Thermal History Coatings: Part I — Influence of Atmospheric Plasma Spray Parameters on Performance

Firing temperatures in gas turbines have seen a steady increase over the years to allow for higher engine efficiencies and a decrease in hazardous emission levels. Conversely, these harsh conditions severely challenge the component lifetime, requiring a trade-off during the design process. Thus, it is crucial to understand temperature distribution across the majority of a component surface (>80%) to evaluate the presence of hotspots. Accurate knowledge of temperatures allows the design engineer to evaluate new component prototypes as well as validate thermal models and complex cooling designs. While a range of temperature measurement techniques are available, these are primarily focused on lower temperatures, exhibit low durability (thermal paints), require line of sight (pyrometers), are destructive (thermal crystals) and only provide point measurements (thermocouples, thermal crystals).

To overcome this challenge, Sensor Coating Systems (SCS) have developed Thermal History Coatings (THCs) to measure temperature profiles in the 900-1600°C range. This new temperature profiling capability records the past maximum exposure temperature in such a way that it can be determined once the component has already cooled down. THCs are composed of optically active ions in a ceramic host material that start to luminesce when excited by light. When exposed to very high temperatures, the host material gradually and irreversibly changes at the atomic level, affecting the luminescent properties. Through calibration, the luminescent characteristics are then related to a single past temperature of exposure, enabling the evaluation of thermal history at discrete locations.  Measurement of the luminescent properties over a full component surface using custom-made optical equipment allows for full surface coverage of the temperature data.

THCs are comprised of oxide ceramics deposited via Atmospheric Plasma Spraying (APS) to create a robust coating. APS is a versatile thermal spray deposition processes whereby raw spray powders are  fired through a plasma, where they are exposed above their melting temperature and accelerated towards a substrate. The particles impact the substrate where they flatten and solidify forming a layering of splats which rapidly quench, leading to high amorphous content. APS deposition is a complex process with a large number of variable parameters; spray settings such as gun power, gas flow or scan rate can affect the particle exposure and thus, the microstructure of the coating and its temperature sensing performance.

This two-part paper covers the THCs principles and demonstrates their capabilities for high-temperature applications. This first part shows, for the first time, the influence of APS parameters on luminescent measurements due to changes in the material microstructure and properties. The THC was applied to a Thermal Barrier Coating (TBC) as a representative material found in gas turbine applications. Extensive calibration data showing the evolution of the luminescent performance over temperature when varying APS settings was studied. The results were used to develop a new model to relate the APS spray parameters to the luminescent properties in the as-deposited condition and consequent performance as a temperature sensor. The model identified the optimum spray parameters and was used to demonstrate THCs can achieve measurements in excess of 1500°C.