The use of thermal barrier coatings (TBCs) allows advanced gas turbine engines to operate at a temperature higher than the incipient melt temperature of the superalloy from which the engine components are made, thereby enhancing the performance and efficiency of the engine. However, delamination cracks initiated in these coatings during service limit their applications.
This investigation analysed the effects of thermal cycling on the structure, thermo-mechanical properties and lifetime of Ni-based superalloy samples coated with a TBC. The results indicate that the coating system exhibits substantial changes during its life, with the thermo-mechanical properties of the TBC layers being highly sensitive to temperature and cyclic oxidation.
The current study also presents a new finite element model that describes the evolution of the stress state within a thermal barrier coating subjected to thermal cycling loading. This computational framework was used to identify the optimal design parameters through a newly proposed sensitivity index, so that TBCs can be engineered with improved lifetime.
Photoluminescence piezo-spectroscopy has been used to identify non-destructively the onset of microcracks and monitor their propagation through a proposed local damage factor that combines spectral shape evolution with peak shift. The computational spectral simulation was based on coupling the finite element model for the calculation of stress with an external routine for the prediction of luminescence spectra.
A new non-destructive multi-sensor diagnostics procedure based on the combination of imaging- and laser-based techniques was presented. It has been demonstrated that it can accurately determine the remaining life of high-temperature coatings, and therefore it represents an important development direction for improving the reliability of TBCs.
It is concluded that the results obtained in this research were quite satisfactory, which suggests that further model development and field testing of the non-destructive methodology are warranted for predictive failure assessment of thermal barrier coating systems.
Keywords
Thermal barrier coating
Material properties
Finite element modeling
Photoluminescence piezo-spectroscopy
Non-destructive evaluation