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

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.

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.

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.

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.