Influence of material models on the stress state in thermal barrier coating simulations

The stress state and failure mechanisms of thermal barrier coating systems are frequently studied using finite element simulations. One problem in evaluating the results of such simulations is that the material behaviour of the top coat and especially of the thermally grown oxide is not well-known. Either plasticity or viscoplasticity is frequently assumed to be the dominating stress relaxation mechanism with the implicit assumption that both mechanisms will lead to similar stress patterns. In this contribution, this assumption is critically evaluated. Different combinations of plasticity or viscoplasticity for the top coat and the TGO are studied. If viscoplasticity is not present, the stress evolution is dominated by plastic ratchetting, but this is counteracted by creep relaxation if the creep strength is sufficiently low.