Modelling of Crack Growth Near the Metallic-Ceramic Interface during Thermal Cycling of Air Plasma Sprayed Thermal Barrier Coatings

Abstract

The lifetime under thermal cycling of a system consisting of an air plasma sprayed thermal barrier coating (TBC) deposited on a metallic bondcoat (BC) is determined by the subcritical growth of micro-cracks near the interface between both coatings. This growth mainly occurs during the cooling down phase, as shown by the acoustic emission monitoring during the thermal cycling. The factors controlling the stress level leading to the crack growth are the local curvature of the metallic-ceramic interface, the growth of an oxide scale (TGO) at such interface and the sintering of the TBC, the two last processes occurring during the high temperature cycle phase. Implementing all these factors, a model based on Finite Element Method (FEM) calculations is presented where growing cracks are incorporated by assigning soft properties to the FEM cells occupied by the cracks. Determining the growth direction for the maximum energy release rate at every cooling down step, the current crack extension during the cycling is tracked until it reaches a characteristic length corresponding to the TBC failure. The influence by the metallic-ceramic interface roughness and by the temperature gradient across the TBC is discussed.