Laser Thermal Gradient Testing and Fracture Mechanics Study of a Thermal Barrier Coating

It is critical for thermal barrier coating (TBC) development that a testing method be used to understand the potential and limitation of a coating's durability and integrity under gas turbine engine operating conditions. To this end, a TBC-coated button is tested using a laser high-heat flux facility. The ceramic coating is ZrO2-8 wt.% Y2O3 applied via the air plasma spraying process on top of a NiCoCrAlY bond coating and an Inconel alloy 617 substrate button of 25.4 mm diameter. The coated button is subject to precisely controlled laser heating on the top side (1150 degrees C) and a temperature gradient of 63.9 degrees C/mm through the button overall thickness. The coated button lasts 160.9 h or 570 cycles of laser heating. The void fraction change before and after the test, the thermal conductivity change during the laser test and the failure assessment are presented. After the test, significant horizontal cracks exist in the top coating close to the thermally grown oxide (TGO) layer and near the button center. Based on the cracks and the TGO layer geometry, the stress intensity factor and strain energy release rate are computed. The combined experimental and computational approach can lead to a TBC lifetime model.