Behavior of air plasma sprayed thermal barrier coatings, subject to intense thermal cycling

Microstructural changes, damage initiation and spallation of thermal barrier coatings (TBC), which were deposited by air plasma spraying (APS) and consisting of ZrO2–7 wt.% Y2O3; (YSZ) thermally grown oxide (TGO); MCrAlY bond coat; and a nickel base super alloy, Hastelloy-X, were examined as a function of thermal cycles at 1020 °C. Two types of thermal shock testing were adopted. Firstly, thermal cycling consisted of a directly inserting samples to 1020 °C and then 5 min holding and water quenching. In the other cycling method, forced air was applied for quenching or cooling. Microstructural characterization was carried out by scanning electron microscopy (SEM) and optical microscopy whereas phase analysis was carried out by X-ray diffractrometry (XRD). Spallation of YSZ coatings was initiated by edge cracking after 50 cycles in water quenching and sustained upto 220 cycles, 20% spalling was adopted the criteria for this test. In case of air quenching up to 1000 cycles only 10% of the sample spalled. In both cases, no vertical cracking was observed throughout the cycling. In the water-quenched samples, after 220 cycles the thickness of the top coating was half of its original thickness, while in case of air-quenched samples no such surface deterioration was observed. Peaks for calcium zirconium oxide (CaZrO3) were found during XRD studies for water-quenched samples. Rumpling-induced micro-cracking at the YSZ/TGO interface and the formation of voids decreasing the toughness between the TGO and YSZ.