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Research Papers: Gas Turbines: Ceramics

# Compressive Creep Testing of Thermal Barrier Coated Nickel-Based Superalloys

[+] Author and Article Information
Ventzislav G. Karaivanov, Sean Siw, Minking K. Chyu

Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261

William S. Slaughter1

Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261wss@pitt.edu

Mary Anne Alvin

National Energy Technology Laboratory, Pittsburgh, PA 15236

1

Corresponding author.

J. Eng. Gas Turbines Power 133(9), 091301 (Apr 14, 2011) (9 pages) doi:10.1115/1.4002816 History: Received June 04, 2010; Revised June 30, 2010; Published April 14, 2011; Online April 14, 2011

## Abstract

Turbine airfoils have complex geometries and, during service operation, are subjected to complex loadings. In most publications, results are typically reported for either uniaxial, isothermal tensile creep or for thermal cyclic tests. The former generally provides data for creep of the superalloy and the overall performance, and the later provide data for thermal barrier coating (TBC) spallation as a function of thermally grown oxide thickness, surface roughness, temperature, and thermal mismatch between the layers. Both tests provide valuable data but little is known about the effect of compressive creep strain on the performance of the superalloy/protective system at elevated temperatures. In conjunction with computational model development, laboratory-scale experimental validation was undertaken to verify the viability of the underlying damage mechanics concepts for the evolution of TBC damage. Nickel-based single crystal René N5 coupons that were coated with a $∼150–200 μm$ MCrAlY bond coat and a $∼200–240 μm$ 7-YSZ APS top coat were used in this effort. The coupons were exposed to $900°C$, $1000°C$, and $1100°C$, for periods of 100 h, 300 h, 1000 h, and 3000 h in slotted silicon carbide fixtures. The difference in the coefficients of thermal expansion of the René N5 substrate and the test fixture introduces thermally induced compressive stress in the coupon samples. Exposed samples were cross sectioned and evaluated using scanning electron microscopy. Performance data were collected based on image analysis. Energy-dispersive X-ray was employed to study the elemental distribution in the TBC system after exposure. To better understand the loading and failure mechanisms of the coating system under loading conditions, nanoindentation was used to study the mechanical properties (Young’s modulus and hardness) of the components in the TBC system and their evolution with temperature and time. The effect of uniaxial in-plane compressive creep strain on the rate of growth of the thermally grown oxide layer, the time to coating failure in TBC systems, and the evolution in the mechanical properties are presented.

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## Figures

Figure 7

Young’s modulus (GPa) at 900°C: (a) SB and SI, (b) BCB and BCI, and (c) TC

Figure 8

Hardness (GPa) at 900°C: (a) SB and SI, (b) BCB and BCI, and (c) TC

Figure 9

SEM images of cross sectioned samples exposed to 1100°C for 100 h, 300 h, and 600 h

Figure 10

Compact TGO thickness at 1100°C, measured and data from literature (8)

Figure 13

Hardness (GPa) at 1100°C: (a) SB and SI, (b) BCB and BCI, and (c) TC

Figure 12

Young’s modulus (GPa) at 1100°C: (a) SB and SI, (b) BCB and BCI, and (c) TC

Figure 11

Coalescent cracks within the oxide scale at the edge of the sample at 600 h, 1100°C

Figure 6

Compact TGO thickness at 900°C, measured and data from literature (8)

Figure 5

SEM images of cross sectioned samples exposed to 900°C for 100 h, 300 h, and 1000 h. Unstressed samples are shown at left column, stressed at the right.

Figure 4

Indentation pattern: ten indents, 5 μm apart within the β-phase depleted zone of the BCI

Figure 3

Indentation locations shown on the SEM image of a sample exposed to 1100°C for 300 h

Figure 2

Test fixture and sample placement

Figure 1

SEM image of as processed sample

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