The Failure of Protective Oxides on Plasma-Sprayed NiCrAlY Overlay Coatings

The oxidation behavior in air of air-plasma sprayed (APS) overlay coatingsof Ni–25Cr–6Al–Y have been studied at 1100°C. Aprotective alumina scale developed after 5- to 10-hr exposure with, initially,parabolic growth kinetics. With protracted exposures (>100 hr),subparabolic behavior developed, associated with aluminum depletion withinthe coating caused, principally, by internal oxidation of the low-densityAPS structure. This depletion caused intrinsic chemical failure, manifestedby the formation of a layer of Cr,Al,Ni-rich oxide beneath the residualalumina layer. Associated with this process of chemical failure was theformation of a layer of porous Ni,Cr-rich oxide above the aluminalayer. Oxide spallation occurred by delamination within this layer duringcooling; the spallation sites tended to lie above protuberances in theunderlying coating. Initial spallation occurred at a critical temperaturedrop, which decreased rapidly with increasing exposure time. A nonrigorousmodel of this spallation process has been developed which envisages thatdelamination occurs by the propagation of an oxide void under the action ofout-of-plane tensile stresses developed during cooling. Agreement with thespallation data is encouraging and shows that the deterioration ofspallation resistance with exposure time arises not only because oxidethickness increases but also because the maximum void size within the porousoxide layer increases.