Fatigue cracks in a thermal barrier coating system on a superalloy in multiaxial thermomechanical testing

Thermal barrier coatings (TBC) are applied on cooled gas turbine components. Between the heated and cooled surfaces a thermal gradient develops, resulting for constraint components in high multiaxial stresses, which may exceed stresses due to mechanical loading. In order to investigate the damage behaviour of TBC systems under close to reality conditions, thermal mechanical fatigue tests with controlled thermal gradients (TGMF-tests) were performed on coated tubular specimens. The specimen substrate was made from directionally solidified nickel base superalloy IN100 DS, and the coating system comprised a NiCoCrAlY bond coat and a ceramic top coat from partially stabilized zirconia. The stress distribution over the specimen wall, which was generated in the course of one TGMF cycle, has been analysed by linear elastic Finite Element calculations.In TGMF testing specific damages occurred underneath the adherent ceramic top coat, evolving into fatigue cracks, which propagated primarily in the metallic bond coat parallel to the surface. In length sections the crack shape in this stage resembles a ‘smiley’. During further cycling the crack path deviated to the interface between BC and top coat, enhancing spallation. The ‘smiley’-crack patterns, which only occurred in TGMF but not in isothermal low cycle fatigue or thermal fatigue, are discussed with respect to the cyclic local stress distribution.