The evolution of oxidation stresses in zirconia thermal barrier coated superalloy leading to spalling failure

The evolution of the stresses in the aluminum oxide formed beneath a zirconia thermal barrier coating during high-temperature oxidation has been measured from the piezospectroscopic shift in the R-line photoluminescence from Cr 3+ impurities incorporated into the growing aluminum oxide scale. The early stages of oxidation are associated with concurrent phase transformations in the aluminum oxide leading to the stable alpha-alumina phase. On the basis of microstructural observations and the broadening of the luminescence lines, it is suggested that spalling failure occurs as a result of linking together of localized decohesion of the oxide/bond-coat interface, a process accelerated by moisture enhanced sub-critical crack growth at the interface.