Models for thermal barrier coating lifetime prediction are often based on bondcoat oxidation models leading to an end of life criterion either based on bondcoat full consumption or a critical thermally grown oxide thickness. Such models can be satisfactory on turbine parts where the most common coating delamination modes are black or grey failure which are linked to the bondcoat behaviour. Such models are not reliable for combustor parts with thick thermal barrier coating systems where the most common life limiting factor is the formation of cracks appearing in the ceramic layer few tens of microns above the bondcoat interface. This behaviour is linked to the TBC layer mechanical properties and should be described by a model taking into account the evolution of the TBC mechanical properties during engine operation, the mechanical loads in the ceramic layer and a crack propagation model in the TBC. A study of the strain tolerance of TBC from combustor parts after engine operation was performed by taking samples from combustor liners at various locations having different TBC surface temperature. The strain tolerance of TBC samples was measured by four-point bending and correlated with the TBC microstructure and various engine operation parameters. It was shown that the TBC microstructure has an influence on TBC strain tolerance, and that the evolution of the TBC strain tolerance during engine operation is linked to the TBC temperature as well as the operating hours. The data have been used to develop a predictive model of the evolution of the TBC strain tolerance during engine operation. This model allows optimization of parts reconditioning interval, and provides tools for determining the residual life of coated components.
Bending-driven failure mechanism and modelling of double-ceramic-layer thermal barrier coating system
