Modélisation de l'interdiffusion et du comportement en oxydation cyclique de superalliages monocristallins à base de nickel revêtus d'une sous-couche γ-γ’ riche en platine. Extension aux systèmes barrière thermique

Les systèmes barrière thermique actuels connaissent une importante dispersion de durées de vie liée principalement aux ondulations de surface du revêtement métallique β-(Ni,Pt)Al provoquant l’écaillage du dépôt céramique. Les revêtements γ-γ’ riches en platine sont étudiés en tant qu’alternative au système actuel. Ce travail de thèse s’est intéressé à l’élaboration des revêtements γ-γ’ riches en platine sur un superalliage à base de nickel, l’AM1 à partir de procédés conventionnels : dépôt électrolytique de platine et aluminisation courte. Les mécanismes de dégradation par oxydation cyclique à 1100°C ont été étudiés sur des systèmes revêtement/AM1 et sur des systèmes barrière thermique. Pour comparaison, trois types de revêtement ont été élaborés : γ-γ’ Pt seul, γ-γ’ Pt+Al et β-(Ni,Pt)Al. Ces essais ont mis en évidence une meilleure tenue à l’oxydation cyclique des systèmes revêtus γ-γ’ Pt+Al comparée aux deux autres systèmes revêtus. L’importance de l’ajout d’aluminium dès l’élaboration sur la tenue à l’oxydation cyclique a été soulignée. La modélisation p-kp a mis en avant une augmentation de la proportion d’écaillage au cours du temps du fait de la dégradation de l’interface métal/oxyde et une augmentation du kp du fait de la formation d’un oxyde à croissance plus rapide. Outre l’oxydation, les phénomènes d’interdiffusion lors des tous premiers instants à haute température ont été étudiés à partir de matériaux modèles (Ni13Al et Ni11Al10Cr) et de revêtements de Pt et/ou de Pt-Ir. Ces essais ont mis en avant la rapide formation de la phase α-NiPtAl, les transformations de phases et les chemins de diffusion à 1100°C dans les systèmes Ni-Al-Pt et Ni-Al-Cr-Pt. L’effet du chrome et de l’iridium sur les cinétiques de diffusion a été évalué. La modélisation de l’interdiffusion a mis en évidence les interactions chimiques entre les espèces et une sursaturation en lacunes dans la zone d’interdiffusion prouvant que l’effet Kirkendall est responsable de la formation des pores. ABSTRACT : TBC systems currently used in aircraft engines with a Pt-modified aluminide coating β-(Ni,Pt)Al show an important lifetime dispersion due to the surface undulations of the bond-coating. This phenomenon called rumpling leads to the ceramic scale spallation and is the most common degradation mechanism. Pt-rich γ-γ’ bond-coatings have been extensively studied for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings. The aim of this work was to fabricate Pt-rich γ-γ’ bond-coatings on a first generation Ni-based superalloy, the AM1. Conventional processes were used as a platinum electroplating and a short aluminizing step. The failure mechanisms occurring by cyclic oxidation at 1100°C were studied on coating/superalloy systems and on TBC systems. Three kinds of coatings were fabricated: Pt-only γ-γ’, Pt+Al γ-γ’ and β-(Ni,Pt)Al. These tests highlighted the best oxidation resistance for the Pt+Al γ-γ’/AM1 systems when compared with the two other systems. Al addition during the coating fabrication is necessary to improve the lifetime. The p-kp modeling results pointed out that the oxide scale spalling probability p increases due to the metal/oxide interface degradation with time. If the spallation increases, a breakaway locally occurs with the formation of a fast-growing oxide explaining the kp progression. The interdiffusion phenomena were also investigated during the first times at high temperature from model alloys (Ni13Al and Ni11Al10Cr) and Pt and/or Pt-Ir coatings. These investigations emphasized the rapid formation of the α-NiPtAl phase, the phase transformations and diffusion paths at 1100°C in the ternary Ni-Al-Pt and quaternary Ni-Al-Cr-Pt systems. Chromium and iridium effect was evaluated on the diffusion kinetics. Interdiffusion modeling highlighted the chemical interactions between the species and a vacancy supersaturation in the interdiffusion zone proving that Kirkendall effect is responsible for void formation

Chromium and iridium effects on the short-term interdiffusion behaviour between Pt rich γ-γ′ bond-coatings and a Ni-Al-Cr alloy

The interdiffusion behaviour of a 5 μm thick layer of Pt deposited by electroplating on a γ-Ni-12Al-10Cr model alloy was studied in order to assess the effect of Cr. Heat treatments were performed for 1 min up to 1 h at 1100 °C under argon. Cr addition increased the uphill diffusion of Al to the surface when compared with Pt/γ-(Ni,Al) systems. Al and Cr had a positive chemical interaction in presence of Pt, as shown by the positive values of the DAlCrNi and DCrAlNi diffusion coefficients determined by modelling. Pt had a negative chemical interaction with Al and with Cr in such a way that Pt decreased their activities. According to the diffusion coefficient values, Pt had a greater influence on the Al activity than on the Cr one. Similarly, 2 μm of Pt and 3 μm of Pt-25Ir were deposited by electroplating on the same model alloy to investigate the effect of Ir. Heat treatments were performed in the same conditions as for Cr. Iridium slowed down the interdiffusion when compared with systems with Pt only. Iridium diffused slower toward the substrate than Pt and a lower Pt + Ir flux toward the substrate was found. As voids formed at the interdiffusion zone/substrate interface due to Kirkendall effect, this lower inward Pt + Ir flux resulted in a lower outward vacancy flux and then Ir reduced Kirkendall voids formation. Moreover, Ir decreased the Pt effect on Al activity by dilution or even gave an opposite contribution to the Pt one. This reduced the uphill diffusion of Al, delaying the α-NiPtAl phase formation. Diffusion paths of each model system were also identified after 15 min at 1100 °C and all highlighted the α-NiPtAl phase formation and its aptitude to be used in TBC systems

Cyclic Oxidation Behavior of TBC Systems with a Pt-Rich γ-Ni+γ′-Ni3Al Bond-Coating Made by SPS

To obtain long-lasting thermal barrier coating (TBC) systems, two types of Pt-rich γ-Ni+γ′-Ni3Al bond-coatings (BC) were fabricated by spark plasma sintering (SPS). The former had the highest possible Pt content (Ni-30Pt-25Al in at.%) while the latter had the highest possible Al level (Ni-28Al-17Pt in at.%). Hf was added as a reactive element. TBCs were fabricated on different superalloys (AM1, René N5 and MCNG) with the aforementioned BCs and with zirconia stabilized with yttria top coats made by SPS or electron beam physical vapor deposition (EBPVD). The cyclic oxidation resistance of these systems was studied at 1,100 °C in air. Most TBCs with a Pt-rich γ–γ′ BC showed better thermal cycling resistance when compared to the reference TBCs (β-(Ni,Pt)Al diffusion BC and EBPVD ceramic top coat), with lifetimes up to 1,745 cycles instead of 700 for the reference, and despite the fabrication defects observed within the SPS BCs. Cu was tested as an addition in the BCs and proved to have a slight negative effect on the system lifetime. Moreover, the fourth generation MCNG substrate led to the best cyclic oxidation behavior

High-temperature cyclic oxidation behaviour of Pt-rich γ-γ’ coatings. Part I: Oxidation kinetics of coated AM1 systems after very long-term exposure at 1100 °C

The cyclic oxidation behaviour of several compositions of Pt-rich γ-γ’ bond-coatings on AM1 superalloy was studied at 1100 °C and was compared to the β-(Ni,Pt)Al coated and uncoated superalloy. AM1 superalloy exhibited an outstanding performance due to an optimized Hf doping and a low sulfur content. The Pt-rich γ-γ’ bond-coatings showed a better cyclic oxidation resistance than the reference system with a β-(Ni,Pt)Al coating. Aluminium addition during fabrication was found to be beneficial to improve the oxidation behaviour of Pt-rich γ-γ’ bond-coatings. Their breakaway resulted from an insufficient aluminium content below the TGO whereas the reference system suffered from rumpling

A direct comparative study of the corrosion behaviour of Si-free and Si-rich slurry aluminide coatings in molten carbonate melts

ABSTRACT: This study is focused on the evaluation of the hot corrosion behaviour of two low-cost slurry aluminide coatings with and without Si addition deposited on ferritic-martensitic steels, comparing them with uncoated steel, after 1000-h of exposure to Li, K, Na molten carbonates at 650ºC, under static and dynamic conditions with a high linear velocity (1.3 m/s). Both coatings evidenced a high performance increase in comparison with uncoated substrates after exposure in both conditions. Both coatings behaved in a similar way according to the gravimetric results. However, after dynamic exposure, the Si-free aluminide maintained its morphology and composition after 1000 h with α-LiAlO2 as the only corrosion product while the Si-rich coating showed a higher attack extent in the outer part and developed a large quantity of voids at the coating/substrate interface, leading to substrate corrosion.info:eu-repo/semantics/publishedVersio

High-temperature cyclic oxidation of Pt-rich γ-γ’ bond-coatings. Part II: Effect of Pt and Al on TBC system lifetime

Three kinds of Pt-rich γ-γ’ bond-coating were processed with different contents in Pt and Al. The cyclic oxidation tests performed at 1100 °C on TBC systems showed the superiority of the Pt-rich γ-γ’ coatings when compared with the β-(Ni,Pt)Al reference system. TBCs with a Pt-only bond-coating provided the highest performance. Whatever the bond-coating, the failure occurred at the TGO/bond-coating interface which appeared to be the weak point of these γ-γ’ bond-coating based systems. Al addition during bond-coating fabrication did not improve the durability. A decrease of 2 μm of electroplated Pt thickness led to a higher performance than the reference systems

Observation and modeling of α-NiPtAl and Kirkendall void formations during interdiffusion of a Pt coating with a γ-(Ni-13Al) alloy at high temperature

During the last 15 years, Pt-rich γ–γ′ bond-coatings have been studied extensively for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings in thermal barrier coating systems. To optimize their fabrication and durability, it is essential to investigate their interdiffusion with Ni-based superalloys. This study reports on experimental results and modeling of the interdiffusion of the model Pt/γ-(Ni-13Al) alloy system. Pt coatings were deposited either by electroplating or by spark plasma sintering using a Pt foil. Heat treatments at 1100 °C for 15min to 10 hwere performed either in a high-temperature X-ray diffraction device under primary vacuum or in a furnace under argon secondary vacuum. The α-NiPtAl phase with L10 crystal structure formed very rapidly, implying fast uphill Al diffusion toward the surface. For Pt electroplating, α-phase transformed to γ′-(Ni,Pt)3Al after only 45 min–1 h at 1100 °C. The resulting two-phased γ–γ′ microstructure remained up to 10 h. When using a Pt foil coating, the continuous layer of α-NiPtAl phase disappeared after 10 h and the γ′-(Ni,Pt)3Al or γ-(Ni,Pt,Al) phase appeared, resulting in two different diffusion paths in the Ni–Pt–Al phase diagram. Voids also formed at the interdiffusion zone/substrate interface for both systems after 1 h or more. Composition analyses confirmed that voids were located at the Pt diffusion front corresponding to the Al-depleted zone. Experiments performed with the samples coated with a Pt foil confirmed that voids are due to a Kirkendall effect and not to the Pt deposition process. Numerical simulations including the cross-term diffusion coefficients in the diffusion flux equations reproduced the experimental concentration profiles for the γ-phased systems

Modeling of the interdiffusion and cyclic oxidation behavior of Ni-based superalloy / Pt-rich γ-γ’ bond-coating. Application to TBC systems

Les systèmes barrière thermique actuels connaissent une importante dispersion de durées de vie liée principalement aux ondulations de surface du revêtement métallique β-(Ni,Pt)Al provoquant l’écaillage du dépôt céramique. Les revêtements γ-γ’ riches en platine sont étudiés en tant qu’alternative au système actuel. Ce travail de thèse s’est intéressé à l’élaboration des revêtements γ-γ’ riches en platine sur un superalliage à base de nickel, l’AM1 à partir de procédés conventionnels : dépôt électrolytique de platine et aluminisation courte. Les mécanismes de dégradation par oxydation cyclique à 1100°C ont été étudiés sur des systèmes revêtement/AM1 et sur des systèmes barrière thermique. Pour comparaison, trois types de revêtement ont été élaborés : γ-γ’ Pt seul, γ-γ’ Pt+Al et β-(Ni,Pt)Al. Ces essais ont mis en évidence une meilleure tenue à l’oxydation cyclique des systèmes revêtus γ-γ’ Pt+Al comparée aux deux autres systèmes revêtus. L’importance de l’ajout d’aluminium dès l’élaboration sur la tenue à l’oxydation cyclique a été soulignée. La modélisation p-kp a mis en avant une augmentation de la proportion d’écaillage au cours du temps du fait de la dégradation de l’interface métal/oxyde et une augmentation du kp du fait de la formation d’un oxyde à croissance plus rapide. Outre l’oxydation, les phénomènes d’interdiffusion lors des tous premiers instants à haute température ont été étudiés à partir de matériaux modèles (Ni13Al et Ni11Al10Cr) et de revêtements de Pt et/ou de Pt-Ir. Ces essais ont mis en avant la rapide formation de la phase α-NiPtAl, les transformations de phases et les chemins de diffusion à 1100°C dans les systèmes Ni-Al-Pt et Ni-Al-Cr-Pt. L’effet du chrome et de l’iridium sur les cinétiques de diffusion a été évalué. La modélisation de l’interdiffusion a mis en évidence les interactions chimiques entre les espèces et une sursaturation en lacunes dans la zone d’interdiffusion prouvant que l’effet Kirkendall est responsable de la formation des pores.TBC systems currently used in aircraft engines with a Pt-modified aluminide coating β-(Ni,Pt)Al show an important lifetime dispersion due to the surface undulations of the bond-coating. This phenomenon called rumpling leads to the ceramic scale spallation and is the most common degradation mechanism. Pt-rich γ-γ’ bond-coatings have been extensively studied for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings. The aim of this work was to fabricate Pt-rich γ-γ’ bond-coatings on a first generation Ni-based superalloy, the AM1. Conventional processes were used as a platinum electroplating and a short aluminizing step. The failure mechanisms occurring by cyclic oxidation at 1100°C were studied on coating/superalloy systems and on TBC systems. Three kinds of coatings were fabricated: Pt-only γ-γ’, Pt+Al γ-γ’ and β-(Ni,Pt)Al. These tests highlighted the best oxidation resistance for the Pt+Al γ-γ’/AM1 systems when compared with the two other systems. Al addition during the coating fabrication is necessary to improve the lifetime. The p-kp modeling results pointed out that the oxide scale spalling probability p increases due to the metal/oxide interface degradation with time. If the spallation increases, a breakaway locally occurs with the formation of a fast-growing oxide explaining the kp progression. The interdiffusion phenomena were also investigated during the first times at high temperature from model alloys (Ni13Al and Ni11Al10Cr) and Pt and/or Pt-Ir coatings. These investigations emphasized the rapid formation of the α-NiPtAl phase, the phase transformations and diffusion paths at 1100°C in the ternary Ni-Al-Pt and quaternary Ni-Al-Cr-Pt systems. Chromium and iridium effect was evaluated on the diffusion kinetics. Interdiffusion modeling highlighted the chemical interactions between the species and a vacancy supersaturation in the interdiffusion zone proving that Kirkendall effect is responsible for void formation

Hot Corrosion Behavior of Slurry Sprayed Aluminide Coatings in a Simulated Dynamic Molten Carbonate Environment [Comunicação oral]

ABSTRACT: One of the key challenges associated with the use of molten salts in Concentrated Solar Power (CSP) plants is to reduce or even suppress corrosion phenomena occurring at elevated temperatures. Molten nitrate mixtures are currently seen as the most viable storage medium for Thermal Energy Storage. But the upper temperature limit of the currently used “Solar Salt” is restricted by salt decomposition at ~580ºC. Prieto et al. estimated that increasing the temperature to 650ºC could increase the solar-to-electric conversion up to 18.5%, so alternative salts operating at higher temperature are required. Molten carbonates which are less corrosive and allow an operation up to 800ºC are an appealing breakthrough for future CSP plants.N/