On-line temperature measurement inside a thermal barrier sensor coating during engine operation

Existing thermal barrier coatings (TBC) can be adapted enhancing their functionalities such that they not only protect critical components from hot gases, but also can sense their own material temperature or other physical properties. The self-sensing
capability is introduced by embedding optically active rare earth ions into the thermal barrier ceramic. When illuminated by light the material starts to phosphoresce and the phosphorescence can provide in-situ information on temperature, phase changes, corrosion or erosion of the coating subject to the coating design. The integration of an on-line temperature detection system enables the full potential of TBCs to be realised due to improved accuracy in temperature measurement and early warning of degradation. This in turn will increase fuel efficiency and will reduce CO2 emissions. This paper reviews the previous implementation of such a measurement system into a Rolls-Royce jet engine using dysprosium doped yttrium-stabilised-zirconia as a single layer and a dual layer sensor coating material. The temperature measurements were carried out on cooled and uncooled
components on a combustion chamber liner and on nozzle guide vanes respectively. The paper investigates the interpretation of those results looking at coating thickness effects and temperature gradients across the TBC. For the study a specialised cyclic thermal gradient burner test rig was operated and instrumented using equivalent instrumentation to that used for the engine test. This unique rig enables the controlled heating of the coatings at different temperature regimes. A long-wavelength pyrometer
was employed detecting the surface temperature of the coating in combination with the phosphorescence detector. A correction
was applied to compensate for changes in emissivity using two methods. A thermocouple was used continuously measuring the
substrate temperature of the sample. Typical gradients across the coating are less than 1K/µm. As the excitation laser penetrates the coating it generates phosphorescence from several locations throughout the coating and hence provides an integrated signal.
The study successfully proved that the temperature indication from the phosphorescence coating remains between the surface and substrate temperature for all operating conditions. This demonstrates the possibility to measure inside the coating closer to the bond coat. The knowledge of the bond coat temperature is relevant to the growth of the thermally grown oxide which is linked to the delamination of the coating and hence determines its life. Further, the data is related to a one dimensional phosphorescence model determining the penetration depth of the laser and the emission. Note: a video of the measurement system can be watched
under: [http://www.youtube.com/watch?v=T6uXN1__Z7I]