Nondestructive Testing of Thermal Barrier Coated Turbine Blades Using Microwave Techniques

Zirconia-based thermal barrier coatings (TBCs) are widely used to protect turbine blades against hightemperature and pressurized steam, increasing the lifetime of the whole system. A turbine blade crack, which normally begins from the metal surface due to corrosion and stress, must be inspected nondestructively to avoid catastrophic failure. Although there are several established nondestructive testing (NDT) techniques available, microwave testing using a broad frequency sweep with an open-ended rectangular waveguide energy launch, in conjunction with signal processing, is proposed in this paper as an inspection technique. Electromagnetic signals at microwave frequencies are capable of providing inspection of metals coated with dielectric materials such as TBC. In this paper, the principle of this technique is described, along with 3D electromagnetic software simulation using various measurement scenarios and a number of measurements of representative turbine blade models with discontinuities on the metal surface, visibly hidden by the TBC. This research is aimed at delivering an in-situ microwave test capable of detecting surface and subsurface discontinuities and may form part of quality control in manufacturing as well as portable field service inspection. As such, the system is comparable in size to current NDT systems, as well as robust, affordable, and capable of providing real-time data about the nature of an anomaly such as discontinuity location and size.