National Aeronautics and Space Administration
Date of this Version
2011
Citation
Surface & Coatings Technology 206 (2011) 1577–1585; doi:10.1016/j.surfcoat.2011.07.015
Abstract
Delayed failure of TBCs is a widely observed laboratory phenomenon, although many of the early observations went unreported. “The weekend effect” or “DeskTop Spallation” (DTS) is characterized by initial survival of a TBC after accelerated laboratory thermal cycling, followed by failure when exposed to ambient humidity or water. Once initiated, failure can progress to completion quite dramatically in less than a second. To this end, the water drop test and digital video recordings have become useful techniques in studies at NASA (Smialek, Zhu, Cuy), DECHMA (Rudolphi, Renusch, Schütze), and CNRS Toulouse/SNECMA (Déneux, Cadoret, Hervier, Monceau). In the present study the results for a commercial turbine blade, with a standard EB-PVD 7YSZ TBC top coat and Pt-aluminide diffusion bond coat are reported. Cut sections were intermittently oxidized at 1100°, 1150°, and 1200 °C and monitored by weight change and visual appearance. Failures were distributed widely over a 5–100 hr time range, decreasing with oxidation temperature. At some opportune times, failure was captured by video recording, documenting the appearance and speed of the moisture-induced spallation process. The failure interfaces exhibited, on the TBC side, alumina scale grains (decorated with Ta-rich oxide particles) and, on the exposed bare metal surface of the blade, imprints from the scale grains (with alumina islands and streamers). The phenomenon is based on moisture-induced delamination at the alumina scale - bond coat interface. Cycling damage is a contributing factor as cracking allows access of moisture to this interface, while high strain energy provides the driving force for spallation. It has been proposed that moisture reacts with aluminum in the bond coat and releases hydrogen atoms that ‘embrittle’ the interface. A modified chemical viewpoint of scale adhesion results, including a negative synergistic effect with interfacial sulfur.