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Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

# Comparative Performance of a Thermal Barrier Coating System Utilizing Platinum Aluminide Bond Coat on Alloys CMSX-4® and MAR M® 002DS

[+] Author and Article Information
H. M. Tawancy, Luai M. Al-Hadhrami

Center for Engineering Research and Center of Research Excellence in Corrosion, Research Institute,  King Fahd University of Petroleum and Minerals, P. O. Box 1639, Dhahran 31261, Saudi Arabia

J. Eng. Gas Turbines Power 134(1), 012101 (Oct 28, 2011) (8 pages) doi:10.1115/1.4004131 History: Received April 11, 2011; Revised April 12, 2011; Published October 28, 2011; Online October 28, 2011

## Abstract

It is known that the relative performance of thermal barrier coatings is largely dependent upon the oxidation properties of the bond coat utilized in the system. Also, the oxidation properties of diffusion-type bond coats (aluminides and their modifications) are functions of the superalloy substrate used in blade applications. Therefore, the performance of a given coating system utilizing a diffusion-type bond coat can significantly vary from one superalloy to another. Toward the objective of developing coating systems with more universal applicability, it is essential to understand the mechanisms by which the superalloy substrate can influence the coating performance. In this study, we examined the relative performance of yttria-stabilized zirconia/platinum aluminide coating system on alloys CMSX-4 and MAR M 002DS representing single-crystal and directionally-solidified alloy systems respectively using thermal exposure tests at 1150 °C with a 24-h cycling period to room temperature. Changes in coating microstructure were characterized by various electron-optical techniques. Experiment showed that the coating system on alloy MAR M 002DS had outperformed that on alloy CMSX-4, which could be related to the high thermal stability of the bond coat on alloy MAR M 002DS. From a detailed microstructural characterization, this difference in behavior could be explained at least partially in terms of variation in chemical composition of the two alloys, which was also reflected on the exact failure mechanism of the coating system.

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## Figures

Figure 1

Characteristic microstructural features in the as-deposited condition. (a) and (b) are backscattered SEM composition images along a cross-section of the bond coat and into alloys CMSX-4 and MAR M 002DS, respectively; the three bond coat layers i, ii, and iii are indicated. (c) X-ray diffraction pattern representative of the outermost layer of the Pt-aluminide bond coat on both alloys; standard patterns of β-NiAl and PtAl2 are also shown. (d) Concentration profiles of Pt along a cross-section of the bond coat and into the substrates.

Figure 2

An example derived from the bond coat on alloy CMSX-4 illustrating the microstructure of PtAl2 in the outermost coating layer in the as-deposited condition. (a) Dark-field TEM image formed with the [200] reflection; the inset is a microdiffraction pattern in <001> orientation consistent with the structure of PtAl2 . (b) Corresponding energy dispersive X-spectrum illustrating the elemental composition of PtAl2 .

Figure 3

Analysis of the composition of interdiffusion zone of the coating on alloy CMSX-4 in the as-deposited condition. (a) Backscattered SEM composition image showing the microstructure of the interdiffusion zone. (b) Energy dispersive X-spectrum showing the elemental composition of the matrix phase (regions marked 1). (c) Composition of the matrix phase as determined by electron probe microanalysis. (d) Energy dispersive X-spectrum representative of the precipitates (regions marked 2).

Figure 4

Analysis of the composition of interdiffusion zone of the coating on alloy MAR M 002DS in the as-deposited condition. (a) Backscattered SEM composition image showing the microstructure of the interdiffusion zone. (b) Energy dispersive X-spectrum illustrating the elemental composition of the matrix phase (region marked 1). (c) Composition of the matrix phase as determined by electron probe microanalysis. (d), (e), and (f) are energy dispersive X-spectra illustrating the elemental compositions of the precipitates marked 2, 3, and 4, respectively, in (a).

Figure 10

Schematics illustrating possible mechanisms leading to decohesion between the thermally grown oxide and underlying bond coat. (a) Alloy CMSX-4: coalescence of voids formed around Ta,Ti-rich oxide particles near the oxide-bond coat interface. (b) Alloy MAR M 002DS: fracture of Hf-rich oxide pegs.

Figure 9

Microstructural features of the bond coat surface exposed by failure of the coating system on alloys CMSX-4 (a), (b) and MAR M 002DS (c), (d), and (e) during thermal exposure at 1150 °C with a 24-h cycling period to room temperature. (a) Secondary SEM image showing the microstructure of the bond coat surface exposed by failure of the coating system on alloy CMSX-4 after 72 h of exposure. (b) Corresponding secondary SEM image at higher magnification and energy dispersive spectrum showing the presence of Ta,Ti-rich oxide particles within voids at the coating surface. (c) Backscattered SEM composition image showing the microstructure of the bond coat surface exposed by failure of the coating system on alloy MAR M 002DS after 192 h of exposure. (d) and (e) are corresponding energy dispersive X-spectra showing the elemental compositions of regions a and 2 in (c).

Figure 8

Comparative oxidation behavior of the bond coat during thermal exposure at 1150 °C. (a) Thickening rate of the thermally grown oxide. (b) and (c) are backscattered SEM composition image showing the microstructures of the thermally grown oxide corresponding to alloys CMSX-4 and MAR M 002DS, respectively, after 48 h of exposure.

Figure 7

Temperature dependence of the parabolic rate constant (K) of interdiffusion between the bond coat and the superalloy substrates

Figure 6

Examples illustrating comparative thermal stability characteristics of the bond coat during thermal exposure at 1150 °C. (a) and (b) are backscattered SEM composition images illustrating the effect of 48 h of exposure at 1150 °C on the microstructure of the bond coat on alloys CMSX-4 and MAR M 002DS, respectively. (c) Bright-field TEM image showing the presence of γ′-phase in the outermost coating layer on alloy CMSX-4. (d) and (e) are characteristic diffraction patterns of γ′-phase and β-phase in <111> orientation, respectively.

Figure 5

Comparative performance of the coating system on alloys CMSX-4 and MAR M 002DS as determined from thermal exposure tests at 1150 °C in air with a 24-h cycling period to room temperature

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