Spallation of two thermal barrier coating systems: experimental study of adhesion and energetic approach to lifetime during cyclic oxidation

To understand the degradation of two thermal barrier coating (TBC) systems, we determined the adhesion energy between the bondcoat and the topcoat and its evolution during cyclic oxidation at 1,100 °C, by means of a modified 4-point bending test. An yttria stabilized zirconia (YSZ) ceramic topcoat was deposited by electron beam physical vapour deposition (EBPVD) on a Ni-based superalloy with either an intermediate β-(Ni,Pt)Al bondcoat or a newly developed Zr-doped β-NiAl bondcoat. Although a similar evolution of the adhesion energy during cyclic oxidation has been recorded for both systems, observations of the fracture surfaces combined with a microstructure study revealed different degradation mechanisms. An energetic model of spallation is applied to predict their lifetime. According to this approach, the TBC failure is induced by the accumulation of strain energy in the ceramic layers and resisted by the interfacial fracture toughness. The predicted lifetime is consistent with experiments for both systems.