Spallation mechanism of thermal barrier coatings with real interface morphology considering growth and thermal stresses based on fracture phase field

Abstract

The interface of atmospheric plasma spray (APS) thermal barrier coatings (TBCs) is extremely rough and uneven, causing extremely complicated laws in thermally grown oxide (TGO) growth, stress distribution and evolution, as well as crack initiation and expansion. In this paper, a fracture phase field theory of TBCs interfaces with considerations of growth and thermal stresses is established. A two-dimensional geometric model of APS TBCs was constructed based on the real interface morphology. Next, the TGO growth laws and interface spallation mechanism during the service of TBCs were studied through numerical simulation and experiment. Results demonstrated that the TGO growth rate at the convex position on the interface is higher than those at other positions, especially at the local convex positions. TGO at the left and right sides of concave positions approach continuously with oxidisation of the interface, causing the TGO to wrinkle and the interface to become rougher. With an increase in TGO thickness, stresses in the TGO at peak areas perpendicular to the interface increase significantly and their maximum reaches to 1.58 GPa. Interface cracks initiated in peak areas around the TGO/BC interface at 877 h, and then formed in flat positions in the TGO at 912 h as well as valley areas around the TGO/TC interface at 928 h. Final, TBCs failed at 959 h due to the coalescence of these cracks. All four types of interface cracks could be found in the simulation results.