Propiedades de transporte en recubrimientos cerámicos utilizados en turbinas a gas

Abstract

Los sistemas de barrera térmica TBCs (Thermal Barrier Coatings) son ampliamente utilizados en los componentes de las turbinas a gas sometidos a altas temperaturas, con el fin de reducir la transferencia de calor hacia el sustrato. El uso de estos recubrimientos ha permitido incrementar la eficiencia de las turbinas a gas como consecuencia de mayores temperaturas de operación. Además éstos proveen protección contra los agentes corrosivos y erosivos a altas temperaturas. Un sistema TBC convencional está compuesto por cuatro capas: el recubrimiento cerámico TC (Top Coat) de ZrO2 estabilizado con Y2O3 el cual está en contacto directo con los gases de combustión y provee aislamiento a las demás capas. Este recubrimiento es aplicado sobre una capa metálica conocida como capa de anclaje BC (Bond Coat), la cual, a su vez, es depositada sobre un sustrato base níquel. La BC proporciona adherencia entre la TC y el sustrato, además actúa como una fuente de aluminio para la formación de una cuarta capa de Al2O3 en la interface BC/TC. Esta capa de óxido es conocida como TGO (thermally grown oxide) se forma y crece como consecuencia de la difusión del aluminio en la BC y el paso de oxígeno en la TC y su posterior reacción. La formación y el crecimiento de esta capa juega un papel determinante en la vida útil de las turbinas, ya que durante la operación ésta puede alcanzar espesores críticos entre 5 a 7 a partir de los cuales la delaminación de la TC comienza. En este trabajo, TBCs depositados mediante APS (Air Plasma Spray) fueron estudiados. Un modelo en dos dimensiones acoplado de transferencia de masa y de calor fue desarrollado el cual se consideró la variación temporal y espacial de las propiedades térmicas y difusionales. Este modelo numérico fue solucionado a través del método de Funciones de Base Radial RBF (Radial Basis Functions). A partir de este modelo, se pudo observar una reducción de alrededor de 300ºC entre la superficie del cerámico y la interface TGO/TC con un recubrimiento de 120 μ. El análisis de la difusión y reacción del aluminio y el oxígeno fue realizado utilizando argumentos de escala como la relación de difusividades y número de Damköhler; a partir de éstos, se pudo concluir que la reacción O2-Al es varios órdenes de magnitud superior a la difusión y que el crecimiento de la TGO es gobernada por la difusión de aluminio. De los resultados numéricos, se pudo observar que esta capa presenta un crecimiento parabólico, es decir, la TGO crece en función del tiempo al exponente ½. Para validar estos resultados, se realizaron medidas experimentales en muestras de TBC tratadas térmicamente a 1100ºC con tiempos de sostenimiento entre 1 y 1700 horas. Experimentalmente, se pudo ver que la TGO alcanza un espesor crítico después de 400 horas de operación a 1100ºC; sin embargo, para este tiempo no se observó delaminación de la TC ni grietas cerca o dentro de la TGO. Los cambios de fase, composicionales y microestructurales con la temperatura en los TBC fueron también estudiados por medio de técnicas de caracterización como microanálisis químico EDXS (Energy Dispersive X-ray Spectroscopy) y WDXS (Wavelenth Dispersive X-ray Spectroscopy), difracción de rayos x (DRX), microscopía electrónica de barrido (SEM), microscopía electrónica de transmisión (TEM) y delatometría (DIL). Estos cambios fueron analizados en relación con las propiedades térmicas y mecánicas. Adicionalmente, debido a que la porosidad en la TC tiene un efecto importante en su conductividad térmica, se utilizó la técnica de análisis de imágenes para cuantificar los cambios en la porosidad con diferentes tiempos de exposición a 1100ºC. Se observó una reducción de alrededor del 4% en la fracción volumétrica de la porosidad después de 1700 horas. / Abstract. Thermal barrier coatings (TBCs) are commonly used in hot components in gas turbines to reduce the heat transfer to the substrate. They also provide protection against oxidation and erosion at high temperature. The use of TBCs has increased the gas turbines efficiency because of the higher service temperatures that can be reached. A conventional TBC system is comprised of four layers: the TC (top coat) which is a ceramic compound typically formed by ZrO2 – Y2O3 that is in contact with hot combustion gases; it provides thermal isolation to the others layers. The TC is deposited over a metallic bonding layer, the BC (Bond Coat), which is, in turn, deposited onto the nickel base substrate. The BC gives adherence between the TC and the substrate and acts as an aluminum source for the formation of the TGO layer in the interface BC/TC. The TGO is an oxide layer of Al2O3 which forms and growths because of the aluminum and oxygen diffusion in the BC and TC respectively and their subsequent reaction.The TGO formation and growth is a fundamental factor in the turbine’s lifetime, since during operation this layer can reach a critical thickness between 5 to 7, where the delamination of the TC begins to happen. In this research, a ZrO2 – 6% Y2O3 top coat deposited by APS (Air Plasma Spray), NiCoCrAlY type bond coat, Hastelloy X substrate and Al2O3 TGO were studied. A two-dimensional coupled model of heat and mass transfer was developed considering the temporal and spatial dependence of diffusion and thermal properties. This numerical model was solved through Radial Basis Function (RBF) collocation method. From this model, a reduction of around 300ºC between TC surface and TGO/TC interface is achieved using a TC with 120. Analysis of Aluminum and Oxygen diffusion - reaction using simple scaling arguments, such as diffusivities relation and Damköhler number, indicates that O2-Al reaction is several orders of magnitude higher than diffusion and the TGO growth is governed by aluminum diffusion. From numerical solution, it can be seen that the TGO exhibit a parabolic growth law, e.g. TGO grows with dependence of time as. To validate these results, experimental measurements were carried out on TBC samples thermally treated at 1100ºC and soaking times from 1 to 1700 hours. Experimentally, it was observed that critical TGO thickness is reached after 400 hours at 1100ºC. The phase, compositional and microstructural changes in the TBC system with temperature and soaking time were also studied by means of characterization techniques such as Microanalysis EDXS (Energy Dispersive X-ray Spectroscopy) and WDXS (Wavelenght Dispersive X-ray Spectroscopy), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and dilatometry (DIL). These changes were analyzed in relation with thermal and mechanical properties. In addition, since porosity in the TC has a mayor effect in its thermal conductivity, an analysis image approach was used to quantify porosity changes with different holding times at 1100ºC. A reduction of around 4% in porosity volume fraction after 1700 hours was observed

Abstract: Thermal barrier coatings (TBCs) are commonly used in hot components in gas turbines to reduce the heat transfer to the substrate. They also provide protection against oxidation and erosion at high temperature. The use of TBCs has increased the gas turbines efficiency because of the higher service temperatures that can be reached. A conventional TBC system is comprised of four layers: the TC (top coat) which is a ceramic compound typically formed by ZrO2 – Y2O3 that is in contact with hot combustion gases; it provides thermal isolation to the others layers. The TC is deposited over a metallic bonding layer, the BC (Bond Coat), which is, in turn, deposited onto the nickel base substrate. The BC gives adherence between the TC and the substrate and acts as an aluminum source for the formation of the TGO layer in the interface BC/TC. The TGO is an oxide layer of Al2O3 which forms and growths because of the aluminum and oxygen diffusion in the BC and TC respectively and their subsequent reaction.The TGO formation and growth is a fundamental factor in the turbine’s lifetime, since during operation this layer can reach a critical thickness between 5 to 7 __, where the delamination of the TC begins to happen. In this research, a ZrO2 – 6% Y2O3 top coat deposited by APS (Air Plasma Spray), NiCoCrAlY type bond coat, Hastelloy X substrate and Al2O3 TGO were studied. A two-dimensional coupled model of heat and mass transfer was developed considering the temporal and spatial dependence of diffusion and thermal properties. This numerical model was solved through Radial Basis Function (RBF) collocation method. From this model, a reduction of around 300ºC between TC surface and TGO/TC interface is achieved using a TC with 120__. Analysis of Aluminum and Oxygen diffusion - reaction using simple scaling arguments, such as diffusivities relation and Damköhler number, indicates that O2-Al reaction is several orders of magnitude higher than diffusion and the TGO growth is governed by aluminum diffusion. From numerical solution, it can be seen that the TGO exhibit a parabolic growth law, e.g. TGO grows with dependence of time as ____. To validate these results, experimental measurements were carried out on TBC samples thermally treated at 1100ºC and soaking times from 1 to 1700 hours. Experimentally, it was observed that critical TGO thickness is reached after 400 hours at 1100ºC. The phase, compositional and microstructural changes in the TBC system with temperature and soaking time were also studied by means of characterization techniques such as Microanalysis EDXS (Energy Dispersive X-ray Spectroscopy) and WDXS (Wavelenght Dispersive X-ray Spectroscopy), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and dilatometry (DIL). These changes were analyzed in relation with thermal and mechanical properties. In addition, since porosity in the TC has a mayor effect in its thermal conductivity, an analysis image approach was used to quantify porosity changes with different holding times at 1100ºC. A reduction of around 4% in porosity volume fraction after 1700 hours was observed. Thermal Barrier Coating (TBC), Efficiency, Lifetime, Characterization, Heat Transfer, Diffusion, Radial Basis Functions