Failure mechanisms of coin-type plasma-sprayed thermal barrier coatings with thermal fatigue

The aim of this study is to investigate the failure mechanisms of coin-shaped plasma-sprayed thermal barrier coatings (TBCs) for gas turbine blades due to cyclic thermal fatigue. For this purpose, cyclic thermal fatigue tests at 1100 °C were performed for a commercial TBC system that was used for the first-stage blade of a gas turbine. The failure location was near the interface between the top coat and the thermally grown oxide (TGO). For thermally aged TBC specimens, bond tests were performed to investigate the failure modes and to observe the decrease in the bond strength. The spallation of a TBC was preceded by external crack initiation at the edge of a specimen, and the bond strength gradually decreased with increasing thermal cycles due to the expansion in the delaminated zone. Through the thermal stress analysis it was found that edge delamination originated from repeated “cooling shock”, which occurs right after forced air quenching, where the normal stress the edge increases sharply within a few seconds.