Strength degradation and failure mechanisms of electron-beam physical-vapor-deposited thermal barrier coatings

Failure mechanisms were determined for electron-beam physical-vapor-deposit ed thermal barrier coating (TBC) systems from the degradation of mechanical properties and microstructural changes in a furnace cycle test. Bond stren gth degradation for TBCs resulted from the initiation and growth of interfa cial delamination defects between the yttria-stabilized zirconia topcoat an d the thermally grown alumina (TGO). It is proposed that defects started fr om concave depressions in the bondcoat surface created by the grit-blast-cl eaning process and that defect growth was driven by the reduction in compre ssive strain in the TGO as the alumina deformed into and displaced the bond coat during the cooling cycles. Inclusion of yttrium in the substrate resul ted in a doubling of the furnace cycle life of the TBCs because of enhanced fracture toughness of the TGO-bondcoat interface.