Oxidation characteristics of a platinized MCrAlY bond coat for TBC systems during cyclic oxidation at 1000 °C

The scale growth mechanisms of a platinized NiCoCrAlY bond coat (BC) beneath a ceramic thermal barrier coating (TBC) were studied during cyclic oxidation in air at 1000 °C. The thermally grown oxide scales (TGO) after exposure times ranging from 100 to 9100 h were characterized by scanning electron microscopy and laser fluorescence spectroscopy. The TGO appeared to consist of alumina and no oxides of the other elements present in the NiCoCrAlY coating were detected. Up to exposure times of 1000 h substantial amounts of metastable alumina were found in the outer part of the TGO. This is probably the reason for the formation of a porous, mixed alumina/zirconia layer at the TBC/TGO interface. This layer was prone to micro-crack formation, which did, however, not result in macroscopic TBC failure. The latter occurred after crack formation at the TGO-BC interface.