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Substitution of TiO2 for ZrO2 into single‐phase tetragonal (t′) 7YSZ has been shown to induce a twofold increase in toughness. This enhancement has been achieved by increasing the tetragonality of the unit cell, upon substituting Ti4+ for the larger Zr4+ cation. The observed behavior is consistent with ferroelastic toughening, but direct evidence of the mechanism is lacking. Adding TiO2 to 7YSZ has the additional benefit that it diminishes the transformability of the depleted tetragonal form: specifically, no monoclinic phase was observed even after equilibration at 1600°C, followed by low‐temperature annealing. The combination of properties bodes well for potential application as a thermal barrier coating in gas turbine engines.

Compositions in the ZrO2-Y2O3-Ta2O5 system are of interest as low thermal conductivity, fracture resistant oxides for the next generation thermal barrier coatings (TBC). Multiple phases occur in the system offering the opportunity to compare the thermal properties of single, two-phase, and three-phase materials. Despite rather large variations in compositions almost all the solid solution compounds had rather similar thermal conductivities and, furthermore, exhibited only relatively small variations with temperature up to 1000 o C. These characteristics are attributed to the extensive mass disorder in all the compounds and, in turn, small interfacial Kapitza (thermal) resistances. More complicated behavior, associated with the transformation from the tetragonal to monoclinic phase, occurs on long-term annealing in air of some of the compositions. However, the phases in two of the compositional regions do not change with annealing in air and their thermal conductivities remain unchanged suggesting they may be suitable for further exploration as thermally stable TBC overcoats.

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The correlation between microstructural and phase evolution in aged, yttria‐partially‐stabilized zirconia, air plasma‐sprayed coatings is discussed. Freestanding coatings with the dense, vertically cracked structure were isothermally aged at 1482°C (2700°F) in air. Characterization of the resulting microstructures was conducted using transmission electron microscopy, then compared with a parallel analysis of the phase evolution via synchrotron X‐ray diffraction (XRD) described in Part I. Additional context was provided by related studies on vapor‐deposited coatings. Several salient points can be extracted from these assessments. XRD was further validated as a practical method for studying phase stability after clarification of how the possible phases are defined, including the following: (i) the nature of the t′ phase observed in XRD after phase decomposition has begun and (ii) the relationship between the Y‐rich tetragonal (t″) and Y‐rich cubic (c) phases reported to coexist via XRD. A strong relationship between the initial microstructure and the subsequent phase destabilization is also reported. As a result, phase evolution is proposed to proceed via two competing routes. The interplay between these mechanisms dictates the incubation time for monoclinic formation within a given coating.