Photo-stimulated phosphorescence for thermal condition monitoring and nondestructive evaluation in thermal barrier coatings

This article describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and on-line condition monitoring in gas turbines. New and enhanced instrumentation to measure surface temperature distributions and heat flux and to monitor TBC health are regarded as a priority by the industry. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving such measurements. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia) with a rare earth activator to provide the coating with luminescence when excited with ultraviolet (UV) light. The article describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented from a coating produced using a production standard spray system. 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. Preliminary experimental data are presented that indicate that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers. 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 1.33 mm thickness.