Characterization of alumina scales formed during isothermal and cyclic oxidation of plasma-sprayed TBC systems at 1150°C

The isothermal- and cyclic-oxidation behavior of thermal barrier coating (TBC) systems consisting of vacuum plasma-sprayed (VPS) Ni-22Cr-10Al-1Y (wt.%) bond coatings and air plasma-sprayed (APS) Y2O3-stabilized ZrO2 (YSZ) top coatings (on single-crystal superalloys) was investigated. The microstructures, flaw contents, and fracture behavior of the Al2O3 scales formed during oxidation testing at 1150 degrees C were characterized (by analysis of coating and scale fracture surfaces and metallographic cross sections). Significant localized fracture and buckling of the Al2O3 scales that formed along the bond-coat-top-coat interfaces were observed after cyclic oxidation of TBCs. However, substantial amounts of localized scale damage did not induce rapid TBC failure. Decohesion of the columnar alumina scales on the rough bond-coat surfaces occurred by both internal Al2O3 fracture (parallel to the metal surface) and oxide-metal delamination. There were microstructural indications of Al2O3 scale crack healing by sintering into planar arrays of voids. Alumina scales that formed on convex NiCrAlY surfaces (with radii of 50 mu m or less) often contained significant amounts of internal voids (along grain boundaries) after cyclic oxidation, whereas scales formed by isothermal oxidation contained few visible voids. Accelerated void growth in Al2O3 scales on the irregular NiCrAlY surfaces appeared to be creep-related and was attributed to the synergistic effects of geometric and thermal stresses.