Interface thermal fracture in functionally graded zirconia-mullite-bond coat alloy thermal barrier coatings

Thermally activated time-dependent (viscoplastic) deformation is a dominant cause of surface and interface thermal fracture in thick thermal barrier coatings (TBC). In this paper, the possibility of inhibiting the time-dependent deformations in zirconia based TBCs through the addition of mullite is considered. Five different TBC architectures including pure ceramic as well as functionally graded coatings were studied. Mean field micromechanics models were used to compute the effective thermo-elastic properties and time-dependent response of each of the layers in these TBCs. These effective properties were then utilized in fracture mechanics computations to assess the magnitudes of driving force for TBC-bond coat interface crack growth under transient thermal loading. The effects of coating architecture and surface crack morphology on interface crack growth were assessed. It is shown that the driving force for interface crack growth in zirconia-bond coat alloy functionally graded TBCs may be reduced by the addition of mullite. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.