Application of Photothermal Emission Analysis (PopTea) to quality control of thermal barrier coatings

Phase of Photothermal Emission Analysis (PopTea) is well suited for in-production quality control of thermal barrier coatings.  PopTea is a measurement technique that employs periodic heating to probe heat transfer characteristic of coatings by looking at thermal emission.    PopTea exploits the thermal contrast between the coating and substrate materials to measure two independent thermal properties of the coating. This allows the thermal diffusivity, thermal conductivity and volumetric heat capacity (directly related to density) of the coating to be established.  Since measurements are conducted entirely from the front surface, it is ideally suited for noninvasive measurements of coating thermal properties on serviceable parts.  However, some difficulties for the PopTea technique can arise when diffusion times for heat transfer through the coating are not short compared with diffusion times for heat transfer through the underlying wall structure of the part.  In such a situation, the heat transfer description on which the standard PopTea measurement is based becomes poorly defined and can adversely affect measurements of coating thermal properties.  To address this, the “differential” PopTea measurement has been developed to ensure that heat transfer through the wall structure is correctly accounted for.  This paper demonstrates the successful application of this technique to a situation in which poorly understood heat transfer conditions exist beyond a certain depth in the wall material.