Numerical study on interfacial delamination of thermal barrier coatings with multiple separations

Coating film generally fails by spalling on cooling after prolonged thermal cycling. Cross-section SEM images show that numerous separations exist at the coating-substrate interface even in the as-processed condition. Upon cooling from the high deposition temperature to room temperature, the difference in the thermal expansion coefficients between the substrate and the coating film makes these multiple isolated separations grow, coalescence and linking up, and, finally leads to coating buckling upon a critical size. In this paper, the interfacial degradation of thermal barrier coatings with multiple separations is investigated by combining the virtual crack closure technique with the user element subroutine "UEL" of Abaqus. The effects of interfacial separation morphology on the energy release rate (ERR) are quantified. Results show that the propagation and coalescence of local separations will dramatically alter the load capacity of the studied structures. It revealed that the larger the initially bonded zone is, the larger the steady ERR value is and the later the local maximum ERR value appears. It is believed that the concerted modeling technique will greatly benefit the understanding of the mechanics associated with the linking-up of subcritical flaws to form a critical flaw size for buckling.