Damage process of the high-temperature-exposed ceramic thermal barrier coatings under tensile loading

Abstract

In this study, the damage process of Thermal Barrier Coatings (TBC), which were exposed at 1273K for 500hours in advance, under tensile loading was observed continuously, and the influence of exposure temperature on crack initiation strength and interface strength was clarified quantitatively based upon interfacial fracture mechanics. TBC/IN738LC specimen, which is consisted of yttria-stabilized zirconia as top coat (TC) and CoNiCrAlY as bond coat (BC) in TBC, was prepared and the high-temperature exposure treatment was then conducted before tensile test. Continuous observation of damage process was revealed that (i) interfacial crack propagated along the interface between TC and BC in exposed TBC tested at room temperature however the crack propagated along BC/substrate interface in as-sprayed TBC, and (ii) interfacial crack propagated along TC/BC interface in the exposed TBC specimen as well as as-sprayed one. It was considered that the crack propagation path was changed by strong effect of a high-temperature exposure treatment. Quantitative evaluation also revealed that critical strain up to crack initiation in TC was increased by the exposed treatment and number of cracks per length was decreased in contrast with the critical strain. The exposure treatment stiffened top coat layer, which means that cohesive strength between splats became stronger. The interfacial fracture energy of the exposed TBC specimen was lower than that of as-sprayed one. Therefore, in order to assess precisely remaining life of main hot-parts of gas turbine, data for as-sprayed TBC is needed as initial information.