The influence of coating compliance on the delamination of thermal barrier coatings

Low relative density yttria stabilized zirconia (YSZ) thermal barrier coatings have been deposited on NiCoCrAlY over-lay bond-coated Hastelloy-X substrates by an electron beam-directed vapor deposition (EB-DVD) method. The coatings have been applied to both roughened and smooth bond coat surfaces. During thermal cycling, those deposited on roughened surfaces spalled by delamination with the failure located in the YSZ layer close to the thermally-grown oxide (TGO). The cracks were initiated at “corn kernel” defects in the YSZ layer by a bond coat rumpling mechanism. Coatings applied to smooth bond coat surfaces had much longer (factor of two) spallation lifetimes and delamination occurred at the bond coat/TGO interface by the nucleation and coalescence of interfacial voids. In both cases, the delamination lifetime decreased with coating density and significantly exceeded the lifetimes of higher density coatings made by conventional electron beam deposition methods. The enhanced life of the more porous coatings is consistent with an increased compliance. This reduces the YSZ layers stored strain energy contribution to the driving force for interface delamination.