ICMCTF2010 Session A1-1: Coatings to Resist High Temperature Oxidation

Tuesday, April 27, 2010 8:00 AM in Room Sunrise
Tuesday Morning

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Start Invited? Item
8:00 AM Invited A1-1-1 Oxidation and Hot Corrosion of MCrAlY Alloys
Gerald Meier, Frederick Pettit (University of Pittsburgh)
The MCrAlY (M= Ni and/or Co) coatings have been used for many years to protect superalloys from oxidation and corrosion. Nevertheless, there is still considerable research activity relative to the oxidation and hot corrosion mechanisms of these coatings. This presentation will describe recent results from the authors and others regarding the effects of composition, microstructure and surface preparation on the oxidation and hot corrosion behavior of MCrAlY coatings and bulk alloys. Particular attention will be given to the concentration and distribution of Y in the alloys with regard to cyclic oxidation resistance. The effect of composition and heat treatment (and the resultant phase constitution) on Type I and Type II hot corrosion will be described. Finally, the effect of intermittent exposure to hot corrosion conditions on cyclic oxidation behavior will be discussed.
8:40 AM A1-1-3 Characterization of the Alumina Scale Formed on a Commercial MCrAlY Coating
K.A. Unocic, Bruce Pint (Oak Ridge National Laboratory)
A commercial MCrAlY (M = Ni,Co) coating deposited on a Ni-base superalloy 1484 substrate was characterized before and after high temperature oxidation. The coating contained small additions of Hf and Si that are reported to improve its high temperature oxidation performance. High resolution characterization techniques including scanning transmission electron microscopy will be used to study the segregation behavior of Y and Hf to the alumina grain boundaries after isothermal exposure at 1100°C. Comparisons will be made to prior observations of the alumina scale formed on model NiCrAl+(Y,Hf) alloys.
9:00 AM A1-1-4 Thermodynamic Investigation into the Equilibrium Phase Fractions, Phase Compositions and Phase Transformation in the NiCoCrAl System at Elevated Temperatures
Kaka Ma, Julie Schoenung (University of California, Davis)

NiCoCrAlY alloys have been widely applied as the bond coat layer in thermal barrier coatings. Extensive microstructural characterization has been carried out experimentally on NiCoCrAlY bond coat by various researchers. It is generally agreed that the major phases in the NiCoCrAlY are γ phase and β phase. Additionally, γ’ precipitates, Cr- and Co-rich σ precipitates, and α-Cr precipitates have been reported to be present in the NiCoCrAlY bond coat. It must be noted, however, that the chemical compositions of the NiCoCrAlY alloys vary from one study to another. Thermodynamic calculations have demonstrated that even minor changes in composition and in temperature can lead to major changes in the resulting phases. The present work reviews the literature on the experimental descriptions of the phases present in NiCoCrAlY alloys and the corresponding phase transformations. To complement this literature review, the computational tool, Thermo-calc® is applied to investigate, as a function of temperature, the equilibrium phase fractions, phase compositions and phase transformations in the NiCoCrAl system, allowing for a wider and continuous variation in composition (Co, Cr or Al content). Comparisons between the experimental and computational findings are provided.

9:20 AM A1-1-5 Modeling Aluminum Depletion Kinetics in MCrAlY Coatings
Mario Rudolphi, Daniel Renusch, Michael Schütze (Karl-Winnacker-Institut der DECHEMA e.V., Germany)

MCrAlY coatings are widely used in high temperature applications due to their ability to form a dense protective alumina layer during high temperature exposure. By doing so, the underlying superalloy material is protected from corrosive attack and the component life-lime is extended. However, as a result of the alumina growth, aluminum is consumed from the coating reservoir and eventually the coating may become depleted from aluminum. This finally leads to a change in oxidation mechanisms and the growth of voluminous and non-protective spinels and consequently a breakdown of the protective behavior of the coating.

Life-time prediction is therefore of high interest to ensure safe service inspection intervals of component parts. This leads to the demand of a detailed understanding of the complex depletion kinetics in a coating/substrate system. In the course of this project, modeling equations have been developed to calculate the aluminum profile of a coating/substrate system as a function of exposure time and temperature. The equations take the most important aspects of Al depletion into account: (i) Al outward diffusion due to alumina scale growth and (ii) Al interdiffusion between the coating and the substrate material. The calculated aluminum profiles have been validated with experimental EPMA line scan data from several different systems with varying aluminum content. Finally, the developed equations have been built into a software tool to allow user friendly comparison of modeled and measured Al profiles. Furthermore, the software calculates the average Al concentration of the coating as a function of exposure time and temperature to provide information about coating life-time.

9:40 AM A1-1-6 Reactivity and Microstructure Evolution of SPS Prepared CoNiCrAlY/Talc Cermet
Nicolas Ratel, Daniel Monceau, Djar Oquab, Claude Estournès (CIRIMAT, France)

A mixture of CoNiCrAlY and talc powders is considered as a new candidate composition for abradable seal coatings applications. Dense specimen having the composition of 1:20 weight ratio of talc with respect to CoNiCrAlY was prepared using the Spark Plasma Sintering (SPS) technique. The aim of the present investigation is the determination of the reactivity and microstructure evolution of the β/γ-CoNiCrAlY based cermet. The resulting microstructures were analysed and their compositions determined using standard analytical techniques such as SEM, TEM and X-Ray diffraction. After fabrication, the bulk of the material is shown to contain a continuous oxide layer of MgAl2O4 at the periphery of metallic particles, resulting from the reaction between aluminium, which has diffused from the bulk of CoNiCrAlY grains, with magnesium and oxygen delivered during the high temperature decomposition of the talc phase. Thermodynamic calculations were conducted to support the experimental observations.

10:00 AM Invited A1-1-8 The Effects of Chemical Composition on the Oxidation Behavior of Platinum-Modified Aluminide Coatings
Vladimir Tolpygo (Honeywell Aerospace)
The effects of chemical composition on the cyclic oxidation behavior of platinum-modified nickel-aluminide CVD coatings are described. While these coatings typically demonstrate excellent oxide adhesion, they are prone to plastic deformation during thermal cycling, also known as surface rumpling. Another form of microstructural degradation that occurs in the course of high temperature exposure is the development of pores or cavities within the coating. Both phenomena are shown to be influenced by chemical composition of the coating and superalloy substrate. This paper summarizes various observations and measurements of rumpling and pore formation and provides comparison of the extent of degradation between different coatings. The significance of bond coat rumpling in the thermal barrier coating systems is also discussed.
10:40 AM A1-1-10 Early Stages of Oxidation of NiPtAl Coatings in Low and High pO2 Gases
Peng Song, Dmitry Naumenko (Forschungszentrum Jülich GmbH, Germany); W Braue (German Aerospace Center (DLR), Germany); Lorenz Singheiser, WillemJoe Quadakkers (Forschungszentrum Jülich GmbH, Germany)

NiPtAl-coatings produced by low Al-activity pack cementation process on CMSX-4 were studied. Isothermal oxidation experiments were performed in Ar-20%O2 and Ar-4%H2-2%H2O for durations up to 100 hours at 1150°C. The specimens after oxidation were analyzed by optical metallography, laser-induced fluorescence spectroscopy and scanning electron microscopy (SEM). From selected areas of the oxidized surfaces thin lamella were prepared using a focused ion beam (FIB) facility, which were analyzed in a transmission electron microscope (TEM). The analytical studies revealed that in both oxidizing environments alumina scales were formed, whereas transformation of the metastable theta alumina into thermodynamically stable alpha alumina was accelerated in the low pO2, Ar-H2-H2O gas as compared to the high pO2, Ar-O2 atmosphere. Furthermore, indications were found that the rate of theta to alpha alumina transformation has a relationship with the grain orientation of the NiPtAl-coatings.

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