TBC systems rely on microstructural properties and yield different types of porosity. These microstructural properties can influence the thermal conductivity of TBC systems. The purpose of this thesis is to assess the thermal conductivity of TBC systems based on microstructural attributes, particularly the effect of different types of porosity. Thus, the first component of this thesis investigates the microstructural characterization of various TBC systems using image analysis (IA) technique. In this technique, scanning electron microscopy (SEM) and light optical microscopy (LOM) micrographs were used to measure the porosity level of different TBC materials. The volumetric fraction of porosity along with orientation, shape, and morphology have a considerable impact on the total thermal conductivity of TBCs. The second component of this thesis evaluates the thermal conductivity of these porous TBC systems by taking into account the effect of the heat treatment process. The IA results reveal that as long as the porosity content increases, the thermal conductivity decreases for all of the TBC materials studied in this thesis. Further, while the content of microcracks and non-flat porosity play a crucial role in reducing the thermal conductivity of TBC materials, the other types of porosity (open randomly oriented, penny-shaped, and interlamellar) exert less impact on the thermal conductivity of TBCs. Comparing the results of the proposed six-phase model to experimental values and finite element analysis (FEA) values showed a relatively good agreement. The proposed six-phase model can predict the thermal conductivity of porous microstructure of TBC systems close to real measured values; therefore, the proposed six-phase model may be utilized to fabricate the porous microstructure of TBCs.

Thermal barrier coatings (TBCs) of ZrO2-7wt% Y2O3 were deposited by electron beam physical vapor deposition (EB-PVD) onto stationary flat plates and cylindrical surfaces in a multiple ingot coater. Crystallographic texture, microstructure, and deposition rate were investigated in this thesis. The crystallographic texture of EB-PVD TBCs deposited on stationary flat surfaces has been experimentally determined by comparing pole figure analysis data with actual column growth angle data. It was found that the TBC coating deposited directly above an ingot exhibits <220> single crystal type crystallographic texture. Coatings deposited between and off the centerline of the ingots the exhibited a <311>-type single crystal texture. For coatings deposited in the far corners of the coating chamber either a <111> fiber texture or a <311> single crystal type texture existed. The crystallographic texture of EB-PVD TBCs deposited on cylindrical surfaces was characterized using x-ray diffraction (XRD) at different angular positions on the cylinder substrate. XRD results revealed that crystallographic texture changes with angular position. Changes in crystallographic texture are attributed to the growth direction of the columns and substrate temperature. Growth direction is controlled by the direction of the incoming vapor flux (i.e. vapor incidence angle), in which competition occurs between crystallites growing at different rates. The fastest growing orientation takes over and dominates the texture. Substrate temperature variations throughout the coating chamber resulted in different growth rates and morphology. Morphology differences existed between cylindrical and flat plate surfaces. Flat cross sectional surfaces of the coatings exhibited a dense columnar structure in which the columns grew towards the closest vapor source. Surface features were found to be larger for coatings deposited directly above an ingot than coatings deposited away from the ingots. Morphological differences result from substrate temperature changes within the coating chamber, which influences growth kinetics of the coating. Cylindrical surfaces revealed a columnar structure in which columns grew towards the closest vapor. Porosity of the coating was found to increase when the angular position changed from the bottom of the cylinder. Change in angular position also caused the column diameter to decreases. Morphology changes are attributed to self-shadow effects caused by the surface curvature of the cylinder and vapor incidence angle changes. Overall, the microstructure and crystallographic texture of EB-PVD coatings was found to depend on the position in the coating chamber which was found to influence substrate temperature, growth directions, and shadowing effects. The coating thickness profiles for EB-PVD TBCs deposited on stationary cylinders have been experimentally measured and theoretically modeled using Knudsen's cosine law of emissions. A comparison of the experimental results with the model reveals that the model must to be modified to account for the sticking coefficient as well as a ricochet factor. These results are also discussed in terms of the effects of substrate temperature on the sticking coefficient, the ricochet factor, and coating density.