Bond coats used in thermal barrier coatings (TBCs) for gas-turbine engine applications are studied in this thesis, with a focus on oxidation behaviour, surface rumpling and stress evolution. Bond coats made of γ/γ’ Ni-Al-Pt alloys have been widely used in TBCs and it has been found that addition of platinum greatly improves the oxidation resistance of the coatings. The mechanisms behind this benefit, however, are not well understood. For this reason, the oxidation behaviour of four γ/γ’ Ni-20Al-xPt (x= 0, 5, 10 and 15 at. %) alloys at 1150 °C is studied and compared in terms of oxide spallation, oxide microstructure and growth, residual stress in the oxide scale and oxide/alloy interface morphology. The progressive increase of platinum addition into the alloys results in (1) greater resistance to oxide spallation, (2) reduction in oxidation of nickel, (3) lower stresses in the α-Al2O3 scale and (4) more planar oxide/alloy interfaces. It is found that the selective oxidation of aluminium promoted by platinum plays a central role in the evolution of the oxidation behaviour of the alloys. Surface rumpling of a NiCoCrAlY bond coat deposited on a Ni-base superalloy during cyclic oxidation at 1150 °C is studied. The extent of rumpling is found to depend on thermal history, coating thickness and exposure atmosphere. While the coating surface progressively roughens with cyclic oxidation, bulk NiCoCrAlY alloys with the same nominal composition show a much less tendency to rumple under the same thermal cycling condition. The coatings, especially the thin ones, experience substantial degradation (e.g. β to γ phase transformation and exhaustion of yttrium) induced by oxidation and coating/substrate interdiffusion during thermal exposure. The observations together suggest that rumpling is driven by the lateral growth of the thermally grown oxide (TGO) and the coating deforms in compliance with the TGO. While the dependence of rumpling development on experimental conditions is generally in agreement with the prediction of the existing model, it is suggested that the degradation of the NiCoCrAlY coating and its dependence on coating thickness need to be taken into consideration when predicting the rumpling development of NiCoCrAlY coatings. The residual stresses in a NiCoCrAlY bond coat deposited on a Ni-base superalloy are studied by X-ray diffraction using the sin2Ψ technique. The stresses at room temperature are found to be tensile; they first increase and then decrease with oxidation time. The stress develops and builds up upon cooling, predominantly within the temperature range from 1150 °C to 600 °C. Due to the limited penetration depth into the bond coat, the X-ray only probes the stress in a thin surface layer consisting of a single γ-phase formed through aluminium depletion during oxidation. Above 600 °C, the volume fraction of the β-phase in the bond coat increases with decreasing temperature. The mechanisms of stress generation in the coating are examined and discussed based on experiments designed to isolate the contribution of possible stress generation factors. It is found that the measured bond coat stresses are mainly induced by the volume change of the bond coat associated with the precipitation of the β-phase upon cooling.

No description available.

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.

No description available.

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.

Les revêtements barrières thermiques sont de plus en plus employés dans l'industrie afm d'accroître lesperformances thermiques et de diminuer les émissions de NOx des turbines aéronautiques et de générationd'électricité. Au cours de cette étude, le système constitué d'un substrat en superalliage base nickelmono cristallin AMI, d'une sous-couche NiAlPt et d'une barrière thermique en zircone yttriée élaborée par voieEBPVD (Electron Bearn Phase Vapor Deposition) a été étudié. Le but de cette thèse était de caractériser lesmodes d'endommagements en conditions représentatives de celles rencontrées en service sur le profil des aubesde turbines hautes pressions. Pour pallier au manque de données dans la littérature ouverte, des essais à différentsniveaux de température et de chargement mécanique ont été réalisés avec pour objectif de découpler les diversmodes d'endommagement qui entrent en jeu dans la ruine des barrières thermiques. Ces essais mécaniques detype fluage isotherme, fluage cyclé thermiquement, fatigue oligocyclique et fatigue avec temps de maintien ontété réalisés dans la gamme de température 950°C-1200°C avec des moyens d'essais classiques de laboratoire. Ilsont permis de mettre en évidence l'impact du chargement mécanique sur la cinétique de croissance de la couched'alumine inter faciale à llOO°C(notamment en fluage) mais aussi le délaminage à l'interface sous-coucheinterne/sous-couche externe en conditions de forte vitesse de déformation viscoplastique du substrat et le rôlemajeur de l'interdiffusion entre sous-couche et substrat sur la durée de vie en fatigue oligocyclique à 950°C. Desessais technologiques ont aussi été réalisés sur le banc MAATRE. / Thermal barrier coatings are widely used in the industry to enhance thermal performances and to decrease NOxemissions of aeronautic gas and electricity generating turbines. During this study, the system consisting of asingle crystal nickel based superalloy AMI (substrate), of a NiAIPt bondcoat and an yttria-stabilized zirconiatopcoat made by columnar EBPVD (Electron Bearn Phase Vapor Deposition) was studied. The aim ofthis thesiswas to characterize the damage mechanisms in conditions representative of those encountered in service on theprofile of high pressure turbine blades. To overcome the lack of data in the open literature, tests at differentlevels oftemperature and mechanicalloading has been made with the aim to decouple the various damage modesthat are involved in degradation of thermal barrier coatings. Thermomechanical testings such as isothermal creeptests, thermally cycled creep tests, low cycle fatigue with or without holding time have been made in thetemperature range 950°C-1200°C with conventional laboratory testing methods. They allowed to demonstratethe impact of the mechanical loading on the oxidation kinetics of the thermally grown oxide at IIOO°C(particularly during creep tests) but also a new delamination mode in the bond coat (near process defaults) in caseof high viscoplastic deformation rate of the substrate and the major role of interdiffusion between bondcoat andsubstrate on the low cyclic fatigue lifetime at 950°C. Finally, technological testings have also been realized onthe MAA TRE test bench.

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.