Mechanisms and mechanics of early growth of debonding crack in an APSed Ni-base superalloy TBCs under cyclic load

Nucleation and early growth of debonding cracks in a Ni-based superalloy thermal barrier coating (TBC) system were investigated. The TBC system consisting of CoNiCrAlY bond and ZrO₂ top coat stabilized by 8%Y₂O₃, was subjected to cyclic load at high temperature. During the study, special effort was made to understand both mechanistic and mechanical behavior on the nucleation and subsequent crack propagation process. The experimental observations clearly demonstrated that both the lamellar structure of top coat and the interlamellar cracks at the splat boundaries played intrinsic role in the nucleation of the spallation cracks. It was also shown that the naturally-initiated small spallation cracks in the smooth specimen revealed propagation rates considerably higher than long crack in the notched specimen under the same remote external loading conditions. This behavior reminds us the fatigue small crack problems, same as in metallic materials. Some fracture mechanics aspects of these characteristics and behaviors are discussed on the basis of FEM calculations.