Effect of TGO creep on top-coat cracking induced by cyclic displacement instability in a thermal barrier coating system

The displacement instability of the thermally grown oxide (TGO) under thermal cycling is a fundamental source of cracking within the ceramic top-coat (TC) in a thermal barrier coating system. The effect of TGO creep on TC cracking induced by cyclic instability is numerically investigated in this work. Temperature-dependent elastic-plastic properties of each coating layer are considered in a finite element analysis. A power law is adopted to model the creep behavior of TGO and bond-coat (BC), and TGO growth at high temperature with the consideration of both lateral and through-thickness growth strains is simulated. The numerical results show that during thermal cycling TGO displacement instability occurs in the zone having a geometric imperfection and the accumulation of TGO displacements (downward at the center and upward around the periphery) is evident. The tensile stress (perpendicular to the TGO/TC interface) in TC mainly arises above the center of the instability zone and causes a mode I crack, whereas the shear stress is predominant around the periphery of the instability zone and causes a mode II crack. Both the tensile and shear stresses can be reduced by TGO creep which restrains the morphology distortion of the instability zone during thermal cycling. Furthermore, TGO creep can significantly decrease the energy release rate of TC cracking no matter whether the crack is located in the tensile-stress dominating zone or the shear-stress dominating zone.