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) 2 O 3 d Ni(Cr,Al) 2 O 4 d NiO, formed heterogeneously at a very early stage during oxidation in air, and in the meantime, a layer of predominantly Al 2 O 3 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) 2 O 4 and NiO particles embedded in the chromia. D

In thermal barrier coating (TBC) systems, a continuous alumina layer developed at the ceramic topcoat/ bond coat interface helps to protect the metallic bond coat from further oxidation and improve the durability of the TBC system under service conditions. However, other oxides such as spinel and nickel oxide, formed in the oxidizing environment, are believed to be detrimental to TBC durability during service at high temperatures. It was shown that in an air-plasma-sprayed (APS) TBC system, postspraying heat treatments in low-pressure oxygen environments could suppress the formation of the detrimental oxides by promoting the formation of an alumina layer at the ceramic topcoat/bond coat interface, leading to an improved TBC durability. This work presents the influence of postspraying heat treatments in low-pressure oxygen environments on the oxidation behavior and durability of a thermally sprayed TBC system with high-velocity oxy-fuel (HVOF)-produced Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat. Oxidation behavior of the TBCs is evaluated by examining their microstructural evolution, growth kinetics of the thermally grown oxide (TGO) layers, and crack propagation during low-frequency thermal cycling at 1050°C. The relationship between the TGO growth and crack propagation will also be discussed.

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