EFFECTS OF BOND COAT SURFACE ROUGHNESS AND PRE-OXIDATION THERMAL CYCLING LIFETIME OF THERMAL BARRIER COATINGS

摘要

Effects of surface roughness and pre-oxidation heat treatment for the NiCoCrAlY bond coat (PWA276), carried out prior to the electron beam physical vapor deposition (EB-PVD) of ZrO_2-7 wt. percent Y20s (7YSZ) ceramic topcoat, on the thermal cycling lifetime of thermal barrier coatings (TBCs) were examined. As a function of bond coat processing and thermal cycling, TBCs were characterized by using optical profilometry, photo-stimulated luminescence speetroscopy (PSLS), optical microscopy, and scanning electron microscopy. TBC specimens with pre-oxidized bond coats, in general, exhibited longer lifetime than those without pre-oxidation. The longest TBC lifetime was observed when hand-polished (i.e., smoothest) bond coats were pre-oxidized prior to EB-PVD of 7YSZ. However, without pre-oxidation, TBC specimen with hand-polished and barrel-finished bond coats failed prematurely due to underdeveloped thermally grown oxide (TGO) and/or failure at the YSZ/TGO interfaces. For TBCs specimens with pre-oxidized bond coats, a greater lifetime was observed with a higher initial compressive residual stress within the TGO.