A mechanistic study of microcracking in transversely isotropic ceramic–metal systems

A mechanistic study of the local stresses required to nucleate microcracks at or near ceramic–metal bimaterial interfaces subjected to high temperature oxidation is presented. A coupled diffusion–constitutive framework is used which accounts for the phase transformations associated with the oxidation reaction, and incorporates the effects that the anisotropic local volumetric expansion of the newly formed phases have in the generation of inelastic volumetric strains and stresses. The initial morphology of the ceramic material is assumed to have the typical columnar structure obtained from electron-beam physical vapour deposition (EB-PVD) of yttria stabilised zirconia. A new constitutive model is also proposed for the ceramic material which accounts for its transversely isotropic elastic properties and the stiffening of the material due to sintering. The coupled constitutive framework is used to study the local oxidation induced stresses in a ceramic–metal bimaterial typical of an EB-PVD thermal barrier coating system. Parametric finite element studies are conducted using periodic unit cell finite element techniques to study the effects of oxidation and sintering on the local stresses responsible for microcrack nucleation during cooling.