Failure analysis of high-temperature oxidation for plasma sprayed thermal barrier coating systems with different coating characteristics

In order to evaluate the durability of plasma sprayed thermal barrier coating (TBC) systems with different coating characteristics such as the coating defect structures and interfacial natures, high-temperature oxidation tests were conducted at 1000 and 1100degreesC under both the isothermal and thermal cycle conditions. High-temperature oxidation behavior was found to differ depending strongly on the top-coat (TC) microstructure and the reheat-treatment (RHT) conditions after spraying; namely on a combination of the treatment temperature and atmosphere. It was clarified, in particular, that the TC microstructure with the improved strain accommodation (stress relief) function, such as the porous or segmented TC layer is much effective in suppressing the TC spalling. For TBC systems with Y2O3-stabilized ZrO2 as the TC, furthermore, the RHT at an appropriate temperature in Ar was found to be also effective for an improvement of the oxidation property, since the RHT in Ar is attainable for the continuous Al2O3 layer at the TC/ bond-coat (BC) interface so as to reduce effectively the growth rate of thermally gown oxide (TGO) during high-temperature oxidation. For TBC systems with TC of CaO-SiO2-ZrO2, on the contrary, it was found to be difficult to improve the oxidation property by any RHT processes, mainly due to the high reactivity of CaO in TC with the CoNiCrAlY-BC. The affecting factors for the high-temperature oxidation property were discussed on the basis of such a failure analysis.