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 coat
ing are the same superalloy as the substrate and partially stabilized
zirconia. The volume fraction of zirconia is assumed to vary continuou
sly from 0% at the interface to 100% at the surface. The surface of th
e coating is exposed to a high-temperature environment, the surface of
the substrate is forced-cooled, and the ends of the specimen are subj
ected to natural convection. The surfaces df the crack are assumed to
be partially insulated. Thus, the mechanically unconstrained inhomogen
eous medium is under a steady-state heat conduction with convective bo
undary conditions. The thermal stress problem is solved for various co
mposition profiles in the coating, including 100% ceramic, and for var
ious values of the heat conductivity index describing the insulation c
ondition 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 t
he surfaces of the substrate and coating, along the interface and on t
he crack surfaces, the total heat flow across the substrate surface, m
odes I and II stress intensity factors, and the strain energy release
rate. Noting that in the case of a homogeneous ceramic layer crack sur
faces always interfere giving negative KI, a statically indeterminate
problem is also solved to determine the contact force and to recalcula
te the stress intensity factors. (C) 1998 Elsevier Science Ltd.