Development of Nondestructive Evaluation Methods for Ceramic Coatings and Membranes

Nondestructive evaluation (NDE) methods are being developed for thermal barrier coatings (TBCs) and for gas-separation membranes. For TBC applications, current NDE development is focused on infrared thermal imaging methods. Compared with optical and other methods, thermal imaging is quantitative so can be used to characterize TBCs during their entire life cycle, from as-processed condition to service degradation and eventual delamination/spallation. Two thermal imaging methods are being developed: multilayer thermal modeling and thermal tomography. The multilayer-modeling method may determine the thermal property distribution and the thermal tomography method can image the TBC structure in 3D. Recent development in experimental procedure and data processing algorithms has allowed for measurement of TBC thermal conductivity with accuracies comparable to the standard laser flash method. Thermal conductivity is the most important TBC thermophysical property and exhibits characteristic changes during service lifetime. A conductivity-based model to monitor TBC degradation has been proposed. For membrane application, synchrotron x-ray microCT technology was investigated for potential identification of membrane parameters and structural defects based on constructed 3D microstructure data. This paper describes recent developments and experimental results to demonstrate the potential application of these NDE methods. INTRODUCTION Advanced structural and functional ceramics have been developed for fossil energy applications to improve efficiency and reduce emission. These include thermal barrier coatings (TBCs) applied on turbine engine components and high-temperature gas-separation membranes. As the materials are improved and their applications are extended, it becomes necessary to establish new characterization methods capable to examine the fundamental material properties and their degradation during service. The present research is focused on developing nondestructive evaluation (NDE) methods for TBCs and ceramic membranes. Work at Argonne National Laboratory (ANL) is underway to develop quantitative NDE methods for TBCs and membranes. TBCs, applied by electron beam–physical vapor deposition (EB-PVD) or air plasma spraying (APS), allow for metallic components to be utilized at higher temperatures in the hot-gas path of gas turbines, including syngas fired turbines. 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. TBC thermal properties are important in this degradation process because they determine the thermal gradient that affects stress distribution in crack development. Recent effort at ANL was focused on optical and thermal-imaging methods to measure TBC thickness and thermal conductivity. Optical methods are effective for relatively thin TBCs because of limited optical penetration depth. Thermal-imaging methods, on the other hand, are applicable for both thin and thick