Luminescence for the non-destructive evaluation of thermal barrier coatings

Abstract

Gas turbines provide an efficient source of power for aviation and electricity generation and future demand is set to increase. Efforts to further improve efficiencies have led to the integration of thermal barrier coatings (TBCs) in advanced turbines. In order to maximise the efficiency gains offered by the coatings, accurate lifing models must be developed based on reliable measurements of the operating conditions and supported by quantitative non-destructive evaluation (NDE) of coating degradation in service.

Although a range of NDE techniques have been studied for this application, the nature of the coatings present particular practical challenges. A novel approach, introduces rare-earth dopants into the TBC ceramic to make the material luminescent. This approach has several technical advantages including the capability for in-situ measurement and assessment of damage before it becomes critical. The present study investigates the use of luminescence for NDE of TBCs through two applications, erosion detection and thermal history measurement. The former detects partial failure of the ceramic coating while the latter records the thermal exposure of the coating material which is related to the primary failure mode of TBCs.

The addition of different dopants in layers can be used to determine the remaining coating thickness. The first quantitative study of this approach, conducted in this project, has demonstrated, through two methods of image processing, that multi-layered doped coatings provide a detailed, precise and accurate profile of the erosion damage. An estimated precision of ±5μm was achieved while the accuracy of the depth profile was comparable to alternative, sophisticated thickness profiling techniques. Furthermore, the addition of dopants did not alter the failure mechanisms compared to standard TBC architectures and coatings have exhibited no damage after operation on a turbine blade in a Rolls-Royce Viper jet engine.

The luminescence is affected by the microstructure of the host material which can be used to record the extent of the thermal exposure of the coating. A greater understanding of the link between the microstructural and luminescence properties was achieved by comparing luminescence measurements to results from standard materials characterisation techniques. Powder samples were synthesized by the sol-gel route to provide the first evidence of a link between the phosphorescent decay time and the crystallite size, explained by energy transfer to quenching sites at the edges of crystallites.

YAG:Eu coating samples were produced using the same method as for TBCs. Heat treatment of the coatings instigated crystallisation which caused changes to the emission spectrum and an increase in the decay time, enabling temperature measurements between 300 and 880°C. A similar coating was applied and successfully tested in an engine test bed and measurements of the phosphorescence indicated the thermal profile during operation.

When embedded in a TBC, the changes in the phosphorescence of the YAG:Eu altered such that temperature measurement range extends to at least 1150°C, covering the typical operating range of TBCs. This suggested that the mixed material is a favourable candidate for thermal history measurements in TBCs.