The evolution of residual stress in the thermally grown oxide on Pt diffusion bond coats in TBCs

Abstract

The evolution of residual stress in the thermally grown oxide (TGO), formed on bond coats (BC) produced by diffusion of Pt into a Ni-based superalloy, has been studied as a function of thermal cycling using photo-luminescence piezo-spectroscopy through the yttria-stabilised zirconia (YSZ) thermal barrier coating. The luminescence spectra were analysed in terms of a high stress and a low stress contribution to the residual stress sampled at a single analysis point (approx. 20 μm in size); the low stress contribution being caused by either local TGO fractures or interfacial delamination The mean compressive residual stress increases during the first two thirds of the lifetime due to gradual stiffening of the non-planar TGO as it thickens and becomes more resistant to bending. In the last third of life the mean stress tends to decrease due to relaxation by increasingly numerous local damage events. This is more pronounced as a dramatic increase in the parameters that quantify the extent of the low stress contributions. Spatial mapping of the low stress contributions shows that they are isolated at first, but become more numerous towards the end of life and coalesce into larger regions of damage a few hundred μm in size. Eventual spallation is by unstable delamination at the BC/TGO interface and the stored elastic energy released is between 40 and 80 J m −2. The lifetime is governed by the growth of the local damage regions until one (probably near an edge) reaches a critical size above which it propagates in a highly unstable way over most of the interface.