Differential phase of photothermal emission analysis for thermal property measurement of thermal barrier coatings

•Thermal properties measurements of thermal barrier coatings.•Model characterizing heat transfer beyond well-defined substrate thickness.•Differential technique enables coating measurement on complex subsurface structure.

The utility of phase of photothermal emission analysis (PopTea) for thermal property measurements of coatings is extended by the use of differential measurements. In PopTea, the phase of thermal emission from a periodically heated coating is measured to determine thermal properties of the coating. Measurements become differential when characteristics of heat transfer into the substrate system are measured in addition to the coating properties. For typical thermal barrier coating systems, the need for differential measurements arises when the coating thickness approaches or exceeds roughly one third of the underlying metal thickness. In such a situation, heat transfer into the subsystem becomes poorly defined and can adversely affect measurements of coating thermal properties. In this paper, a model for a substrate system is explored in the context of mitigating the effects of poorly understood heat transfer conditions beyond a certain thickness of the substrate material. The substrate system model is used in differential PopTea (dPopTea) measurements and is shown to be an effective way of addressing measurement conditions when engine hardware has relatively thin structural walls.