Parameters affecting TGO growth and adherence on MCrAlY-bond coats for TBC's

Electron Beam–Physical Vapor Desposited Thermal Barrier Coatings (EB–PVD TBC) on the Ni-base superalloy IN738LC were tested in respect to non-isothermal and cyclic oxidation resistance at 1100 °C. Two types of MCrAlY's (M = Ni, Co), a Co-base and a Ni-base, were used as bond coats (BC) for the TBC's. Additionally, free standing MCrAlY specimens of 2 mm thickness were manufactured by vacuum plasma spraying. The results of the present studies strongly indicate, that an important lifetime governing factor of the TBC is the yttrium incorporation into the alumina based Thermal Grown Oxide (TGO) which results in an increase of the TGO growth rate and in parallel in a decrease of the yttrium concentration in the coating. If the yttrium concentration has been decreased beneath a critical level, its positive effect on TGO adherence is lost, resulting in TGO spallation. The time required for yttrium exhaustion will not only depend on the initial yttrium content but also on the yttrium reservoir, which is directly proportional to the BC thickness. The transport of yttrium into the TGO seems to occur slower in a Co-based than in a Ni-based coating, resulting in a longer life time for the TBC on the CoNiCrAlY-BC.