The temperature capability of yttria-stabilized zirconia thermal barrier coatings (TBCs) is ultimately tied to the rate of evolution of the ''nontransformable'' t 0 phase into a depleted tetragonal form predisposed to the monoclinic transformation on cooling. The t 0 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.

The relationship between yttria concentration and the unit cell parameters in partially and fully stabilized zirconia has been reassessed, motivated by the need to improve the accuracy of phase analysis upon decomposition of t′-based thermal barrier coatings. Compositions ranging from 6 to 18 mol% YO 1.5 were synthesized and examined by means of high-resolution Xray diffraction. Lattice parameters were determined using the Rietveld refinement method, a whole-pattern fitting procedure. The revised empirical relationships fall within the range of those published previously. However, efforts to achieve superior homogeneity of the materials, as well as accuracy of the composition and lattice parameters, provide increased confidence in the reliability of these correlations for use in future studies. Additional insight into the potential sources for scatter previously reported for the transition region (~12-14 mol% YO 1.5 ), where tetragonal and cubic phases have been observed to coexist, is also provided. Implications on the current understanding of stabilization mechanisms in zirconia are discussed.

Phase evolution accompanying the isothermal aging of free‐standing air‐plasma sprayed (APS) 7–8 wt% yttria‐stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) is described. Aging was carried out at temperatures ranging from 982°C to 1482°C in air. The high‐temperature kinetics of the phase evolution from the metastable t′ phase into a mixture of transformable Y‐rich (cubic) and Y‐lean (tetragonal) phases are documented through ambient temperature X‐ray diffraction (XRD) characterization. A Hollomon–Jaffe parameter (HJP), T[27 + ln(t)], was used to satisfactorily normalize the extent of phase decomposition over the full range of times and temperatures. Comparison to vapor deposited TBCs reveal potential differences in the destabilization mechanism in APS coatings. Furthermore, the lattice parameters extracted from Rietveld refinement of the XRD patterns were used to deduce the stabilizer concentrations of the respective phases, which suggest a retrograde tetragonal solvus over the temperature range studied. In concert with a complementary microstructural study presented in Part II, this effort offers new insights into the mechanisms governing the phase evolution and raises implications for the high‐temperature use of 8YSZ ceramics.

Water vapor condensation is a ubiquitous process in nature and industry. Over the past century, methods achieving dropwise condensation using a thin (<1 µm) hydrophobic 'promoter' layer have been developed, which increases the condensation heat transfer by 10 times compared to filmwise condensation. Unfortunately, implementations of dropwise condensation have been limited due to poor durability of the promoter coatings. Here, we develop thin film condensation which utilizes a promoter layer not as a condensation surface, but rather to confine the condensate within a porous biphilic nanostructure, nickel inverse opals (NIO) with a thin (<20 nm) hydrophobic top-layer of decomposed polyimide. We demonstrate filmwise condensation confined to thicknesses <10 µm. To test the stability of thin film condensation, we performed condensation experiments to show that at higher supersaturations (0.975 < S < 2.05), droplets coalescing on top of the hydrophobic layer are absorbed into the superhydrophilic layer through coalescence induced transitions. Through detailed This article is protected by copyright. All rights reserved.3 thermal-hydrodynamic modeling, we show that thin film condensation has the potential to achieve heat transfer coefficients approaching ≈100 kW m -2 while avoiding durability issues by significantly reducing nucleation on the hydrophobic surface. The work presented here develops an approach to potentially ensure durable and high performance condensation comparable to dropwise condensation.

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. †