Interaction of isothermal low cycle fatigue and ageing in an EB-PVD thermal barrier coating system

Kurzfassung

Crack growth behaviour in a thermal barrier coating system has been investigated in isothermal low cycle fatigue (LCF) testing of specimens in as coated condition and after ageing in order to achieve information on interaction between fatigue and ageing. Cracks were initiated in most cases at the interface between the metallic bond coat of the TBC system and the adjacent thermally grown oxide. The observed crack growth was assisted by severe oxidation once the crack reached the nickel-based substrate, generating voluminous oxides. Ageing of specimens before the LCF test changed the crack feature and crack growth rate drastically; pre-aging at 950°C for 250 h in air resulted in crack deviation at the diffusion zone between bond coat and substrate with a significant slow down of the crack propagation into the substrate, while extended pre-aging for 500 h did not cause expressed crack deviation but compared to the as coated condition a deceleration of crack propagation. Microstructural analyses showed that formation of a chain of Cr-rich intermetallic precipitates caused the crack deviation after 250 h pre-ageing, while after 500 h a wide meshed network of precipitates formed, which rather slowed down the crack propagation than causing crack deviation. The obtained results will be discussed with respect to strategies for accelerated testing by separation of ageing and fatigue and set in context to lifetime assessment methods.