A Method for Measuring Thermal Properties of Coatings on Engine Hardware

A new method based on Phase of Photothermal Emission Analysis (PopTea), a nondestructive test method for determining the transport properties of thermal barrier coatings (TBCs), was developed to find the thermal properties (diffusivity, conductivity, and heat capacity) of thick coatings on relatively thin substrates. TBCs are ceramic coatings (100 – 1000 µm) that are commonly used in gas turbines to provide thermal protection to underlying superalloy substrates. The ability to quantitatively characterize the heat transfer performance of a TBC is necessary to realizing a predictable life operation or “prime reliance” of these coatings. One difficulty of applying PopTea to engine hardware is when heat transfer characteristics become “ill-defined” beyond a certain finite thickness of the coated structural material. Coating thickness greater than a third of the substrate thickness demonstrated ill-defined behavior. To overcome this difficulty, a strategy for differential measurements is developed where characteristics of the heat transfer into the substrate system are measured in addition to the coating properties. A model for an ill-defined substrate system is explored in the context of characterizing the heat transfer beyond the well-defined extent of the substrate material. Differential PopTea (dPopTea) measurements were made of a TBC system mounted on a test-structure that introduced a complex heat transfer path. These measurements were contrasted with standard PopTea measurements of the TBC system. It was demonstrated that dPopTea measurements on an ill-defined substrate system is able to determine the thermal properties of the coating to within 10% of the value obtained from standard PopTea measurements.