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

Abstract

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 verify the design and component durability. 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 comprised of oxide ceramics deposited via Atmospheric Plasma Spraying (APS) to create a robust coating. APS deposition employs several 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. Extensive calibration data was 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 powder composition and the power and gas flow used during deposition were found to be the most influential parameters. The model identified the optimum spray parameters and was used to demonstrate THCs can achieve measurements in excess of 1600°C.