This research was directed at developing the fundamentals that will facilitate the fabrication of air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC) with controlled thickness and density combinations and bond coat optimization for maximum TBC durability. It includes research on the yttrium content and surface roughness of NiCoCrAlY bond coats deposited by argon-shrouded plasma spraying and on the microstructural control of APS topcoats to maximize the coating thicknesses that can be applied without spallation and to minimize the thermal conductivity of the TBC. The specimens used for this research were prepared by Praxair Surface Technologies and have been evaluated using cyclic oxidation and thermal shock tests. Important properties of the YSZ TBCs, such as resistance to sintering and phase transformation and the thermal conductivity were determined. Coefficients of thermal expansion were measured for the superalloy substrate materials. The properties of high-purity, low-density (85%) APS TBCs on NiCoCrAlY bond coats were evaluated. The high purity resulted in topcoats which are highly resistant to sintering and transformation from the metastable tetragonal phase to the equilibrium mixture of monoclinic and cubic phases. The thermal conductivity of the as-sprayed topcoats was measured to be less than 1 (W/m-K). The high sintering resistance means this low value will increase only slowly with high temperature exposure. The porous topcoat microstructure also resulted in significant durability during thermal cycling. A 375μm thick APS coating was found to have a comparable furnace cyclic life to that of a standard 100μm EBPVD coating. The actual failure mechanisms of the APS coatings were found to depend on topcoat thickness and the nature of the thermal exposure. The failure times and mechanisms were found to depend strongly on the superalloy substrate used. This effect was found to be associated with the coefficient of thermal expansion (CTE) mismatch between the superalloy and topcoat. The bond coat topography modifications implemented in this work did not have a significant effect on the cyclic behavior. However the yttrium content and distribution in the bond coat did have a significant effect on TBC lifetime.
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