In Situ Diffraction Study of the High‐Temperature Decomposition of t′ ‐Zirconia

The mechanisms of phase destabilization upon aging of the metastable t′ phase of yttria‐stabilized zirconia ( YSZ ) are poorly understood, despite its broad application in thermal barrier coatings. To provide insight, synchrotron X‐ray diffraction ( XRD ) with a quadrupole lamp furnace is used to examine the temperature response, including thermal expansion and phase evolution, of a 9 mol% (8 wt%) YO 1.5 t′‐ 8 YSZ . The thermal expansion of equilibrated YSZ powders ranging from 0 to 18.4 mol% YO 1.5 is also investigated to better understand the effect of composition on the thermal expansion anisotropy. The T 0 ( c/t ) temperature for t′‐ 8 YSZ is estimated to be 1640°C. Full decomposition of the t′ phase into a coherent mixture of a Y‐lean tetragonal phase ( t ) and a Y‐rich cubic phase ( c ) that coarsen over time is observed at elevated temperatures; however, upon quenching, the t′ phase reappears in the diffraction profile. This supports our evolving understanding that the t′ phase observed by XRD in aged samples is a microstructural artifact due to the coherency strain between the t and c phases.