Protective pre‐deposited slurry dip‐coating alumina layer on TBC system

Abstract

In the present study, a thin intermediate protective α‐alumina layer was pre‐deposited by slurry dip‐coating technique on APS–CoNiCrAlY bond coat prior depositing APS–YSZ as top coat. The alumina layer acts as barrier for oxygen diffusion to protect the metallic bond coat from excessive oxidation. The new TBC was compared with the standard TBC system which comprises of Ni‐superalloy substrate, CoNiCrAlY bond coat and YSZ top coat concerning the behavior under thermal cycling at 1150 °C. The samples were investigated before and after thermal cycling by optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction analysis (XRD). Image analysis was used to compare between the microstructure by measuring the crack lengths and the thermally grown oxide layer (TGO) average thickness after thermal cycling. It was observed that a more uniform and compact α‐Al2O3 TGO was formed in the new TBC system after thermal cycling comparing to the commercial one. It was, also, found that the alumina intermediate layer has the potential to reduce the length of the cracks and the average thickness of TGO by suppressing the detrimental mixed oxides, like Cr2O3, CoO, NiO, and (Ni, Co)(Cr, Al)2O4, thus, improving the TBC durability.