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Research Papers: Gas Turbines: Ceramics

# Optical Nondestructive Condition Monitoring of Thermal Barrier Coatings

[+] Author and Article Information
A. L. Heyes

Department of Mechanical Engineering,  Imperial College London, Exhibition Road, London, SW7 2AZ, UK

J. P. Feist

Southside Thermal Sciences Ltd., c/o IC Innovations, Imperial College London, Exhibition Road, London, SW7 2AZ, UK

X. Chen, Z. Mutasim

Solar Turbines Inc., 2200 Pacific Highway, P.O. Box 85376,   San Diego, CA 92186-5376

J. R. Nicholls

Cranfield University, College Road, Cranfield, Bedfordshire, MK43 0AL, UK

J. Eng. Gas Turbines Power 130(6), 061301 (Aug 28, 2008) (8 pages) doi:10.1115/1.2940988 History: Received April 27, 2007; Revised September 04, 2007; Published August 28, 2008

## Abstract

This paper describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and online condition monitoring in gas turbines. Increases in turbine inlet temperature in the pursuit of higher efficiency will make it necessary to improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation, it will also be necessary to devise techniques for online condition monitoring and NDE. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving NDE for TBCs. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia (YSZ)) with a rare-earth activator to provide the coating with luminescence when excited with UV light. This paper describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented along with the results of comparative thermal cycle testing of TBSCs, produced using a production standard air plasma spray system. The latter show the durability of TBSCs to be similar to that of standard YSZ TBCs and indicate that the addition of the rare-earth dopant is not detrimental to the coating. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer, which in turn destabilizes the coating system as it becomes thicker. Experimental data are presented, indicating that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers covered by as much as $500 μm$ of standard TBC material. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made, and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.56 mm thickness.

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## Figures

Figure 1

Temperature dependent emission decay of thermographic phosphors. For each decay curve, an exponential decay function is fitted to the data and the decay characterized by the time constant τ.

Figure 2

Proposed embodiments of the thermal barrier sensor coating technology. (a) Average temperature/ location independent degradation. (b) Bond coat/TGO interface temperature or metal surface temperature. (c) Heat flux gauge or thermal conductivity measurement. (d) Erosion sensor.

Figure 3

(a) Microstructure of the sensor TBC in as-coated condition and (b) after failure along ceramic/bond coat interface

Figure 4

Experimental setup for phosphor/sensor coating calibration

Figure 5

Exponential decay time constant versus temperature for coatings with a YSZ overcoat of various thicknesses

Figure 6

Emission spectra of TBSCs at room temperature and excited at 355 nm

Figure 7

Phosphorescent emission integrated over the emission period and normalized by the background radiation over the same period

Figure 8

Emission ratio as a function of coating thickness under constant illumination at 400°C.

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