Influence of yttria on the cyclic lifetime of YSZ TBC deposited on EB-PVD NiCoCrAlY bondcoats and its contribution to a modified TBC adhesion mechanism

In the present study IN100 substrates have been coated with a 100 μm NiCoCrAlY bondcoat (BC) by electron beam physical vapor deposition (EB-PVD). Prior to thermal barrier coating (TBC) deposition the bondcoats have been annealed in vacuum applying various annealing temperatures and durations in order to form a thermally grown oxide (TGO). Subsequently, a 175 μm yttria partially stabilized zirconia (YSZ) TBC has been deposited by EB-PVD. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) have been used to investigate TGO microstructure and morphology on as coated NiCoCrAlY bondcoats after TBC deposition. Furthermore, the TGO growth during the early stages of TBC deposition has been correlated with TBC thermal cyclic lifetime. STEM investigations reveal the nucleation of an outer yttria TGO layer during different bondcoat pre-treatments that was located between alumina and ceramic topcoat after TBC deposition. The amount of yttria formed as an outer TGO layer increases with annealing temperature and annealing duration. The growth of this yttria layer has been correlated with TBC lifetime. The current results reveal that longer cyclic TBC lifetimes have been observed on those specimens that form a significant quantity of yttria at the TGO outer surface prior to TBC deposition. When disregarding further oxidation during pre-heating of the samples right prior to TBC deposition it is getting obvious that the ceramic topcoat mainly binds to yttria and not to alumina in the initial stages of TBC deposition. It is assumed that the yttria layer acts as a bi-functional primer promoting a) the formation of a chemical compounds such as YAG and YAP between alumina and yttria and additionally b) towards TBC the formation of solid solutions between yttria and YSZ that are indicated by interdiffusion curves. Within a chemical approach a modified TBC adhesion mechanism has been assessed implementing a compound alumina–yttria TGO into the current understanding of TBC adhesion.