Modelling of catastrophic stress development due to mixed oxide growth in thermal barrier coatings

In thermal barrier coatings (TBCs) of heavy-duty gas turbines, thermally grown oxide (TGO) develops in two stages, i.e. firstly, a thin layer of dense protective alpha-Al2O3 forms slowly, and then, a layer of porous detrimental mixed oxide (MO) between top coat (TC) and alpha-Al2O3 appears. During long-term isothermal oxidation at high temperature, the failure of TBCs usually occurs when a critical thickness of MO is reached, but the exact failure mechanism is still largely unclear, let alone the related stress development. In this paper, we analyze the stress evolution and the resultant failure modes due to the whole-layer growth of uniform MO. The results show that it is MO, rather than alpha-Al2O3, that is mainly responsible for the micro-cracking and/or delamination in TBCs. The fast growth of expansive MO induces catastrophic stresses, which leads to micro-cracking in the alpha-Al2O3 layer. The cracking of alpha-Al2O3 layer reduces the oxidation resistance and further accelerates the MO growth. Our theoretical analysis provides a reasonable explanation of the experimental results.