Evaluation of Plasma Sprayed Thermal Barrier Coatings Using NDE and SEM

Thermal barrier coatings (TBC) are used to protect the hot section components of gas turbine engines from high temperatures. A TBC system consists of a ceramic topcoat and a metallic bond coat sprayed or deposited onto the metal substrate. TBC failure is often associated with oxidation of the metallic bond coat at elevated temperatures via formation of thermally grown oxides (TGO) that cause internal stresses leading to the final spallation of the TBC. The present study explores the application of eddy current and infrared thermal imaging techniques for the detection of TGO in thermally-exposed TBC with a view of finding the damage criteria and a suitable solution for nondestructive evaluation (NDE) of TBC. The eddy current technique is based on the induction of an electromagnetic field and is sensitive to minute changes in electrical or magnetic properties of the test piece while infrared thermal imaging is based on thermal diffusion process and measures small differences in surface temperature. The NDE results are validated through destructive testing and microscopic examination of the TBC samples in as-sprayed condition and after exposure to elevated temperatures.