Effect of oxygen content in NiCoCrAlY bondcoat on the lifetimes of EB-PVD and APS thermal barrier coatings

The effect of oxygen content in NiCoCrAlY bondcoat on the cyclic oxidation lifetimes of EB-PVD and APS thermal barrier coatings (TBC) has been studied. The EB-PVD TBC system with an oxygen content of 0.05 wt. % in NiCoCrAlY bondcoat shows five times longer lifetimes compared to a TBC system with 0.2 wt% oxygen in the bondcoat. In the bondcoat with the high oxygen content the minor (0.3 wt.%) yttrium addition was found to be tied up by oxygen into fine precipitates of yttrium aluminates. Thereby the beneficial effect of yttrium onto the adherence of the alumina scale was significantly reduced. The critical scale thickness at failure was by about a factor of two lower for the high oxygen bondcoat than for the low oxygen bondcoat. In contrast to EB-PVD TBC systems, no detrimental effect of increasing oxygen content on the lifetime of APS-TBC systems was observed. This can be explained by a different failure mechanism of APS-TBC systems, whereby the lifetime is mainly determined by the rate of crack propagation through the ceramic topcoat.