Topcoat Transmission Measurement and Sensitivity Evaluation for Detection of Thermally Grown Oxide Layer of Thermal Barrier Coating

The thermally grown oxide (TGO) layer, which forms below the topcoat of thermal barrier coating (TBC), is an important factor which causes topcoat delamination, and its detection is important for health monitoring of TBC. Photoluminescence detects the emission from Cr3+ in the TGO layer upon laser excitation, and is an effective, nondestructive method to detect the TGO layer. In this method, the laser light and Cr3+ photoluminescence must transmit through the topcoat, so the topcoat transmittance is a limiting factor of the measurement sensitivity. In this report, the topcoat transmittance was measured by comparing the photoluminescence intensities from the TGO layer in the presence and absence of the topcoat. The round-trip transmittance of the topcoat of thickness 300 µm was 1.26 %, corresponding to an attenuation coefficient of 7.3 mm-1. The sensitivity of the luminescence measurement was evaluated by placing ND filters of known attenuation in front of the TGO layer, to simulate attenuation for different topcoat thickness. The results showed that luminescence measurement is possible for a maximum topcoat thickness of about 700 µm. A method to adjust the photoluminescence intensity for different topcoat thickness is presented.