Spalling failure of a thermal barrier coating associated with aluminum depletion in the bond-coat

A plasma-sprayed thermal barrier coating is observed to spall after oxidation at 1121 degrees C from a CoNiCrAlY bond-coated superalloy at the interface between the thermally grown oxide (TGO) and the zirconia thermal barrier coating (TBC). Phase characterization by photostimulated luminescence and X-ray diffraction, as well as microstructural characterization by scanning electron microscopy, indicates that the spalling is associated with the conversion of the initially formed alpha-alumina thermally grown oxide to alpha-chromia and a (CoNi)(CrAl) spinel. It is proposed that the phase conversion occurs after the alumina TGO has cracked on thermal cycling and the underlying bond-coat alloy is depleted of aluminum with concurrent enrichment of the oxide by Cr, Co and Ni. The observations suggest that monitoring the luminescence intensity as a function of oxidation time might form the basis of a nondestructive tool for detecting the onset of failure based on the disappearance of alpha-alumina. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.