Thermal Cycle Damage and Residual Adhesion Strength in a Thermal Barrier Coating

Abstract

In this work, the thermal cycle fatigue behavior of an air plasma sprayed thermal barrier coatings was investigated in order to improve the thermal fatigue properties. In addition, the residual interfacial strength after the thermal cycle fatigue was also evaluated by means of the modified 4-point bending test. From the measurement of the AE signals during the thermal cycle fatigue tests, micro-cracking occurred in the cooling stage of each thermal cycle. Therefore, such damage depends on the number of thermal cycle. In addition, the thermal grown oxides (TGO) grew at the interface with the exposed time at elevated temperature, i.e. it is the time dependent damage. Thermal barrier coating undergoes both of the time dependent damage and the cycle dependent one under the thermal fatigue condition. The failure mode by the thermal cycle fatigue depended on the thermal cycle wave form ; in thermal fatigue without a hold, the spallation failure occurred in the top-coating near the interface by the coalescence of the microcracks, on the other hand, in thermal fatigue with a hold, delamination occurred through the microcracks and/or along the top-coating/TGO interface. Based on these experimental results and discussions, the damage equation, which was formed by summation of the time-dependent damage and the cycle-dependent damage, was proposed to evaluate the thermal cycle fatigue life of the thermal barrier coatings.