Delamination Susceptibility of Coatings Under High Thermal Flux

Abstract

Delamination of coatings initiated by small cracks paralleling the free surface is investigated under conditions of high thermal flux associated with a through-thickness temperature gradient. A crack disrupts the heat flow thereby inducing crack tip stress intensities that can become critical. A complete parametric dependence of the energy release rate and mode mix is presented in terms of the ratio of the crack length to its depth below the surface and coefficients characterizing heat transfer across the crack and across the gaseous boundary layer between the surface and the hot gas. Proximity to the surface elevates the local temperature, which in turn, may significantly increase the crack driving force. A detailed assessment reveals that the energy release rates induced by high heat flux are capable of extending subsurface delaminations in thermal barrier coatings, but only when the modulus has been elevated by either calcium-magnesium-alumino-silicate (CMAS) penetration or sintering. Otherwise, the energy release rate remains well below the toughness.