Phase transformations of thermally grown oxide on (Ni,Pt)Al bondcoat during electron beam physical vapor deposition and subsequent oxidation

Phase constituents, morphology and microstructure of thermally grown oxide (TGO) as a function of electron beam physical vapor deposition (EP-PVD) time, (Ni,Pt)Al bondcoat surface treatment and subsequent oxidation in air were examined for a series of thermal barrier coatings (TBCs). Photostimulated luminescence spectroscopy (PSLS) and focused ion beam (FIB) in-situ lift-out (INLO) specimen preparation for (scanning) transmission electron microscopy were employed to characterize the TBCs. Photostimulated luminescence only from α-Al2O3 was observed on grit-blasted bondcoat for all YSZ deposition times, while that from metastable Al2O3 phases including α-Al2O3 was observed for the TGO on as-coated (i.e. not grit blasted) bondcoat. Relative to the luminescence intensity of the α-Al2O3, the luminescence intensity from the metastable Al2O3 phases decreased with an increase in deposition time and corresponding thickness of ZrO2–Y2O3 (YSZ). On subsequent oxidation in air at 1000, 1050 and 1100 °C for 0.5, 10 and 50 h, the luminescence intensity from the metastable Al2O3 phases decreased while that of α-Al2O3 increased. These observations regarding the phase constituents of TGO were confirmed by diffraction analysis using high resolution TEM (HR-TEM) via FIB INLO. The initial development of the TGO during EB-PVD deposition consisted of a mixed-oxide zone. The relative thickness of the mixed-oxide zone for TGO formed on the as-coated bondcoat surface was greater than that of TGO formed on grit-blasted bondcoat surface. The variation in the presence of metastable Al2O3 in the TGO for as-processed TBCs was observed to influence the overall growth of TGO during short-term isothermal oxidation, particularly as the temperature decreased