This feature article explores the concept of creating functionally graded metal‐ceramic composite microstructures for thermal barrier coatings used in gas‐turbine applications. From a thermomechanical perspective, this concept offers the possibility of significantly improving the life and reliability of thermal barrier coatings. However, prior research reveals that progress has been somewhat limited because of the oxidative instability exhibited by some metal‐ceramic composite microstructures. The present study addresses some of the materials criteria and research issues associated with preparing chemically stable, yet mechanically durable, graded metal‐ceramic microstructures for realistic application environments.

This article describes recent developments of the thermal barrier sensor concept for non-destructive evaluation (NDE) of thermal barrier coatings (TBCs) and on-line condition monitoring in gas turbines. Increases in turbine entry temperature in pursuit of higher efficiency will make it necessary 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 on-line conditions monitoring and NDE. Thermal barrier sensor coatings, which consist of a ceramic doped with rare-earth activator to provide luminescence, may be a possible solution. The thermo-luminescent response of such materials has been shown to be suitable for surface and sub-surface temperature measurement and possibly for material phase determination. Herein we describe a number of steps in the development of the sensor coating technology. For the first time sensor coatings have been successfully produced using a production standard air plasma spray (APS) process. Microscopic analysis of the coatings showed them to be similar to standard TBCs and thermal cycle testing of the coatings to destruction showed them to exhibit durability similar to that of standard TBCs suggesting that the addition of rare earth dopants to produce sensor coatings does not change the material structure or the longevity of coatings. Calibration of the coatings using the lifeteime decay response mode showed them to have a dynamic range for temperature measurement extending to just under 10001C. However, it should be noted that newer compositions have been shown to respond up to 13001C. Finally, a study of surface temperatures and film cooling has been conducted in a research combustor using APS sensor coatings and some preliminary results are presented.Int. J. Appl. Ceram. Technol., 2 [5] 414-421 (2005) Ceramic Product Development and Commercialization *a.heyes@imperial.ac.uk

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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.