Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings

The microstructure and durability of a thermal barrier coating (TBC) produced by the thermal spray method have been characterized. Upon exposure, the bond coat chemistry and microstructure change by inter-diffusion with the substrate and upon thickening of the thermally grown oxide (TGO). A wedge impression test, in conjunction with observations by scanning electron microscopy, has been used to probe the failure mechanisms. At short exposure times, when the TGO thickness is less than about 5 mu m, the growth of the TGO does not affect the crack patterns in the TBC and delaminations induced by wedge impression propagate within the TBC about 30 mu m from the interface. An amorphous phase at the splat interfaces promotes this failure mode. As the thickness of TGO increases during exposure, cracks form in the TBC around imperfections at the interface. Moreover, induced delaminations develop a trajectory close to the interface, propagating not only through the TBC but also within the TGO and along the interfaces. A scaling result based on the misfit around imperfections caused by TGO growth has been used to rationalize the critical TGO thickness when the TBC fails. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.