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

Abstract

The mechanisms of phase destabilization upon aging of the metastable t 0 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 0 -8YSZ. 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 0 -8YSZ is estimated to be 1640°C. Full decomposition of the t 0 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 0 phase reappears in the diffraction profile. This supports our evolving understanding that the t 0 phase observed by XRD in aged samples is a microstructural artifact due to the coherency strain between the t and c phases. †

Structure and Thermal Expansion of YSZ and La2Zr2O7 Above 1500°C from Neutron Diffraction on Levitated Samples

“…Only the cubic phase was observed in the YSZ sample at 1650°C. Measured thermal expansion for cubic YSZ is consistent with data recently reported on cell parameters for similar composition after homogenization (TZ‐6 ~12 × 10 −6 /K for 1500°C–1700°C). Previously reported TEC for LZ (9.6 × 10 −6 /K) obtained by dilatometry in the range 25°C–1400°C agrees with our measurements as well.…”

“…YSZ with composition Y 0.1 Zr 0.9 O 1.95 was chosen due to its stability in the defect fluorite structure above 1500°C. It is also a composition of practical interest due its occurrence during service of protective thermal barrier coatings on turbine blades in airplane jet engines . La 2 Zr 2 O 7 pyrochlore ( Fd ‐3 m ) is an example of an ordered version of DF structure and exhibits lower thermal conductivity and oxygen diffusivity than YSZ at high temperature and may be an alternative to YSZ in thermal barrier coatings for metals .…”

“…It is also a composition of practical interest due its occurrence during service of protective thermal barrier coatings on turbine blades in airplane jet engines. 24 La 2 Zr 2 O 7 pyrochlore (Fd-3m) is an example of an ordered version of DF structure and exhibits lower thermal conductivity and oxygen diffusivity than YSZ at high temperature and may be an alternative to YSZ in thermal barrier coatings for metals. [25][26][27][28] Some formulations of ultra-high-temperature ceramics for hypersonic applications 29 are designed to facilitate LZ formation during oxidation of carbides.…”

Investigation of Annealing Induced Yttria Segregation in Sputtered Yttria‐Stabilized Zirconia Thin Films

“…We believe that the observed trend in microstrain could be due to the thermal microstress introduced by the thermal expansion mismatch of Y-lean and Y-rich phases upon annealing. 27 All the XRD peaks shifted toward higher 2h position with increase in annealing temperature suggesting a shrink in lattice parameter. A reduction in the lattice parameter can be caused either by the strain or compositional changes.…”

“…In our case, although the microstrain values are very small, the values were found to increase with increase in annealing temperature. We believe that the observed trend in microstrain could be due to the thermal microstress introduced by the thermal expansion mismatch of Y‐lean and Y‐rich phases upon annealing …”