Thermal cycling behaviour of thermal barrier coating systems based on first- and fourth-generation Ni-based superalloys

This study deals with the cyclic oxidation behaviour of thermal barrier coating systems. The systems consist of an yttria-stabilised zircona ceramic top coat deposited by EB-PVD, a b-(Ni,Pt)Al bond coat and a Ni-based superalloy. Two different superalloys are studied: a first-generation one and a fourthgeneration one containing Re, Ru and Hf. The aim of this work is to characterise the microstructural evolution of those systems and to correlate it to their resistance to spallation. Thermal cycling is carried out at 1100°C in laboratory air, with the number of cycles ranging between 10 and 1000. Each cycle consists of a 1 h dwell followed by forced-air cooling for 15 min down to room temperature. Among the main results of this work, it is shown that the MCNG-based system is significantly more resistant to spallation than the AM1-based one. Up to 50 cycles, both systems exhibit similar oxidation rate and phase transformations but major differences are observed after long-term ageing. In particular, a Ru-rich b-phase is formed in the bond coat of the MCNG-based system while the AM1- based one undergoes strong rumpling of the TGOybond coat interface due to the loss of the thermal barrier coating

Vieillissement des systèmes barrière thermique : transformation de phases, oxydation et effet du soufre sur l'adhérence

Co-encadrement de la thèse : Cécilie DuhamelThèse confidentielle jusqu'au 1er février 2013A thermal barrier coating system, used for turbine engine blade, is a multilayer system with a superalloy substrate. This substrate is covered by an alumino-former bond coat, which protects the substrate from oxidation by creating an alumina scale, initiated during the process. Finally a ceramic coating, the thermal barrier coating (TBC), covers the bond coat in order to decrease the work temperature. The ceramic, made of yttrium stabilized zirconia, is permeable to oxygen, leading to the development of the alumina scale all along the system life time. The system life time is limited by the spallation of the TBC, which occurs because of interfacial damage, located in particular at the bond coat/oxide interface. The adherence loss is sensitive to the chemical composition and the microstructure of the system. The aim of this work is to correlate the damage mechanisms to the microstructural and chemical evolution of the TBC system after oxidation treatments. For this purpose, the behaviour of TBC systems based on a AM1 or MCNG substrate covered by a NiAlPt bond coat, with and without TBC, has been studied during isothermal and cyclic oxidation. In addition, the influence of the oxide scale grown after short-term oxidation treatment on the long-term adherence has been tested through a pre-oxidation study. Finally, the distribution of impurities, like sulfur and reactive elements like hafnium has been followed after several oxidation treatments. This work shows that the systems based on MCNG substrate are more resistant to spallation than the ones based on AM1 substrate. This behaviour is explained by the lowest oxidation kinetic of the MCNG systems, probably due to the reactive elements presence. It has also been revealed that the formation of transient oxide during the first stages of oxidation is not detrimental for the long-term adherence. A correlation between impurity segregation at the bond coat / oxide interface and spallation has been evidenced.Les systèmes barrières thermiques (BT), utilisés dans les turbines aéronautiques, sont des systèmes multicouches composés d'un substrat en superalliage monocristallin, d'une sous-couche d'alliage alumino-formeur sur laquelle se forme une couche d'alumine, initiée pendant le procédé de fabrication. Le tout est revêtu d'une couche en zircone, la barrière thermique, qui permet de diminuer la température de travail du substrat. Cette couche, perméable à l'oxygène, conduit au développement de l'oxyde tout au long de la vie du système. Les systèmes BT sont soumis à des contraintes thermomécaniques qui créent des endommagements, en particulier aux interfaces, pouvant mener à l'écaillage de la BT. Ces endommagements dépendent de la composition et de la microstructure des différents constituants du système. L'objectif de cette thèse est de corréler les évolutions microstructurales dans la sous-couche et dans l'oxyde à l'adhérence des systèmes BT. Pour ce faire, le vieillissement de systèmes de superalliages AM1 et MCNG revêtus d'une sous-couche NiAlPt, avec et sans barrière thermique, a été suivi sous sollicitations isothermes et cycliques à 1100°C. L'effet d'une étape de pré-oxydation sur la durée de vie à long terme après oxydation isotherme de ces systèmes a été étudié pour différents états de surface initiaux. Ce travail a permis de confirmer l'effet du type de sollicitation et de la présence de la BT sur les mécanismes d'endommagement et de mettre en évidence la meilleure résistance à l'écaillage des systèmes MCNG comparés aux systèmes AM1. Il a également montré que la formation d'alumine de transition au cours des premiers stades d'oxydation ne semblait pas néfaste pour l'adhérence du système. Enfin, la détection et la localisation par SIMS d'impuretés telles que le soufre et le carbone et d'éléments réactifs tels que le hafnium ont révélé le rôle prépondérant joué par ces éléments sur la durée de vie des systèmes BT

Adhesion of thermal barrier coatings systems after long term oxidation : influence of preoxidation temperature and surface state of the bond coat

International audienceThe aim of this study was to determine the effect of the pre-oxidation temperature and surface state of the bond coat on the microstructure of the oxide scale formed at the first stages of oxidation and on its adhesion after subsequent long-term oxidation (1000 h) at 1100°C. Short-term isothermal oxidations of 1 h were performed at several temperatures (900°C, 1100°C) on a Pt-modified NiAl bond coat with two different surface states (as-aluminized or grit-blasted) deposited on a superalloy (AM1). The adherence of the different systems after an additional isothermal ageing treatment in air at 1100°C for 1000 h, was compared in order to deduce the initial oxide scale leading to the best resistance to spallation. Characterization was performed using SEM and analytical TEM. The crystalline structure and the morphology of the as-formed oxide scale were studied as a function of the different parameters

Effect of the superalloy composition on the isothermal oxidation behaviour of TBC systems

International audienceThe isothermal oxidation behaviour of TBC systems based on a first-generation superalloy, AM1 and a fourth-generation one, MCNG, was investigated and compared. The main difference between both is the addition of Re, Ru and Hf in the MCNG composition. The systems consisted of a Pt-modified nickel aluminide (Ni,Pt)Al bond coat deposited on the superalloy and with or without a yttria-stabilized zirconia ceramic top coat. Isothermal oxidations were performed at 1,100 °C in synthetic air for 10, 50, 500 and 1,000 h. The MCNG-based system exhibited a better spallation resistance than the AM1-based one and a lower oxidation rate. Spallation always occurred at the thermally grown oxide/bond coat interface. For both systems, transformation of the β-phase into the γ′-phase was observed. In the MCNG-based samples, the β-(Ni,Pt)Al was enriched in ruthenium and the secondary reaction zone strongly extends, whereas chromium precipitation occurred in the AM1-based ones

Effect of the superalloy composition on the isothermal oxidation behaviour of TBC systems

International audienc

Effect of material and environmental parameters on the microstructure evolution and oxidation behavior of a Ni-based superalloy coated with a Pt-modified Ni-aluminide

International audienceThe present work, performed on nickel aluminides deposited on a single Ni-based superalloy AM1, focuses on the effect of the following several parameters on the microstructural and chemical changes occurring during isothermal heat treatment at 1100°C for 50h : -oxygen pressure by comparing heat treatment under ambient air (PO2 = 0.2 bar) and under secondary vacuum (PO2 = 0.2x10-6 bar). -cooling rate after isothermal heat treatment by comparing furnace cooling (3°C/min) and water quenching (500°C/min). -Pt addition in the coating by comparing NiAl and NiPtAl coatings. Characterizations were performed using SEM, analytical TEM and electron microprobe analyses. The results show that these parameters have a strong influence on both the microstructural evolution and the oxidation of the thermal barrier coating (TBC) system. Appropriate heat treatments are essential to improve interfacial resistance and increase the durability of TBC systems

Sulfur localization in NiPtAl/superalloy systems after high temperature isothermal oxidation

International audienceSulfur distribution and its evolution after isothermal oxidation have been studied using secondary ion mass spectrometry. In this work, the systems consist of a Ni-based single-crystal superalloy with a (Ni,Pt)Al bond coat. Two superalloys are compared: AM1 and MCNG. The latter contains Hf as a reactive element. Isothermal oxidations are performed at 1,100 °C under synthetic air for 1, 10, 50, 500 and 1,000 h. After oxidation, sulfur is detected both in the internal and external zones of the bond coat, at the bond coat/oxide interface and inside the oxide scale. An increase of the sulfur signal with oxidation time is registered at the bond coat/oxide interface for the MCNG-based systems whereas the opposite trend is observed for the AM1-based ones

Sulfur localization in NiPtAl/superalloy systems after high temperature isothermal oxidation

International audienc