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Research Papers

# Stress Distributions in Plasma-Sprayed Thermal Barrier Coatings Under Thermal Cycling in a Temperature Gradient

[+] Author and Article Information
Andi M. Limarga

School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138limarga@seas.harvard.edu

Robert Vaßen

Institut für Werkstoffe und Verfahren der Energietechnik 1, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

David R. Clarke

School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138

J. Appl. Mech 78(1), 011003 (Oct 08, 2010) (9 pages) doi:10.1115/1.4002209 History: Received December 11, 2009; Revised July 08, 2010; Posted July 23, 2010; Published October 08, 2010; Online October 08, 2010

## Abstract

The residual stress distribution in plasma-sprayed zirconia thermal barrier coatings subjected to cyclic thermal gradient testing was evaluated using Raman piezospectroscopy and finite element computation. The thermal gradient testing (approximately $440°C/mm$ at temperature), consisted of repeated front-side heating with a flame and constant cooling of the back-side of the substrate either with front-side radiative cooling only or with additional forced air cooling between the heating cycles. The coatings exhibited characteristic “mud-cracking” with the average crack spacing dependent on the cooling treatment. This is consistent with finite element calculations and Raman spectroscopy measurements in which the sudden drop in coating surface temperature on initial cooling leads to a large biaxial tension at the surface. The key to proper interpretation of the Raman shifts is that the stress-free Raman peaks need to be corrected for shifts associated with the evolution of the metastable tetragonal phase with aging.

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## Figures

Figure 1

Cross-sectional micrograph of the plasma-sprayed thermal barrier coating in its as-sprayed condition

Figure 2

Experimental setup of the cyclic thermal gradient experiment

Figure 3

Measured temperatures of the metal and the coating surface during thermal gradient cycling: (a) with back-side cooling and (b) with front-side cooling with compressed air. The bottom panel shows the temperature difference between the coating and the metal.

Figure 4

Typical Raman spectrum and peak-fitting results from tetragonal zirconia. Residual stress in the coating is determined by evaluating the shift of Raman peak at 465 cm−1.

Figure 5

Temperature difference between the metal and the coating surface measured during experiment with front-side cooling and calculated using finite element analysis

Figure 6

Optical micrograph of the coatings after planarizing: (a) in the as-sprayed condition, (b) after thermal cycling with only backside cooling, and (c) after thermal cycling with back and front-side cooling on the last cycle

Figure 7

Evolution of stresses in the coating and the temperature difference between TBC surface and TBC/TGO interface calculated by finite element analysis for the two cooling scenarios: (a) with only back-side cooling and (b) with back and front sides cooling

Figure 8

Strain-free position and width of the Raman peak around 465 cm−1 of a fragment of APS TBC annealed for 27 h at the various temperatures indicated

Figure 9

Raman peak shift at different location within the coating subjected to cyclic thermal gradient (a) with back-side cooling only and (b) with front-side cooling on the last cycle. The strain-free peak positions at different locations were calculated by combining the temperature-dependent peak position (shown in Fig. 6) and temperature distribution calculated using finite element analysis.

Figure 10

Strain distribution through the coating thickness calculated from the measured Raman peak shift. The lines are for visual guide.

Figure 11

Stress distributions in the coating calculated by finite element analysis and measured by Raman spectroscopy (a) for specimens cooled only from the back-side and (b) for specimen cooled from both sides at the last cycle. The calculated stresses at HT represent those immediately after the temperature of the coating surface drops.

Figure 12

Apparent residual stress distribution measured by Raman spectroscopy when a constant stress-free Raman peak shift is used in the calculation: (a) for specimens cooled only from the back side and (b) for specimen cooled from both sides at the last cycle

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