Modelling and analysis of the oxide growth coupling behaviour of thermal barrier coatings

A chemo-transport-mechanics model is developed to study the growth of thermally grown oxide (TGO) and its impact on deformation and stress in air plasma-sprayed thermal barrier coatings (TBCs). As the driving force for oxygen transport, the chemical potential consists of contributions from both species concentration and hydrostatic pressure. The model suggests that both the concentration boundary condition and the transport process of the oxygen are affected by hydrostatic stress. Since oxygen has smaller diffusion coefficient in TGO than in BC, the retarding effect of the formed TGO on oxygen transport is considered and clarified by the coupled model. The competition between geometrical imperfection (i.e. concave morphology) and the chemo-mechanics coupling to influence the transport of oxygen is also identified numerically. The geometrical imperfection can introduce additional oxygen transport at the margin of the concave imperfection due to the horizontal component of the gradient of the chemical potential of the oxygen, which plays a dominant role in the TGO growth kinetics for the studied TBCs. Consequently, there is a limited effect of the chemo-mechanics coupling on the growth kinetics of a concaved TGO. The amplitude change of the concave portion is found to be up to 0.36 µm after 600-h exposure at 1150 °C, which leads to large tensile stress above the concave portion potentially causing micro-cracks.