Simulation of stresses and delamination in a plasma-sprayed thermal barrier system upon thermal cycling

A thermal barrier system with plasma-sprayed thermal barrier coating (TBC) has been analyzed subject to thermal cycling. The thermally grown oxide (TGO) is allowed to grow at the peak temperature in the cycle, with both thickening and lateral components of the growth strain. The stresses in the TGO are also allowed to relax at high temperature when they attain a critical level. The bond coat is allowed to yield with temperature dependent yield strength. The stresses induced in the TBC and in the TGO have been calculated. The vertical component of the stress in the TBC is shown to have a large tensile domain with a maximum that increases systematically as the system cycles. There is a corresponding increase in the amplitude of the interface undulations. The stresses in the TBC have been used to calculate energy release rates, G, for cracks in the TBC extending parallel to the interface, just above the peaks of the undulations. This analysis reveals a minimum value, Gmin preceding instability. Equating this minimum to the TBC toughness identifies a delamination criticality.