No abstract

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.

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.

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.

The stresses in the aluminum oxide formed during high-temperature oxidation of a bond-coated superalloy are shown to be measurable through zirconia thermal barrier coatings. The basis for the measurements is the piezospectroscopic shift in the R-line fluorescence (photoluminescence) from Cr3+ impurities incorporated into the growing aluminum oxide scale. Measurements through the thermal barrier coating are feasible because (partially stabilized) zirconia coatings have some transparency at both the excitation and at fluorescence frequencies.