Thermal barrier coating systems offer oxidation protection to alloy substrates under heat flux conditions. They consist of an yttria-stabilised zirconia top coat, of low thermal conductivity, bonded to the alloy substrate by an alumina-forming metallic layer. These systems develop considerable strain energy during cooling and are mechanically unstable. The failure by spallation of the top coat is a life-limiting event but its prediction has proved a difficult problem. The growth of the alumina layer on the bond coat surface has been implicated in the failure of the TBC system but complete understanding remains elusive. In this paper, finite-element computations are presented of the out-of-plane stresses that develop both isothermally and during cooling, as a result of oxide growth. A novel feature of the calculations is that oxide thickening and the associated volume expansion are modelled within the computation. It is shown that continuity strains can lead to the development of out-of-plane stresses at the base of the top coat at the oxidation temperature. These are associated with bond coat protuberances and their magnitude increases with surface roughness. The situation will be exacerbated if Al depletion is sufficient to trigger chemical failure and the formation of faster-growing Cr, Ni-rich oxides at bond coat protuberances. In this case, large (> 0.5 GPa) out-of-plane tensile stresses can develop within the top coat at the test temperature for realistic surface roughnesses.
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