Indiana University-Purdue University Indianapolis (IUPUI) / Thermal barrier coatings (TBCs) have been extensively employed as thermal protection in hot sections of gas turbines in aerospace and power generation applications. However, the fabrication of TBCs still needs to improve for better coating quality, such as achieving coating thickness' uniformity. However, several previous studies on the coating thickness prediction and a systematic understanding of the thickness evolution during the deposition process are still missing. This study aims to develop high-fidelity computational models to predict the coating thickness on complex-shaped components. In this work, two types of models, i.e., ray-tracing based and heat transfer based, are developed. For the ray-tracing model, assuming a line-of-sight coating process and considering the shadow effect, validation studies of coating thickness predictions on different shapes, including plate, disc, cylinder, and three-pin components. For the heat transfer model, a heat source following the Gaussian distribution is applied. It has the analogy of the governing equations of the ray-tracing method, thus generating a temperature distribution similar to the ray intensity distribution in the ray-tracing method, with the advantages of high computational efficiency. Then, using a calibrated conversion process, the ray intensity or the temperature profile are converted to the corresponding coating thickness. After validation studies, both models are applied to simulate the coating thickness in a rotary turbine blade. The results show that the simulated validation cases are in good agreement with either the experimental, analytical, or modeling results in the literature. The turbine blade case shows the coating thickness distributions based on rotating speed and deposition time. In summary, the models can simulate the coating thickness in rotary complex-shaped parts, which can be used to design and optimize the coating deposition process.

This thesis deals with the low cycle fatigue nickel-based superalloy IN713LC with applied TBC barrier at high temperature. The theoretical part is divided into four sections. The first one focuses on description of fatigue damage. The second one provides the basic characteristics of nickel-based superalloys. The third section describes the use of the surface layers - diffusion layers and thermal barriers and the fourth section deals with the influence of these layers on fatigue properties. Experimental part is focused on the evaluation of low cycle fatigue tests and on the explanation of the mechanisms of initiation and propagation of fatigue cracks. For the experimental part, fatigue samples were prepared by vacuum precision investment casting. TBC barrier was applied by atmospheric plasma spraying and consists of two sublayers - the lower metallic bond coating type CoNiCrAlY and top ceramic coating type YSZ. Low cycle fatigue tests were conducted under strain control at controlled temperature of 900 ° C. Fractographical analysis of fracture surfaces was carried out by using light and electron microscopy. Effect of applied barrier to fatigue life was determined - the parameters of Manson-Coffin and Basquin curve. A cyclic stress-strain curve was also obtained. The curves softening / hardening and number of transit cycles were determined. The obtained parameters and values from fatigue tests were compared with available data from fatigue tests of superalloy IN713LC without the layer, as applied AlSi type diffusion layer, at high temperatures. The initiation site on the fracture surfaces was determined within the fractographic evaluation and the influence of the layer on the initiation and propagation of fatigue cracks was discussed. A helpful tool was the assessment of longitudinal sections using scanning electron microscopy.

<p> Environmental Barrier Coatings (EBCs) protect ceramic gas turbine engine components from corrosion by high temperature water vapor, but the coatings often form complex metastable microstructures upon plasma spray deposition. In ytterbium disilicate (YbDS) and its yttrium-doped counterpart (Y/YbDS), two coatings compatible with SiC/SiC parts, plasma spray forms a largely cracked, mechanically weak amorphous phase comprising up to ~80% of the coating’s volume. Therefore, the coatings must undergo a pre-service heat treatment to crystallize into stable phases and heal cracks. During the treatment, however, interplay between thermal expansion and crystallization contraction can cause vertical cracks which expose the component to the corrosive atmosphere. Remedial treatments with long, high temperature holds (~1300 ºC) can both crystallize the coating and heal existing cracks. However, these temperatures cause unnecessary grain growth that reduces the structural integrity of the coating over its lifetime.</p> <p>Here we propose an alternate heat treatment informed by experiments and modelling that removes metastable phases, heals cracks, and reduces time at temperature to prevent significant grain growth. First, we determine crystallization and phase change kinetics by applying the Ozawa-Flynn-Wall and Vyazovkin kinetic methods to differential scanning calorimetry (DSC) data. Next, we track locations and microstructural effects of phase evolution using correlative Raman spectroscopic mapping, scanning electron microscopy (SEM), and X-Ray diffraction (XRD). We interpret the formation of three distinct phases – a major phase of stable β-YbDS, and minor phases of stable Χ2-YbMS and metastable α-YbDS – within the existing framework of kinetic theory and quantify differences in their transformations between YbDS and Y/YbDS. We find that cracks in the coating heal through the crystallization of the amorphous phase and the transformation of the metastable phase although the mechanisms remain unclear. Each phase transformation causes a bulk volumetric change which we measure using dilatometry and use to calculate delamination stresses during a simulated heat treatment. Lastly, we determine the viability of our heat treatment compared to the industry standard.</p>

A detailed understanding of failure mechanisms in thermal barrier coatings (TBCs) can help develop reliable and durable TBCs for advanced gas turbine engines. One of the characteristics of failure in electron beam physical vapor deposited (EB-PVD) TBCs is the development of instability, named rumpling, at the interface between (Ni, Pt)Al bond coat and thermally grown oxide (TGO). In this study, thermal cycling at 1100°C with 1 hr dwell time was carried out on 25.4mm disc specimens of TBCs that consisted of EB-PVD coated ZrO2-7wt. %Y2O3, (Pt,Ni)Al bond coat, and CMSX-4 Ni-based superalloy. At specific fraction of lifetime, TBCs were examined by electron microscopy and photostimulated luminescence (PL). Changes in the average compressive residual stress of the TGO determined by PL and the magnitude of rumpling, determined by tortuosity from quantitative microstructural analyses, were examined with respect to the furnace thermal cyclic lifetime and microstructural evolution of TBCs. The combination of elastic strain energy within the TGO and interfacial energy at the interface between the TGO and the bond coat was defined as the TGO energy, and its variation with cyclic oxidation time was found to remain approximately constant ~135J/m2 during thermal cycling from 10% to 80% thermal cyclic lifetime. Parametric study at ~135J/m2 was performed and variation in residual stress with rumpling for different oxide scale thicknesses was examined. This study showed that the contribution of rumpling in residual stress relaxation decreased with an increase in TGO thickness. High pressure turbine blades serviced for 2843 hours and in the as coated form were also examined using electron microscopy and photostimulated luminescence. The difference in iv residual stress values obtained using PL on the suction and pressure sides of as-coated turbine blade were discussed. The presence of a thick layer of deposit on the serviced blade gave signals from stress free α-Al2O3 in the deposit, not from the TGO. The TGO growth constant data from the disc-shape TBCs, thermally cycled at 1100°C, and studies by other authors at different temperatures but on similar EB-PVD coated TBCs with (Pt, Ni)Al bond coat and CMSX-4 Nibased superalloy were used to determine the temperature profile at the YSZ/bond coat interface. The interfacial temperature profiles of the serviced blade and the YSZ thickness profile were compared to document the variable temperature exposure at the leading edge, trailing edge, suction and the pressure side.

In modern turbine engines for propulsion and energy generation, thermal barrier coating (TBCs) protect hot-section blades and vanes, and play a critical role in enhancing reliability, durability and operation efficiency. In this study, thermal cyclic lifetime and microstructural degradation of electron beam physical vapor deposited (EB-PVD) Yttria Stabilized Zirconia (YSZ) with (Ni,Pt)Al bond coat and Hf- and/or Y- modified CMSX-4 superalloy substrates were examined. Thermal cyclic lifetime of TBCs was measured using a furnace thermal cycle test that consisted of 10-minute heat-up, 50-minute dwell at 1135C, and 10-minute forced-air-quench. TBC lifetime was observed to improve from 600 cycles to over 3200 cycles with appropriated Hf- and/or Y alloying of CMSX-4 superalloys. This significant improvement in TBC lifetime is the highest reported lifetime in literature with similar testing parameters. Beneficial role of reactive element (RE) on the durability of TBCS were systematically investigated in this study. Photostimulated luminescence spectroscopy (PL) was employed to non-destructively measure the residual stress within the TGO scale as a function of thermal cycling. Extensive microstructural analysis with emphasis on the YSZ/TGO interface, TGO scale, TGO/bond coat interface was carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning electron microscopy (STEM) as a funcion of thermal cycling including after the spallation failure. Focused ion beam in-situ lift-out (FIB-INLO) technique was employed to prepare site-specific TEM specimens. X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) were also employed for phase identification and interfacial chemical analysis. While undulation of TGO/bond coat interface (e.g., rumpling and ratcheting) was observed to be the main mechanism of degradation for the TBCs on baseline CMSX-4, the same interface remained relatively flat (e.g., suppressed rumpling and ratcheting) for durable TBCs on Hf- and/or Y-modified CMSX-4. The fracture paths changed from the YSZ/TGO interface to the TGO/bond coat interface when rumpling was suppressed. The geometrical incompatibility between the undulated TGO and EB-PVD YSZ lead to the failure at the YSZ/TGO interface for TBCs with baseline CMSX-4. The magnitude of copressive residual stress within the TGO scale measured by PL gradually decreased as a function of thermal cycling for TBCs with baseline CMSX-4 superalloy substrates. This gradual decrease corrsponds well to the undulation of the TGO scale that may lead to relaxation of the compressive residual stress within the TGO scale. For TBCs with Hf- and/or Y-modified CMSX-4 superalloy substrates, the magnitude of compressive residual stress within the TGO scale remained relatively constant throughout the thermal cycling, although PL corresponding to the stress-relief caused by localized cracks at the TGO/bond coat interface and within the TGO scale was observed frequently starting 50% of lifetime. A slightly smaller parabolic growth constant and grain size of the TGO scale was observed for TBCs with Hf- and/or Y- modified CMSX-4. Small monoclinic HfO2 precipitates were observed to decorate grain boundaries and the triple pointes within the alpha-Al2O3 scale for TBCs with Hf- and/or Y-modified CMSX-4 substrates. Segregation of Hf/Hf4+ at the TGO/bond coat interfaces was also observed for TBCs with Hf- and/or Y-modified CMSX-4 superalloys substrates. Adherent and pore-free YSZ/TGO interface was observed for TBCs with Hf- and/or Y-modified CMSX-4, while a significant amount of decohesion at the YSZ/TGO interface was observed for TBCs with baseline CMSX-4. The beta-NiAl(B2) phase in the (Ni,Pt)Al bond coat was observed to partially transform into gama prime-Ni3Al (L12) phase due to depletion of Al in the bond coat during oxidation. More importantly, the remaining beta-NiAl phase transformed into L10 martensitic phase upon cooling even though there was no significant difference in these phase transformations for all TBCs. Results from these microstructural observations are documented to elucidate mechanisms that suppress the rumpling of the TGO/bond coat interface, which is responsible for superior performance of EB-PVD TBCs with (Ni,Pt)Al bond coat and Hf- and/or Y-modified CMXS-4 superalloy.

<p>Thermal barrier coatings (TBCs) made of low-thermal-conductivity ceramic topcoat, metallic bond coat and metallic substrate, have been extensively used in gas turbine engines for thermal protection. Recently, additive manufacturing (AM) or 3D printing techniques have emerged as promising manufacturing techniques to fabricate engine components. The motivation of the thesis is that currently, application of TBCs on AM’ed metallic substrate is still in its infancy, which hinders the realization of its full potential.</p> <p>The goal of this thesis is to understand the processing-structure-property relationship in thermal barrier coating deposited on AM’ed superalloys.</p> <p>The APS method is used to deposit 7YSZ as the topcoat and NiCrAlY as the bond coat on TruForm 718 substrates fabricated using the direct metal laser sintering (DMLS) method. For comparison, another TBC system with the same topcoat and bond coat is deposited using APS on wrought 718 substrates. For thermomechanical property characterizations, thermal cycling, thermal shock (TS) and jet engine thermal shock (JETS) tests are performed for both TBC systems to evaluate thermal durability. Microhardness and elastic modulus at each layer and respective interfaces are also evaluated for both systems. Additionally, the microstructure and elemental composition are thoroughly studied to understand the cause for better performance of one system over the other.</p> <p>Both TBC systems showed similar performance during the thermal cycling and JETS test but TBC systems with AM substrates showed enhanced thermal durability especially in the case of the more aggressive thermal shock test. The TBC sample with AM substrate failed after 105 thermal shock cycles whereas the one with wrought substrate endured a maximum of 85 cycles after which it suffered topcoat delamination. The AM substrates also demonstrated an overall higher microhardness and elastic modulus except for post thermal cycling condition where it slightly underperformed. This study successfully demonstrated the use of AM built substrates for an improved TBC system and validated the enhanced thermal durability and mechanical properties of such a system.</p> <p>A modified YSZ TBC architecture with an intermediate Ti3C2 MXene layer is proposed to improve the interfacial adhesion at the topcoat/bond coat interface to improve the thermal durability of YSZ</p> <p>12</p> <p>TBC systems. First principles calculations are conducted to study the interfacial adhesion energy in the modified and conventional YSZ TBC systems. The results show enhanced adhesion at the bond coat/MXene interface. At the topcoat/MXene interface, the adhesion energy is similar to the adhesion energy between the topcoat and bond coat in a conventional YSZ TBC system.</p> <p>An alternative route is proposed for the fabrication of YSZ TBC on nickel base superalloy substrates by using the SPS technology. SPS offers a one-step fabrication process with faster production time and reduced production cost since all the layers of the TBC system are fabricated simultaneously. Two different TBC systems are processed using the same heating protocol. The first system is a conventional TBC system with 8YSZ topcoat, NiCoCrAlY bond coat and nickel base superalloy substrate. The second system is similar to the first but with an addition of Ti3C2 MXene layer between the topcoat and the bond coat. Based on the first principles study, addition of Ti3C2 layer enhances the adhesion strength of the topcoat/bond coat interface, an area which is highly susceptible to spallation. Further tests such as thermal cycling and thermal shock along with the evaluation of mechanical properties would be carried out for these samples in future studies to support our hypothesis.</p>

No description available.

A haute température, l’interdiffusion entre un superalliage et son revêtement protecteur (ß-NiAl ou ß- NiPtAl) dégrade à la fois la protection contre l’oxydation, par modification de la composition chimique du revêtement, et la microstructure du superalliage (3ième et 4ième générations) par formation de Zones de Réaction Secondaires (SRZ). Le but de cette étude a donc été (1) de développer des barrières de diffusion (BD) constituées d’une dense précipitation de phases a-W après traitement sous vide (BD simple) ou chromisation en phase vapeur (BD enrichie en chrome) (2) de mettre au point une méthode pour en étudier l’efficacité. Des mesures de concentration chimique (à partir de cartographies spectrales EDS), couplées à des ajustements des comportements en oxydation cyclique en utilisant le modèle « p-kp », et le développement d’un modèle « p-kp-ß » ont permis de montrer l’efficacité de la BD selon sa composition et la durée de vieillissement. Pour des longues durées de vieillissement, l’efficacité de la BD se réduit par la dissolution des précipités d’a-W dans les phases y’ et y formées à cause de la dégradation des propriétés protectrices du revêtement ß NiPtAl (augmentation de l’écaillage de l’oxyde formé et de la cinétique d’oxydation). Plusieurs causes probables de cette dégradation ont pu être déterminées, soit dues aux procédés (pollution au soufre) soit liées à la mise en place de la BD : augmentation de la transformation martensitique, enrichissement en tungstène et présence de précipités d’alpha chrome. Enfin, il a été montré que si l’initiation des SRZ est modifiée par l’ajout de la BD, leur cinétique de propagation ne l’est pas et est essentiellement dépendante de la composition de l’alliage. Un modèle de propagation des SRZ décrivant les évolutions chimiques locales de part et d’autres de l’interface « SRZ / superalliage » a été proposé. L’ajout de chrome à la BD permet d’inhiber la formation des SRZ, une couche riche en phases TCP remplace alors la SRZ. / At high temperature, interdiffusion between a superalloy and its protective coating (ß-NiAl or ß- NiPtAl) degrades the oxidation protection by modifying the chemical composition of the coating. It also degrades the 3rd et 4th generation superalloy microstructure due to the formation of Secondary Reaction Zones (SRZ). As a consequence, the aim of this study was (1) to develop diffusion barriers (DB) composed of a dense precipitation of a-W phases after a thermal treatment under vacuum (simple DB) or a vapour phase chromisation (Cr enriched DB), (2) to develop a method for quantifying the DB efficiency. Chemical concentration measurements (with EDS spectral maps) coupled with the « p-kp » modelling of the cyclic oxidation kinetics, and the development of the model « p-kp-ß » have permitted to study DB efficiency as a function of its composition and its high temperature ageing. For long ageing duration, the efficiency of the DB is reduced. Indeed, it is shown that the DB degrades the protection character of the ß-NiPtAl by increasing the oxide scale spallation and of its growth kinetic. This, in turns, accelerates the ß to y’ and y phases transformation and then increases the a-W precipitates dissolution. Some likely causes of this degradation have been determined, either due to the process (sulphur pollution) or intrinsic of the DB addition (increase of the martensitic transformation, enrichment in tungsten and a-Cr formation in the coating). Finally, it has been proved that DB addition modifies the SRZ initiation but not their propagation kinetic, which only depends on the superalloy local composition. A SRZ propagation model which describes local chemical evolutions on both sides of the « SRZ / superalloy » interface was proposed. The addition of chromium to the DB permits to inhibit the SRZ formation. In this case, a layer rich in TCP platelets replaces the SRZ.

Les composites à matrice céramique sont utilisés dans les moteurs aéronautiques en raison de leur stabilité à haute température et de leurs propriétés mécaniques. Cependant, quand ils sont soumis à des environnements sévères (haute température, haute pression, environnement oxydant et humide), ils s'oxydent et se dégradent dû à la volatilisation de la silice protectrice formée en surface par oxydation du CMC. Par conséquent, pour augmenter la durée de vie de ces matériaux, il est nécessaire d'appliquer une protection externe contre la corrosion. Ceci constitue l'objectif de ma thèse. La démarche expérimentale a été la suivante : (i) identification des matériaux de revêtement à étudier ; (ii) validation du choix des matériaux par étude de leur stabilité structurale et de leurs compatibilités chimique et thermomécanique avec le substrat ; (iii) étude de la stabilité des matériaux de revêtement sous atmosphère corrosive et enfin (iv) comportement des revêtements sur composites. / The ceramic matrix composites can be used in aeronautic engines due to their high temperature stability and their mechanical properties. However, under a corrosive environment, an oxidation and then a recession of the CMC occured because of the volatilization of the silica scale formed at the surface of the composite. Consequently, in order to increase the lifetime of such materials, a external protection against corrosion is required. This is the aim of my Ph-D thesis. The experimental approach is the following : (i) identification of the coating materials ; (ii) validation of the selected materials by studying their structural stability and their chemical and thermomechanical compatibilities with the substrate ; (iii) determination of the thermal stability of the materials under a corrosive environment and (iv) behaviour of the coatings onto the CMC.

Coatings are frequently applied on gas turbine components inorder to restrict surface degradation such as corrosion andoxidation of the structural material or to thermally insulatethe structural material against the hot environment, therebyincreasing the efficiency of the turbine. However, in order toobtain accurate lifetime expectancies and performance of thecoatings system it is necessary to have a reliableunderstanding of the mechanical properties and failuremechanisms of the coatings. In this thesis, mechanical and fracture behaviour have beenstudied for a NiAl coating applied by a pack cementationprocess, an air-plasma sprayed NiCoCrAlY bondcoat, a vacuumplasma-sprayed NiCrAlY bondcoat and an air plasma-sprayed ZrO2+ 6-8 % Y2O3topcoat. The mechanical tests were carried out ata temperature interval between room temperature and 860oC.Small punch tests and spherical indentation were the testmethods applied for this purpose, in which existing bending andindentation theory were adopted for interpretation of the testresults. Efforts were made to validate the test methods toensure their relevance for coating property measurements. Itwas found that the combination of these two methods givescapability to predict the temperature dependence of severalrelevant mechanical properties of gas turbine coatings, forexample the hardness, elastic modulus, yield strength, fracturestrength, flow stress-strain behaviour and ductility.Furthermore, the plasma-sprayed coatings were tested in bothas-coated and heat-treated condition, which revealedsignificant difference in properties. Microstructuralexamination of the bondcoats showed that oxidation with loss ofaluminium plays an important role in the coating degradationand for the property changes in the coatings. Keywords:small punch test, miniaturised disc bendingtests, spherical indentation, coatings, NiAl, APS-NiCoCrAlY,VPS-NiCrAlY, mechanical properties