Oxidation and crack nucleation/growth in an air-plasma-sprayed thermal barrier coating with NiCrAlY bond coat

The oxidation behavior of an air-plasma-sprayed thermal barrier coating (APS-TBC) system was investigated in both air and low-pressure oxygen environments. It was found that mixed oxides, in the form of (Cr,Al)2O3·Ni(Cr,Al)2O4·NiO, formed heterogeneously at a very early stage during oxidation in air, and in the meantime, a layer of predominantly Al2O3 grew rather uniformly along the rest of the ceramic/bond coat interface. The mixed oxides were practically absent in the TBC system when exposed in the low-pressure oxygen environment, where the TBC had a longer life. Through comparison of the microstructures of the APS-TBC exposed in air and low-pressure oxygen environment, it was concluded that the mixed oxides played a detrimental role in causing crack nucleation and growth, reducing the life of the TBC in air. The crack nucleation and growth mechanism in the air-plasma-sprayed TBC is further elucidated with emphasis on the Ni(Cr,Al)2O4 and NiO particles embedded in the chromia.