MCrAlY coating design based on oxidation-diffusion modelling. Part I: Microstructural evolution
Introduction
MCrAlY coatings (M for Ni and/or Co) are widely used to protect the base material (superalloys) against high-temperature oxidation and corrosion in modern turbine engines. To improve the engine efficiency, higher combustion temperature is required and more durable MCrAlY coatings should be designed. Coating design should consider many factors, for instance the resistance against the oxidation and corrosion of the coating, and the mismatch of mechanical properties with the superalloy [1], [2]. The mechanical properties are strongly related to the microstructure and the microstructure changes with time in real applications at high temperature. Obviously, the microstructure of materials for high-temperature applications is controlled by the alloy composition and temperature. Therefore, to design durable coatings for superalloy-coating systems, it is necessary to deepen the knowledge of the microstructural evolution with variations in the alloy composition and temperature.
The CALPHAD and other methods have been applied for the calculation of the equilibrium microstructures in Ni-based alloys by Saunders et al. [3], [4]. D.R.G. Achar et al. [5] used commercial software of ThermoCalc [6] to simulate the microstructures in MCrAlY coatings at elevated temperatures by using a developed thermodynamic database; similar work was also carried out by K. Ma and Schoenung [7]. Furthermore, the development of kinetic databases makes it possible to investigate the diffusion issues in alloys. For instance, with the kinetic databases of Ni base, C.E. Campbell et al. [8], [9] simulated the interdiffusion of alloying elements in different multi-component diffusion couples, showing a good agreement with the experimental measurements. The alloying diffusion and the corresponding microstructural evolution in a diffusion couple are due to the composition differences between the two alloys, and, more essentially, due to the activity of alloying elements. Some fundamental investigations on the microstructural evolution in diffusion couples can be found in the work by A. Engstrom et al. [10], [11], [12] who used a “diffusion path” concept to explain the diffusion behaviour of alloying elements in phase diagrams. For the diffusion modelling in superalloys and MCrAlY coatings, which usually contain around ten different alloying elements, the investigation in phase diagrams can be hardly to be proceeded, therefore computer modelling software, like DICTRA, is needed [8], [9].
In the real applications of a superalloy-coating system, not only the interdiffusion between the superalloy and the coating but also the oxidation of the coating should be considered. Many researchers [13], [14], [15] have modelled the development of the microstructures in superalloy-coating systems by combining those two issues. However, those studies showed limitations due to the lack of diffusion parameters for multi-component alloys. By using an advanced diffusion database, M.S.A. Karunaratne et al. [16] simulated the microstructural evolution in a coating with different superalloy substrates at different ageing temperatures; their results showed a good agreement between the simulation and the experiment in a qualitative level. In our recent work [17], a newly developed oxidation-diffusion model was used to predict the microstructure evolution in a superalloy-coating system in oxidizing environments quantitatively by focusing on the depletion of the Al-rich β phase in a high-velocity oxy-fuel (HVOF) sprayed coating which has a typical “splat-on-splat” structure. Another spraying technique for MCrAlY coatings is atmospheric plasma spray (APS) that also creates a “splat-on-splat” structure and has heavy intersplat oxidation for long-term oxidation [18]. The oxidation-diffusion model, after being further developed by including intersplat oxidation, has been used for the life prediction of APS coatings [18].
In this paper, the oxidation-diffusion model, established in previous work [17], will be further tested in different superalloy-coating systems, including three HVOF coatings and two APS coatings. Then such model will be used to simulate the microstructural evolution in several hypothetical superalloy-coating systems with varying alloy compositions and temperatures, to aid the design of MCrAlY-type coatings. There also exists a part II of this paper which deals with the lifing aspects of MCrAlY alloy design [19]. The authors believe that the results in these papers can provide useful information for MCrAlY coating design.
Section snippets
Materials and experiments
Two superalloys and four coatings were used for oxidation testing. The compositions of the alloys are shown in Table 1. The superalloy substrates were 3–5 cm thick and coated with different MCrAlY coatings using two different thermal spraying techniques. The IN-792 superalloy was coated with CoatCo-1, CoatCo-2 or CoatNi-1 by high-velocity oxy-fuel (HVOF) spraying, while Haynes 230 was coated with CoatNi-2 or CoatNi-3 by atmospheric plasma spraying (APS). The HVOF coatings were used to simulate
Modelling
In this paper, an oxidation–diffusion model was built and used for two applications. First, the model was used to simulate the diffusion of alloying elements and the microstructural evolution in real superalloy-coating couples; the purpose of this was to validate the strength of the model for applications combining oxidation (surface oxidation and intersplat oxidation) and alloying element diffusion at high temperatures. The second application of the model, “coating-design modelling”, simulates
Experimental results
The typical microstructures of the superalloy-coating systems at different temperatures are presented in Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 on cross-sections. The different phases in the microstructure are identified by their compositions, quantitatively measured by EDS, and have been denoted in Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8. The identification of the phases was as follows: 1) γ is a Ni, Co, Cr-rich phase and lack Al; 2) β is Al-rich and has a NiAl form; 3) γ′ has a medium Al
Conclusions
An oxidation–diffusion model, that considers the oxidation of coatings (Al oxidation) and the interdiffusion between superalloys and coatings, was applied to simulate the microstructural evolution in superalloy-coating systems in the temperature range 800–1100 °C. The model was mainly used to investigate the elemental effects of Ni, Co, Cr and Al on the microstructural evolution in simplified superalloy-coating couples in high-temperature oxidation processes, to aid the future MCrAlY coating
Acknowledgement
The Siemens Industrial Turbomachinery AB (Finspång, Sweden) and Swedish Energy Agency through the KME consortium — ELFORSK are greatly acknowledged for their financial support in this research.
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