Development of Nondestructive Evaluation Methods for Thermal Barrier Coatings

Nondestructive evaluation (NDE) methods are being developed at Argonne National Laboratory for thermal barrier coatings (TBCs) applied to components in the hot-gas path of advanced high-efficiency and low-emission gas turbines, including syn-gas fired turbines. As TBCs become “prime reliant,” it becomes important to know their conditions nondestructively to assure the reliability of these components. For these applications, quantitative NDE methods to determine the physical and geometrical TBC parameters are essential. This work is focused on developing several quantitative NDE methods for direct measurement and imaging of TBC thickness and thermal conductivity. These NDE methods can be used to assess the reliability of new coating processes, identify defective components that could cause unscheduled outages, monitor degradation rates during engine service, and provide data for reaching rational decisions on replace/repair/re-use of components. INTRODUCTION Thermal barrier coatings (TBCs), applied by two deposition methods, electron beam– physical vapor deposition (EB-PVD) and air plasma spraying (APS), allow for metallic components to be utilized at higher temperatures in the hot-gas path of gas turbines, including syn-gas fired turbines. As TBCs become “prime reliant” to the performance and reliability of the engine components such as vanes, blades, and combustor liners, it becomes important to know their condition nondestructively after coating application and at scheduled or unscheduled outages. For these applications, quantitative nondestructive evaluation (NDE) methods to determine the physical and geometrical TBC parameters are essential. Although various NDE methods have been reported in the literature, most are not capable to deriving quantitative data and/or providing property distributions (images) over the entire surface of TBC-coated components. Work at Argonne National Laboratory (ANL) is underway to develop quantitative NDE methods for TBCs. TBC failure normally starts from initiation of small cracks at the TBC/bond coat interface. These cracks then grow and link together to form delaminations which eventually cause TBC spallation. Two TBC parameters, thickness and thermal conductivity, are important in TBC degradation process because they determine the thermal gradient that affects stress distribution in crack development. Recent effort at ANL was directed to develop optical and thermal-imaging methods to measure TBC thickness and thermal conductivity. An optical coherence tomography method has been used for 3D imaging of TBC cross sections. Direct measurement of TBC thickness and its distribution is achieved when the refractive index of the TBC material is accounted for. However, optical methods are effective only for relatively thin TBCs because of limited optical penetration depth. On the other hand, thermal-imaging methods are applicable for both thin and thick TBCs. Two thermal imaging methods are being developed