ezo-spectroseopy is used to analyse how the residual stress in the alumina thermally grown oxide (TGO) evolves with time as TBCs (with EBPVD YSZ ceramic coatings) are subjected to thermal cycling. Results are presented for two different bond coat (BC) systems; one based on conventional Pt-Al and the other on a Pt diffusion pre-treatment. The Pt diffusion BC has the gamma-gamma' structure which is more resistant to plastic deformation than the beta structure of the Pt-Al BC. The two systems display different TGO stress evolution characteristics and different locations of the eventual failure path. These differences are explained qualitatively by the different mechanical properties of the BCs.