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

# Role of Platinum in Thermal Barrier Coatings Used in Gas Turbine Blade Applications

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

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

J. Eng. Gas Turbines Power 132(2), 022103 (Nov 05, 2009) (6 pages) doi:10.1115/1.3156814 History: Received March 19, 2009; Revised April 12, 2009; Published November 05, 2009; Online November 05, 2009

## Abstract

Current technology of thermal barrier coating systems used in gas turbine blade applications relies on the use of a metallic bond coat, which has a twofold function: (i) it develops a thin layer of aluminum oxide enhancing the adhesion of the ceramic top coat and (ii) it provides an additional resistance to oxidation. It was the objective of this study to develop an understanding of the role of platinum in bond coats of the diffusion-type deposited on a nickel-based superalloy. Two Pt-containing bond coats were included in the study: (i) a platinum-aluminide and (ii) a bond coat formed by interdiffusion between an electroplated layer of platinum and the superalloy substrate. In both cases, the top ceramic coat was yttria-stabilized zirconia. For reference purposes, a simple aluminide bond coat free of Pt was also included in the study. Thermal exposure tests at $1150°C$ with a 24 h cycling period at room temperature were used to compare the coating performance. Microstructural features were characterized by various electron-optical techniques. Experimental results indicated that Pt acts as a “cleanser” of the oxide-bond coat interface by decelerating the kinetics of interdiffusion between the bond coat and superalloy substrate. This was found to promote selective oxidation of Al resulting in a purer $Al2O3$ scale of a slower growth rate increasing its effectiveness as “glue” holding the ceramic top coat to the underlying metallic substrate. However, the exact effect of Pt was found to be a function of the state of its presence within the outermost coating layer. Of the two bond coats studied, a surface layer of Pt-rich gamma prime phase ($L12$ superlattice) was found to provide longer coating life in comparison with a mixture of $PtAl2$ and beta phase. This could be related to the effectiveness of gamma prime phase as a sink for titanium minimizing its detrimental effect on the adherence of aluminum oxide.

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## Figures

Figure 1

Backscattered electron images illustrating characteristic microstructural features of the coating systems in the heat-treated condition: (a) an example derived from the system containing the Pt-aluminide bond coat to illustrate the microstructure along a cross-section of the top coat and into the superalloy substrate showing the initial oxide layer, (b) microstructure along a cross-section of the simple aluminide bond coat, (c) microstructure along a cross-section of the Pt-aluminide bond coat, and (d) microstructure along a cross-section of the Pt bond coat

Figure 5

An example derived from the Pt bond coat to illustrate the growth mode of the thermally grown oxide, (a) backscattered electron image of along a cross-section of the coating after 216 h of exposure at 1150°C, (b) an energy dispersive X-ray spectrum illustrating the elemental composition of the oxide layer near the top coat (region 1 in (a)), (c) an energy dispersive X-ray spectrum illustrating the elemental composition of the oxide layer near the bond coat (region 2 in (a)), and (d) a schematic illustrating the growth mode of the thermally grown oxide

Figure 6

Analysis of the oxide phase near the oxide bond coat interface after 96 h of exposure at 1150°C: (a) bright-field TEM image showing representative grain structure of the oxide developed by the Pt and Pt-aluminide and bond coats, (b) corresponding selected-area diffraction pattern indexed in terms of the structure of α-Al2O3, (c) energy dispersive X-ray spectrum showing the elemental composition of the oxide developed by the Pt bond coat, and (d) energy dispersive X-ray spectrum showing the elemental composition of the oxide developed by the Pt-aluminide bond coat

Figure 7

An example derived from the Pt-aluminide bond coat to illustrate the role of interfacial Ti-rich oxide particles in the loss of adhesion between the thermally grown oxide and bond coat (96 h of exposure at 1150°C with a 24-h cycling period at room temperature): (a) secondary electron image illustrating the morphology of the bond coat surface, voids are marked by the arrows, (b) secondary electron image illustrating the morphology of the bottom surface of top coat covered by the thermally grown oxide containing particles of Ta- and Ti-rich oxides, (c) an example illustrating the presence of Ti-rich oxide particles at the bottom of voids observed in (a), and (d) a schematic illustrating decohesion of the thermally grown oxide by formation of voids around Ti-rich oxide particles at the oxide-bond coat interface

Figure 2

Wavelength dispersive X-ray spectra illustrating the effect of Pt on the composition of the bond coat surface in the heat-treated condition (as-polished condition): (a) spectrum derived from the surface of the simple aluminide bond coat showing the presence of refractory transition metals, (b) spectrum derived from the Pt-aluminide bond coat showing the surface to be relatively free of transition metals

Figure 3

Effect of the type of bond coat on the performance of thermal barrier coating systems at 1150°C with a 24-h cycling period at room temperature

Figure 4

Oxidation properties of the bond coats: (a)–(e) secondary electron images exemplifying the progressive thickening of the thermally grown oxide on the Pt-aluminide bond during thermal exposure at 1150°C (0 h, 24 h, 72 h, 120 h, and 168 h of exposure, respectively) eventually leading to localized decohesion between the oxide and the bond coat, and (f) comparative thickening rates of the thermally grown oxide

Figure 8

Backscattered electron images illustrating comparative thermal stability characteristics of the bond coats at 1150°C as exemplified by the effect of 48 h of exposure on microstructure: (a) simple aluminide bond coat, (b) Pt-aluminide bond coat, and (c) Pt bond coat

Figure 9

Effect of temperature (T) on the parabolic rate constant (K) for interdiffusion between the superalloy substrate and various bond coats

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