Interface cracking of FGM coatings under steady-state heat flow

In this study the plane strain thermal stress problem for an interface crack in a homogeneous substrate with a graded coating is considered. The substrate is a superalloy and the constituents of the graded coating are the same superalloy as the substrate and partially stabilized zirconia. The volume fraction of zirconia is assumed to vary continuously from 0% at the interface to 100% at the surface. The surface of the coating is exposed to a high-temperature environment, the surface of the substrate is forced-cooled, and the ends of the specimen are subjected to natural convection. The surfaces df the crack are assumed to be partially insulated. Thus, the mechanically unconstrained inhomogeneous medium is under a steady-state heat conduction with convective boundary conditions. The thermal stress problem is solved for various composition profiles in the coating, including 100% ceramic, and for various values of the heat conductivity index describing the insulation condition on the crack surface. The main variable in the problem is the length of the symmetrically located edge cracks along the interface. The sample results presented include the temperature distribution on the surfaces of the substrate and coating, along the interface and on the crack surfaces, the total heat flow across the substrate surface, modes I and II stress intensity factors, and the strain energy release rate. Noting that in the case of a homogeneous ceramic layer crack surfaces always interfere giving negative KI, a statically indeterminate problem is also solved to determine the contact force and to recalculate the stress intensity factors. (C) 1998 Elsevier Science Ltd.