Laser Thermal Gradient Test of a Thermal Barrier Coating and Failure Analysis

Abstract

Thermal barrier coatings (TBC) are used to protect the hot components of gas turbine engines to enhance thermal efficiency and component service life. It is critical for TBC development that a testing method be used to understand the potential and limitation of coating’s durability and integrity under the gas turbine engine operation conditions. In this paper, laser high heat flux testing with an applied temperature gradient across TBC coated buttons is presented. 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 substrate button of 25.4 mm diameter. The coated button is subject to precisely-controlled laser heating on the top side (1150°C) and temperature gradient of 63.9°C/mm through the button overall thickness. The TBC button lasts 160.9 hr or 570 cycles of laser heating. Analysis of void fraction change before and after the test, thermal conductivity change during the laser test, and failure assessment are presented. After the test, the top coating has cracks in vertical or oblique directions and significant horizontal cracks near the top coating and bond coating interface. Significant horizontal top coating cracks close to the interface between the top coating and bond coating appear near the button center. Although the coating delamination has not occurred yet, at the end of the laser testing the button is close to delamination. Based on the horizontal cracks and the thermally grown oxide layer geometry, a finite element analysis is conducted to calculate the residual stress and the strain energy release rate. A possible approach to combine laser rig test result and finite element computation for developing a TBC service life model is discussed.