Nondestructive Evaluation of Thermal Barrier Coating Systems Using Ultrasonic Techniques

Nondestructive evaluation (NDE) as a diagnostic tool has been extensively used to inspect the integrity of structure members. In the energy sector, the pre-mature failure of the protective Thermal Barrier Coating (TBC) covering the turbine blade can cause the structural metal base of the blade to be exposed at extremely high temperature fluids, thus reduce the service life. It is highly necessary that an NDE technique be developed for effective inspection of the thermal barrier coated turbine blade.;The ultrasonic Pulse-Echo and Pitch/Catch techniques were used to test the TBC coupons nondestructively after each thermal exposure. The Pulse-Echo traveling time and amplitude measurement, performed on isothermally oxidized Rene N5 coupon coated with commercially available MCrA1Y and 7 wt % Yittra Stabilized Zirconia (7YSZ), received an early warning of delamination at the top coat/bond coat interface. The same traveling time and amplitude evolution were also observed on another three thermal barrier coated Rene N5 coupons subjected to isothermal exposure under 1100°C using the ultrasonic Pulse-Echo technique. The interface delaminations of these coupons mentioned above were successfully predicted before visual confirmation later. In addition, the proposed technique was successfully demonstrated on an as-manufactured turbine blade coated with thermal barriers for thickness and material properties. The Young\u27s modulus values of the 7YSZ top coat at different exposure hours were calculated using the Pulse-Echo traveling time for stressed and non-stressed coupons. Surface wave measurement setup was developed for the estimation of top coat thickness (around 300 microm) and material degradation after thermal exposure. The mass density of the free standing air plasma spray (APS) and electron beam physical vapor deposition (EBPVD) YSZ top coats were measured using Archimedes\u27 principle. Finite Element analysis (FEA) simulation of the ultrasonic wave propagation on a simplified TBC system with nonlinear effects was conducted. The FEA results correlate well with the experimental observation and can clearly show detection of a small embedded void simulating delamination. The FEA was also used to estimate the material properties of different components in the TBC system. Finite Element thermal analysis was also conducted on a TBC model to find out the stress distribution inside TBC after temperature changed from 1100°C to room temperature (25 °C).;Results from this study show that the ultrasonic technique developed can nondestructively detect the internal delamination and degradation of the top coat at different exposure hours. Early warning of delamination was identified before it could be visually confirmed. The ultrasonic signals were found to be very sensitive to the degradation of the 7YSZ top coat and the growth of delamination. The Young\u27s modulus values of APS YSZ top coat TBC coupons at different exposure hours can be predicted using the measured signals