Mechanics-based scaling laws for the durability of thermal barrier coatings

The durability of thermal barrier systems is governed by a sequence of crac k nucleation, propagation and coalescence events that accumulate prior to f inal failure by large scale buckling and spalling. This sequence is governe d by the sigma (zz) stresses that develop normal to the substrate, around i mperfections, as the thermally grown oxide (TGO) thickens. Their effect is manifest in the stress intensity factor, K, caused by the sigma (zz) stress es acting on cracks emanating from them. In turn, these events are governed by scaling laws, ascribed to non-dimensional groups governing sigma (zz) a nd K. In this article the basic scaling relations are identified and used t o gain some understanding of the relative importance of the Various mechani sms that arise for application scenarios with minimal thermal cycling. Thes e mechanisms are based on stresses that develop because of TGO growth strai ns in combination with thermal expansion misfit. The results are used to id entify a critical TGO thickness at failure and express it in terms of the g overning material variables. The changes in behavior that arise upon extens ive thermal cycling, in the presence of TGO ratcheting, are elaborated else where. (C) 2001 Elsevier Science Ltd. All rights reserved.