The ability to measure temperature in extreme environments such as the hot sections of gas turbines is critically important. Several on-line techniques exist but it is often not possible to measure in real-time the temperature of all surfaces of interest. Indeed, some surfaces are so inaccessible as to require complex, costly and intrusive instrumentation for on-line temperature measurement. Here, off-line sensors, also called thermal history sensors, can be used to record the temperatures to which they are exposed, in such a way that they can be extracted later off-line, at room temperature.

Probably the best-known types of thermal history sensor are the colour changing thermal paints, that are widely used in gas turbine development. These have been valuable tools of engine developers for many years, but their use presents a number of challenges so that alternatives would be welcome.

This paper reports the latest developments of a thermal history sensor based on phosphors that undergo permanent changes in their luminescence properties when exposed to high temperatures. Such thermal history sensors have several advantages over and address many of the shortcomings of existing sensors. The paper contains details of the application of a phosphor-based temperature indicating paint based on Y2SiO5:Tb suspended in a chemical binder. The binder was found to influence the optical properties of the phosphor but despite this, a viable sensor paint for temperatures in the range 400°C to 900°C was formed.

A thermal history coating was installed using a thermal barrier coating architecture, applied on various components of a Royce-Rolls Viper 201 engine owned by STS and operated for a number of hours at Cranfield University. Post-operation analysis revealed a temperature distribution on the surfaces/components and enabled hotspots to be identified. Overall the results suggest that phosphor-based temperature indicating paints have the potential to surpass the capability of existing paints.

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