Effect of Cycle Frequency on High Temperature Oxidation Behavior of Alumina-forming Coatings Used for Industrial Gas Turbine Blades

Oxidation kinetics of platinum modified aluminide and overlay coatings on nickel base superalloys were investigated. Isothermal oxidation tests were carried out at 1050°C in synthetic air. Cyclic oxidation tests were performed with 2 cycle frequencies : - Short term cycles : 1h dwells at 1050°C in synthetic air ×1800 cycles - Long term cycles : 300h dwells at 1050°C in laboratory air × 6 cycles (experiment planned to totalize at least 10 000 hours at high temperature) The mass gain curves point out a large effect of the cycle frequency at 1050°C for overlay NiCoCrAlYTa coating whereas the effect is less significant for Pt-modified nickel aluminide coating. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy was used to evaluate the effect of cycle frequency on microstructural evolution. A simple statistical spalling model [1,2], assuming that the parabolic rate constant kp and the spalling probability p are constant, is tentatively applied and discussed in view of the microstructural evolution complexity

Etude de la détérioration par oxydation haute température et interdiffusion de systèmes revêtement/superalliage à base de Nickel. Prévision de durée de vie

L'amélioration des performances des turbines à gaz nécessite d'identifier et de comprendre les mécanismes de détérioration qui ont lieu durant leur utilisation. Les modèles de prévision de durée de vie sont des outils indispensables, pour les choix techniques et la définition des politiques de révision et de réparation des moteurs. Ce travail de thèse s'est intéressé à la détérioration, par oxydation et interdiffusion, de systèmes revêtement/superalliage à base de nickel, utilisés pour les aubes de turbine à gaz. Trois superalliages ont été étudiés : CMSX-4, SCB et IN792. Ils sont revêtus par un aluminiure modifié au platine (RT22 ou CN91) ou par un revêtement NiCoCrAlYTa. Les cinétiques d'oxydation de ces systèmes, ont été déterminées par analyses thermogravimétriques entre 900°C et 1150°C. Des essais d'oxydation cyclique « long terme », de 15000h (cycles de 300h) à 900°C et 1050°C, ainsi que des essais d'oxydation cyclique « court terme » de 1800h (cycles de 1h) à 1050°C sous air synthétique et humide, ont été effectués. Les essais « long terme » sont utilisés pour accélérer l'endommagement des systèmes et fournir des données pour la modélisation. Ces essais ont mis en évidence l'effet de la nature du superalliage, sur la résistance des systèmes RT22 ou CN91/superalliage et le rôle majeur de l'interdiffusion sur leur détérioration. Puis, le suivi des évolutions microstructurales et chimiques des systèmes revêtement/superalliage, a permis d'affiner les données à utiliser pour le modèle de prévision de durée de vie, basé sur la diffusion et l'oxydation. Ces investigations ont mis en évidence la nécessité de prendre en compte, dans le modèle de diffusion, les transformations de phases, la présence de précipités qui peuvent agir comme une barrière de diffusion et le déplacement des interfaces. Enfin, le critère de fin de vie le plus pertinent a été déterminé, pour estimer la durée de vie des systèmes revêtement/superalliage étudiés

Numerical modelling of diffusion coupled with cyclic oxidation. Application to alumina-forming coatings used for industrial gas turbine blades

A model using a finite elements code is developed to simulate degradation due to combined cyclic oxidation and interdiffusion in materials used in high temperature components of gas turbines. Coating recession due to cyclic oxidation (oxide growth and spalling) is also modelled using a statistical approach. Interdiffusion between coating and superallloy is modelled to predict total Al depletion in the coating. The phase transformations in the alumina-forming coating and the effects of the precipitates at the coating / superalloy interface are investigated. The results of simulations are compared with experimental data. Effects of diffusion parameters and of cyclic oxidation kinetics are discussed

Substrate Effect on the High-Temperature Oxidation Behavior of a Pt-Modified Aluminide Coating. Part I: Influence of the Initial Chemical Composition of the Coating Surface

The effect of substrate composition on the oxidation behavior of the industrial NiPtAl coating RT22 TM was investigated by studying the isothermal and cyclic-oxidation behavior of this coating deposited on three different Ni-base superalloys (CMSX-4 TM , SCB TM and IN792 TM ). Isothermal tests were performed at 900, 1050 and 1150°C for 100 h. Cyclic oxidation was studied at 900°C with a holding time of 300 h for up to 52 cycles (i.e, 15,600 h at 900°C). Thermogravimetric analysis (TGA), X-ray diffraction (XRD), microstructural and analytical investigations using scanning-electron microscopy (SEM) and transmission-electron microscopy (TEM), both equipped with energy-dispersive X-ray spectroscopy (EDS) were performed to characterize the oxidation behavior of the systems studied. An effect of the superalloy substrate was observed and related to the initial chemical composition of the coating surface which depends on the superalloy and the associated heat treatments. The effect of the substrate’s alloying elements is discussed. Particularly the influence of Ti and Ta that formed rutile-type oxides inducing oxide-scale cracking and spallation. The excellent resistance to cyclic oxidation of the coating systems studied at 900°C was also demonstrated from very long duration tests of 15,600 h

Application of image analysis and image simulation for quantitative characterization of scale spallation during cyclic oxidation of a Pt-aluminide coating

An image analysis and simulation of the spalled alumina areas at the surface of a Pt modified b-NiAl aluminide coating RT22 during cyclic oxidation in air is presented. The size distribution of the freshly spalled areas and of the unspalled zones is studied for the same coating deposited on three different nickel base superalloys which were exposed to various numbers of 300 h cycles at 1050 8C (up to 10,500 h) in a previous study [Vialas N, Monceau D. Oxid Metals, submitted for publication]. It is shown for this coating that the distribution of the freshly spalled area depends on the nature of the coated substrate and on the number of cycles. It is also shown that spallation of these three systems can be simulated by a twoparameter model which describes the spalling during the cooling step as successive microspalling events through nucleation and growth processes. This approach is consistent with previous in-situ observations and allows the introduction of a time-dependent damaging process in the models of cyclic oxidation kinetics

Effect of Pt and Al content on the long-term, high temperature oxidation behavior and interdiffusion of a Pt-modified aluminide coating deposited on Ni-base superalloys

The present work is devoted to the effect of Al and Pt content on the oxidation behavior and interdiffusion of the industrial NiPtAl coating RT22 deposited on SCB and IN792 Ni-base superalloys. Some specimens of RT22/SCB experienced a defective aluminization resulting in one side with a lower Al content, and some specimens of RT22/IN792 had less platinum than the specification. The effect of both Pt and Al on the initial microstructure of the coating is discussed. Isothermal oxidation tests for 100 h and long-term cyclic oxidation/interdiffusion tests at 1050 °C were performed (up to 51 cycles of 300 h). It is shown that a 50 μm coating with 30 at.% Al instead of a nominal 70 μm coating with 52 at.% Al leads to the full transformation of the β phase after 6×300 h at 1050 °C and to the formation of large voids and spinel oxides after 35×300 h, but also to less surface undulations than the standard coating. A lower Pt concentration in the RT22 coating results in lower aluminization kinetics and increased spalling and Al consumption during long-term cyclic oxidation without decreasing the isothermal oxidation kinetics

Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 C

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclicoxidation tests at 1,050 C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 C for 100 h and cyclicoxidation tests at 900 C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the b to c0 transformation leads to the formation of a two-phase b/c0 layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this b to c0 transformation occurs gradually by an inward transformation of b leading to the formation of a continuous layer of c0 phase, parallel to the metal/scale interface

Étude de la détérioration par oxydation haute température et interdiffusion de systèmes revêtement-superalliage à base de nickel. Prévision de durée de vie

Ce travail de thèse s'est intéressé à la détérioration, par oxydation et interdiffusion, de systèmes revêtement/superalliage à base de nickel, utilisés pour les aubes de turbine à gaz. Trois superalliages ont été étudiés : CMSX-4, SCB et IN792, revêtus par un aluminiure modifié au platine (RT22 ou CN91) ou par un revêtement NiCoCrA1YTa. Les cinétiques d'oxydation ont été déterminés entre 900C et 1150C. Des essais d'oxydation cyclique "long terme" de 15000h à 990C et 1050C, et "court terme" de 1800h à 1050C, ont été effectués. Ils ont mis en évidence l'effet de la nature du superalliage, sur la résistance des systèmes RT22 ou CN91/superalliage et le rôle majeur de l'interdiffuion. Puis, le suivi des évolutions microstructurales et chimiques des systèmes revêtement/superalliage, a permis d'affiner les données à utiliser pour le modèle de prévision de durée de vie. Ces investigations ont mis en évidence la nécessité de prendre en compte, dans le modèle de diffusion, les transformations de phases, la présence de précipités et le déplacement des interfaces. Enfin, le critère de fin de vie le plus pertinent pour estimer la durée de vie des systèmes a été déterminé.TOULOUSE-ENSIACET (315552325) / SudocSudocFranceF