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

# Influence of Titanium in Nickel-Base Superalloys on the Performance of Thermal Barrier Coatings Utilizing $γ−γ′$ Platinum Bond Coats

[+] 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 133(4), 042101 (Nov 19, 2010) (6 pages) doi:10.1115/1.4002154 History: Received April 10, 2010; Revised April 11, 2010; Published November 19, 2010; Online November 19, 2010

## Abstract

Titanium is a key element in nickel-base superalloys needed with aluminum to achieve the desired volume fraction of the strengthening $γ′$-phase. However, depending upon its concentration, titanium can degrade the adherence of aluminum oxide by forming $TiO2$ particles near the oxide-metal interface. This effect is extended to thermal barrier coating systems where in this case, the bond coat replaces the superalloy as the underlying substrate. Noting that the onset of failure of thermal barrier coating systems coincides with the first spall of the thermally grown oxide, titanium level in the superalloy can have an important effect on the useful life of the coating. Therefore, this study was undertaken to examine the effect of titanium on the performance of a thermal barrier coating system. Included in the study were two Ni-base superalloys with similar chemical composition except for the Ti content and a Pt-containing bond coat consisting of $γ′+γ$-phases all top coated with zirconia stabilized by $7 wt %$ yttria. Coating performance was evaluated from thermal exposure tests at $1150°C$ with a 24 h cycling period to room temperature. Various electron-optical techniques were used to characterize the microstructure. The coating system on the low-Ti alloy was found to outperform that on the high-Ti alloy. However, for both alloys, failure was observed to occur by loss of adhesion between the thermally grown oxide and underlying bond coat.

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## Figures

Figure 1

An example derived from alloy 1 to illustrate characteristic microstructural features of the coating system in the as-deposited condition: (a) backscattered electron image along a cross section of the coating and into the alloy substrate showing the gross microstructural features, (b) backscattered electron image showing typical microstructural features of the bond coat, and (c) an X-ray diffraction pattern derived from the surface of the bond coat

Figure 2

Comparative performance of the coating system on the two alloys included in the study as determined from thermal exposure tests at 1150°C with a 24 h cycling period to room temperature: (a) average coating life, (b) secondary electron image along a cross section of the bond coat on alloy 1 after 120 h of exposure at 1150°C showing localized decohesion between the bond coat and thermally grown oxide, and (c) secondary electron image along a cross section of the bond coat on alloy 2 after 120 h of exposure at 1150°C; although the oxide thickness was about the same as for alloy 1, good adherence to the bond coat was still maintained

Figure 3

Comparative thickening rate of the thermally grown oxide during thermal exposure at 1150°C in air with a 24 h cycling period to room temperature

Figure 4

An example derived from alloy 1 to illustrate the thermal stability of the bond coat during thermal exposure at 1150°C. (a)–(e) Backscattered electron images along a cross section of the bond coat: (a) as-deposited condition, (b) 24 h of exposure, (c) 48 h of exposure and 72 h of exposure, (d) 96 h of exposure, and (e) 120 h of exposure.

Figure 5

Effect of exposure time at 1150°C on the concentrations of Pt and Ni in the bond coat on alloy 1: (a) surface layer of γ′-phase, (b) inner layer of γ′-phase, (c) γ-phase near the surface, and (d) inner layer of γ-phase

Figure 6

Effect of exposure time at 1150°C on the concentrations of Pt and Ni in the bond coat on alloy 2: (a) surface layer of γ′-phase, (b) inner layer of γ′-phase, (c) γ-phase near the surface, and (d) Inner layer of γ-phase

Figure 7

Effect of exposure time at 1150°C on the Ti concentration in the bond coat on alloys 1 and 2: (a) surface layer of γ′-phase, and (b) γ-phase near the surface

Figure 8

An example derived from alloy 1 to illustrate typical microstructural features of the surfaces exposed by failure of the coating system (288 h of exposure at 1150°C): (a) exposed surface of the top coat (top: secondary electron image, bottom: backscattered image), (b) exposed surface of the bond coat (top: secondary electron image, bottom: backscattered image), (c) secondary electron image of the exposed surface of bond coat showing voids containing Ti-rich oxide particles, and (d) an energy dispersive X-ray spectrum showing the composition of the particle in (c) as indicated by the arrow

Figure 9

Schematics illustrating possible sequence of events leading to loss of adhesion between the thermally grown oxide and bond coat by formation of Ti-rich oxide particles near the oxide-bond coat interface: (a) formation of voids around oxide particles, (b) coalescence of voids, and (c) morphology of the exposed surface of bond consistent with the image of Fig. 8

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