Stress and cracking behavior of plasma sprayed thermal barrier coatings using an advanced constitutive model

A finite element simulation of a plasma sprayed thermal barrier coating (TBC) system represented as a three-layer structure with an interface asperity is presented. Loading due to thermal expansion mismatch and geometry changes from oxide growth are included. A realistic viscoplastic model is used to represent the behavior of the ceramic layer, which includes both time dependent behavior and strong yield stress asymmetry. Predicted stresses can be up to three orders of magnitude lower than those based on elastic analysis. Elastic-plastic analysis using symmetric yield stresses result in errors up to a factor of 5 on the stress and hence inclusion of tension compression yield asymmetry is essential if better than order of magnitude stress estimates are required. It is shown that residual stresses at room temperature never become large enough to propagate asperity size cracks based on linear elastic fracture mechanics. Instead, failure may need to be described in terms of volumetric damage. A simple representative approach based on Rankine failure theory is presented. (c) 2006 Elsevier B.V. All rights reserved.