Phase transformation and bond coat oxidation behavior of plasma-sprayed zirconia thermal barrier coating

ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 thermal barrier coatings were prepared using the air plasma spray process. Phase transformation in the ceramic top coating, bond coat oxidation and thermal barrier properties were investigated to compare ZrO2–CeO2–Y2O3 with ZrO2–Y2O3 at 1300°C under high temperature thermal cycles. In the as-sprayed condition, both coatings showed a 7∼11% porosity fraction and typical lamellar structures formed by continuous wetting by liquid droplets. In the ZrO2–CeO2–Y2O3 coating, the phase ratio of tetragonal to cubic phase was 75:25 and ZrO2–Y2O3 coating had a 100% non-transformable tetragonal phase. There was no monoclinic phase in either coating. However, the phase ratio of the coatings was changed after 1300°C thermal cycles. In the ZrO2–CeO2–Y2O3 coating, the ratio of tetragonal to cubic was changed to 88:12 and a monoclinic phase was still not detected, but a 10∼19% monoclinic phase had formed in the ZrO2–Y2O3 coating. The life of the coatings was found to be strongly dependent on the temperature which the bond coat experienced during exposure to a peak temperature of 1300°C. When the bond coat experienced a temperature higher than 1100°C, the useful life of both thermal barrier coatings was decreased drastically and this was related to the oxidation behavior of the bond coat. Al2O3 formed preferentially along the bond and top coat interface and the other oxides such as NiO and Ni(Cr,Al)2O4 spinel, which were believed to decrease the coating life by oxide growth stress, formed rapidly at the top coat side of the interface at temperature higher than 1100°C. The thermally shocked ZrO2–Y2O3 coating exhibited a non-linear thermal expansion curve which was probably due to a reversible tetragonal–monoclinic transformation. The ZrO2–CeO2–Y2O3 coating was found to have better thermal cycling behavior than the ZrO2–Y2O3 coating. The reason for this could be that there was no phase transformation from tetragonal to monoclinic phase, which induces volume expansion. The thermal expansion mismatch is smaller and the effect of oxide growth stress is relatively small because of better thermal insulation.