Mechanism of spallation in platinum aluminide/electron beam physical vapor-deposited thermal barrier coatings

The spallation failure of a commercial thermal barrier coating (TBC), consi sting of a single-crystal RENE N5 superalloy, a platinum aluminide (Pt-Al) bond coat, and an electron beam-deposited 7 wt pet yttria-stabilized zircon ia ceramic layer (7YSZ), was studied following cyclic furnace testing. In t he uncycled state and prior to deposition of the ceramic, the Pt-Al bond-co at surface contains a cellular network of ridges corresponding to the under lying bond-coat grain-boundary structure. With thermal cycling, the ridges and associated grain boundaries are the sites of preferential oxidation and cracking, which results in the formation of cavities that are partially fi lled with oxide. Using a fluorescent penetrant dye in conjunction with a di rect-pull test, it is shown that, when specimens are cycled to about 80 pet of life, these grain-boundary regions show extensive debonding. The roles of oxidation and cyclic stress in localized grain boundary region spallatio n are discussed. The additional factors leading to large-scale TBC spallati on are described.