Development and thermal fatigue testing of ceramic thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) will play an increasingly important role in future gas turbine engines because of their ability to effectively protect the engine components and further raise engine temperatures. Durability of the coating systems remains a critical issue with the ever-increasing temperature requirements. Thermal conductivity increase and coating degradation due to sintering and phase changes are known to be detrimental to coating performance. There is a need to characterize the coating thermal fatigue behavior and temperature limit, in order to potentially take full advantage of the current coating capability. In this study, a laser thermal fatigue test technique has been used to study the delamination crack propagation of thermal barrier coatings under simulated engine heat flux heating and thermal cyclic loading. Thermal conductivity and cyclic fatigue behaviors of plasma-sprayed ZrO2–8 wt.% Y2O3 thermal barrier coatings were evaluated under high temperature, large thermal gradient and thermal cycling conditions. The coating degradation and failure processes were assessed by real-time monitoring of the coating thermal conductivity during the testing. The test results showed that the initial average crack propagation rates of ZrO2–8 wt.% Y2O3 coatings were in the range of 3–8 μm/cycle. The crack propagation rates increased to 30–40 μm/cycle at later stages of testing, and the critical spalling crack size ranged from 3 to 5 mm for the coatings. The accelerated crack growth is attributed to the increased driving force for crack propagation under the laser heat flux cyclic test conditions.