Off-Line Temperature Profiling Utilizing Phosphorescent Thermal History Paints and Coatings

Abstract

Temperature profiling of components in gas turbines is of increasing importance as engineers drive to increase firing temperatures and optimize component’s cooling requirements in order to increase efficiency and lower CO2 emissions. However, on-line temperature measurements and, particularly, temperature profiling are difficult, sometimes impossible, to perform due to inaccessibility of the components. A desirable alternative would be to record the exposure temperature in such a way that it can be determined later, off-line. The commercially available thermal paints are toxic in nature and come with a range of technical disadvantages such as subjective readout and limited durability. This paper proposes a novel alternative measurement technique which the authors call thermal history paints and thermal history coatings. These can be particularly useful in the design process, but further could provide benefits in the maintenance area where hotspots which occurred during operation can be detected during maintenance intervals when the engine is at ambient temperature. This novel temperature profiling technique uses optical active ions in a ceramic host material. When these ions are excited by light they start to phosphoresce. The host material undergoes irreversible changes when exposed to elevated temperatures and since these changes are on the atomic level they influence the phosphorescent properties such as the life time decay of the phosphorescence. The changes in phosphorescence can be related to temperature through calibration such that in situ analysis will return the temperature experienced by the coating. A major benefit of this technique is in the automated interpretation of the coatings. An electronic instrument is used to measure the phosphorescence signal eliminating the need for a specialist interpreter, and thus increasing readout speed. This paper reviews results from temperature measurements made with a water-based paint for the temperature range 100–800 °C in controlled conditions. Repeatability of the tests and errors are discussed. Further, some measurements are carried out using an electronic hand-held interrogation device which can scan a component surface and provide a spatial resolution of below 3 mm. The instrument enables mobile measurements outside of laboratory conditions. Further, a robust thermal history coating is introduced demonstrating the capability of the coating to withstand long term exposures. The coating is based on thermal barrier coating (TBC) architecture with a high temperature bondcoat and deposited using an air plasma spray process to manufacture a reliable long lasting coating. Such a coating could be employed over the life of the component to provide critical temperature information at regular maintenance intervals for example indicating hot spots on engine parts.