Zirconia-based ceramics have long been used to provide thermal protection to the structural components of modern gas turbine engines. Economic and environmental considerations have motivated higher engine operating temperatures, potentially leading to more rapid degradation of thermal barrier coating (TBC) systems dependent on a metastable phase, namely t'-8YSZ (ZrO 2+7-8wt%YO1.5) TBCs. The t'-phase was originally thought to decay slowly into the equilibrium yttria-lean tetragonal phase and yttria-rich cubic phase, the former of which may undergo further transformation to the monoclinic phase. The present study shows that the t'-phase rapidly decomposes into a coherent array of Y-rich and Y-lean domains at a small fraction of the time necessary to form the deleterious monoclinic phase. However, because of the constraint imposed by the interfaces and surrounding domains, this is not detectable via XRD until the domains have coarsened considerably. The coherent nature of the interfaces also plays an important role in delaying the onset of the monoclinic transformation by limiting the coarsening rate of Y-lean domains. Coherency strain not only influences the coarsening rate of the microstructure, but may also be responsible for the development of a strain induced spinodal region—a concept that accommodates the currently accepted thermodynamic description of the system as well as the microstructural observations.
The rapid decay of t'-YSZ into a modulated microstructure of coherent domains offers additional insight on the importance of microstructural control in the phase evolution of YSZ TBCs. However, examination of evolution in several typical TBC microstructures reveals that development and retention of the modulated microstructure is subject to the initial microstructural morphology. Larger initial grain size and chemical homogeneity of the initial microstructure were closely correlated to the formation of a modulated microstructure—concomitantly, leading to a delay in the onset of the monoclinic transformation.
Stemming from this comparison, potential pathways for further delaying the onset of the monoclinic transformation have been identified. While the effectiveness of these measures is expected to be modest, lessons have been learned from the t'-8YSZ system. The desire to understand and control phase stability in the 8YSZ system is related to the importance of maintaining a high degree of tetragonality over the entire range of relevant temperatures. This is key to supporting or improving the in service toughness and durability. As such, a novel TBC system has been developed in which a relatively large single phase tetragonal field with exceptional tetragonality is stabilized and has demonstrated comparable or improved toughness, making it a promising alternative for next generation TBCs.
|Advisor:||Levi, Carlos G.|
|Commitee:||Pollock, Tresa M., Seshadri, Ram, van der Ven, Anton|
|School:||University of California, Santa Barbara|
|School Location:||United States -- California|
|Source:||DAI-B 74/02(E), Dissertation Abstracts International|
|Keywords:||Electron microscopy, Phase stability, Phase transformations, Thermal barrier coatings, X-ray diffraction, Zirconia|
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