Numerical simulations of crack formation from pegs in thermal barrier systems with NiCoCrAlY bond coats

The high temperature exposure of thermal barrier systems with MCrAlY bond coats is accompanied by non-uniform thickening of the thermally grown oxide (TGO). The heterogeneities are manifest as intrusions into the bond coat, referred to as “pegs”. The progressive growth and swelling of these intrusions, in conjunction with the thermal expansion mismatch from layer to layer, leads to the development of localized strains upon thermal cycling. A finite element study reports on the evolution of the stress state and of the plastic strains in the vicinity of the pegs, and on the conditions under which cracks may initiate and advance. It is demonstrated that large plastic strains accumulate near the peg. Moreover, large shear stresses arise that could motivate mode II (shear) cracking along the interface between the bond coat and thermally grown oxide. However, for realistic sizes of peg, the energy release rate is always much less than the interface toughness. It is concluded that the pegs do not behave as imperfections that initiate cracks, but rather, serve a beneficial purpose of fastening the TGO to the bond coat.