Thermal History Coatings: Part ӏӏ — Measurement Capability up to 1500 ⁰C

To improve the efficiency of gas turbines, the turbine inlet temperature needs to be increased. The highest temperature in the gas turbine cycle takes place at the exit of the combustion chamber and it is limited by the maximum temperature turbine blades, vanes and discs can withstand. A combination of advanced cooling designs and Thermal Barrier Coatings (TBCs) are used to achieve material surface temperatures of up to 1200°C. However, further temperature increases and materials that can withstand the harsh temperatures are required for next-generation engines. Ceramic Matrix Composites (CMCs) are state-of-the-art structures that can achieve operating temperatures above 1500°C, but accurate data regarding the thermal load on the components must be well understood to ensure the component life and performance. However, temperature data is very difficult to accurately and reliably measure because the turbine rotates at high speed, the temperature rises very quickly with engine start-up and the blades operate under harsh environments.

At the operating temperature range of CMCs, typically platinum thermocouples are used, however, this material is incompatible with silicon carbide CMCs. Other temperature techniques such as infrared cameras and pyrometry need optical access and the results are affected by changes in emissivity that can take place during operation. Offline techniques, in which the peak temperature information is stored and read-out later, overcome the need for optical access during operation. However, the presently available techniques, such as thermal paint and thermal crystals cannot measure above ~1400°C. Therefore, a new measurement technique is required to acquire temperature data at extreme temperatures.

To meet this challenge, Sensor Coating Systems (SCS) is focused on the development of Thermal History Coatings (THC) that measure temperature profiles in the 900-1600 °C range. THC are oxide ceramics deposited via air plasma spraying process. This innovative temperature profiling technique uses optically active ions in a ceramic host material that start to phosphoresce when excited by light. After being exposed to high temperatures the host material irreversibly changes at the atomic level affecting the phosphorescence properties which are then related to temperature through calibration.

This two-part paper describes the THC technology and demonstrates its capabilities for high-temperature applications. In this second part, the THC is implemented on rig components and tested at very high temperatures for the first time. In one test, the THC was implemented on a burner rig assembly on metallic alloys, which was thoroughly instrumented with thermocouples. The measurements acquired with the THC are validated against the thermocouples. In another test, the THC was applied to environmental barrier coatings developed by NASA, as part of a ceramic-matrix-composite system and heat-treated up to 1500°C. The results indicate the THC could provide a unique capability for measuring high temperatures on current metallic alloys as well as next-generation materials. Further analysis of the material through electron microscopy, Raman and X-ray diffraction has improved the understanding of the link between the material properties and their luminescence characteristics.