Phase Stability of t′-Zirconia-Based Thermal Barrier Coatings: Mechanistic Insights


  • A. Heuer—contributing editor

  • This paper is dedicated to the memory of Professor Anthony G. Evans: inspiring leader, generous colleague and good friend.

  • This investigation was financially supported under Grant DMR-0605700 from the National Science Foundation. The research made use of the UCSB-MRL Central Facilities supported by NSF under Grant DMR-0080034. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The scientic input and technical support of the AMMRF node at the University of Sydney, Australia, is gratefully acknowledged. The authors are also grateful to Drs. Y. Gao (GE-GRC) for illuminating discussions and to Dr. R.M. Leckie (UCSB) for assistance during coating fabrication.

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The temperature capability of yttria-stabilized zirconia thermal barrier coatings (TBCs) is ultimately tied to the rate of evolution of the “nontransformable” t′ phase into a depleted tetragonal form predisposed to the monoclinic transformation on cooling. The t′ phase, however, has been shown to decompose in a small fraction of the time necessary to form the monoclinic phase. Instead, a modulated microstructure consisting of a coherent array of Y-rich and Y-lean lamellar phases develops early in the process, with mechanistic features suggestive of spinodal decomposition. Coarsening of this microstructure leads to loss of coherency and ultimately transformation into the monoclinic form, making the kinetics of this process, and not the initial decomposition, the critical factor in determining the phase stability of TBCs. Transmission electron microscopy is shown to be essential not only for characterizing the microstructure but also for proper interpretation of X-ray diffraction analysis.