Temperature Sensing inside Thermal Barrier Coatings using Phosphor Thermometry

Abstract

By introducing a few percent of rare earth oxides into existing Thermal Barrier Coating (TBC) materials, their functionality can be enhanced. The low fraction of the rare earth means the primary function of the coating as a thermal barrier can be maintained. However, it provides additional sensing capabilities. Embedded in the crystal, the rare earth ions act as optical centres making the material phosphorescent. The properties of the phosphorescence depend on temperature, among other parameters, such that the functional coating can be used to conduct phosphor thermometry. Accordingly, the temperature of the coating can be measured by exciting and detecting the phosphorescence from the coating. This technique has been under investigation for many years, although was only recently demonstrated on an operating gas turbine engine. Implementation of a phosphorescent measurement system was carried out in a Rolls-Royce jet engine using dysprosium doped yttrium stabilised zirconia as a sensor. During this implementation, a thermal gradient across the ceramic coating of some components of the engine existed due to cooling effects. This paper complements the previous work by investigating the effect on the temperature measurements accuracy of this thermal gradient.TBCs are translucent therefore the excitation light is expected to penetrate through the coating causing the phosphorescent response to be an integrated signal. When the thermal gradient through the coating thickness is significant relative to the measurement accuracy of the system then it might introduce ambiguity in the measurement. Therefore, the effect of the gradient must be considered independently. The focus of the current contribution is on the interpretation of temperature measurements from phosphorescent TBCs in a thermal gradient, such as those in the engine test. For the study, a specialised cyclic thermal gradient burner test rig was operated 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 longwavelength pyrometer was employed to detect the surface temperature of the coating. A correction was applied to compensate for changes in emissivity using two methods. A thermocouple was continuously used to measure the substrate temperature of the sample. Typical gradients across the coating are less than 1K/μm.The study successfully proved that the temperature indication from the phosphorescent TBC remains between the surface and substrate temperature for all operating conditions. This demonstrates the possibility of measuring inside the coating closer to the bond coat, using a fully doped coating to maintain signal strength.

Upconversion phosphor thermometry for use in thermal barrier coatings

“…excitation wavelength shorter than emission wavelength, with phosphor layers embedded on top of the bond coat layer and on the surface of TBCs, has been used [2][3][4][5]. In addition, downconversion measurements on uniformly lanthanide-doped TBC layers have also been performed [6,7]. However, to the authors' knowledge, no study has been conducted to evaluate the use of upconversion phosphor thermometry, i.e.…”

Phosphor thermometry instrumentation for synchronized acquisition of luminescence lifetime decay and intensity on thermal barrier coatings

“…Currently, the viable techniques for non-contact in situ temperature measurements are infrared thermometry, for which precision is limited by the presence of emissions from the operation of the turbine engines, as well as emissivity variation, and phosphor thermometry, which shows potential as a reliable method for precision temper ature measurements [6,7]. Phosphor thermometry has proven to be effective at high temperatures using rare-earth or transition metal-doped ceramics that can be embedded into TBC configurations to enable real-time temperature monitoring in service conditions [8][9][10][11][12][13]. Among the possible sensors for high temperature measurements integrated into TBCs, rare-earth doped yttria-stabilized zirconia (YSZ:RE) have been largely studied as they offer sensing capabilities with ease of integration in existing standard TBCs and no layer compatibility mismatches.…”