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

Abstract

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-Al 2O 3 was observed on grit-blasted bondcoat for all YSZ deposition times, while that from metastable Al 2O 3 phases including α-Al 2O 3 was observed for the TGO on as-coated (i.e. not grit blasted) bondcoat. Relative to the luminescence intensity of the α-Al 2O 3, the luminescence intensity from the metastable Al 2O 3 phases decreased with an increase in deposition time and corresponding thickness of ZrO 2–Y 2O 3 (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 Al 2O 3 phases decreased while that of α-Al 2O 3 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 Al 2O 3 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