As-deposited thermal barrier coatings (TBC) made of yttria-stabilized zirconia (8YSZ) have the meta-stable, tetragonal-prime crystal structure. Although this is not the stable equilibrium form of 8YSZ, this crystallographic structure is remarkably resistant to transformation to the equilibrium mixture of cubic and monoclinic phases over thousands of hours at typical TBC use temperatures. Nevertheless, with high-temperature aging, Y3+ ions diffuse to their equilibrium positions and the structure slowly evolves into a mixture of t', yttria-poor tetragonal and yttria-rich cubic phase. After cooling, no monoclinic phase is present but it forms slowly over time at low temperatures (including room temperature). The kinetics of this low temperature transformation after heat treatment at 1425 degrees C has been quantified from room temperature to 150 degrees C using both Raman spectroscopy and X-ray diffraction, and appears to be thermally activated. Insufficient data are yet to be accumulated to identify the microscopic mechanism of the transformation but the kinetics is consistent with a diffusion-controlled mechanism.
