| Ni22Cr10Al1.0Y/ZrO₂-25CeO₂-2.5Y₂O₃ and Ni22Cr10Al1.0Y/ZrO₂-8Y₂O₃ duplex(bond+top) thermal barrier coatings were prepared using the air plasma spray process. Properties such as phase transformation in the ceramic top coating, bond coat oxidation and thermal barrier effect were investigated to compare ZrO₂-CeO₂-Y₂O₃ with ZrO₂-Y₂O₃ under the 1300℃ high temperature thermal cycles. In condition of as-sprayed, both coatings showed 7∼11% porosity fraction and typical lamellar structures formed by continuous wetting of liquid droplets. In the ZrO₂- CeO₂- Y₂O₃ coating, the phase ratio of tetragonal to cubic was 75:25, and ZrO₂-Y₂O₃ coating had 100% untransformable tetragonal phase. There was no monoclinic phase in both coatings. However, the phase ratio of coatings was changed after 1300℃ thermal cycles. In the ZrO₂-CeO₂-Y₂O₃ coating, the ratio of tetragonal to cubic was changed to 88:12 and monoclinic phase was not detected, but in the ZrO₂-Y₂O₃ coating 10∼19% monoclinic phase had formed. The life of coatings was found to be strongly dependent upon the temperature of the bond coat experienced during exposure to peak temperature of 1300℃. When the bond coat experienced a temperature higher than 1100℃, the campaign life of bath TBC decreased drastically and this was related to the oxidation behaviour. Al₂O₃ was 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, were formed rapidly at the top coat side of interface at temperature higher than 1100℃. The ZrO₂-CeO₂-Y₂O₃ coating was superior to ZrO₂-Y₂O₃ coating because there was no phase transformation from tetragonal to monoclinic(which induces volume expansion) and the effect of oxide growth stress was relatively small by better thermal insulation.(i.e, bond coat experienced a temperature lower than 1100℃ for a given TBC thickness) |
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