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

The isothermal- and cyclic-oxidation behavior of thermal barrier coating (T BC) 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 microstruct ures, flaw contents, and fracture behavior of the Al2O3 scales formed durin g oxidation testing at 1150 degrees C were characterized (by analysis of co ating and scale fracture surfaces and metallographic cross sections). Signi ficant localized fracture and buckling of the Al2O3 scales that formed alon g the bond-coat-top-coat interfaces were observed after cyclic oxidation of TBCs. However, substantial amounts of localized scale damage did not induc e rapid TBC failure. Decohesion of the columnar alumina scales on the rough bond-coat surfaces occurred by both internal Al2O3 fracture (parallel to t he 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 o f 50 mu m or less) often contained significant amounts of internal voids (a long grain boundaries) after cyclic oxidation, whereas scales formed by iso thermal oxidation contained few visible voids. Accelerated void growth in A l2O3 scales on the irregular NiCrAlY surfaces appeared to be creep-related and was attributed to the synergistic effects of geometric and thermal stre sses.