Phase evolution in the yttrium oxide-titanium dioxide-zirconium oxide system and effects on ionic conductivity and toughness

Compositions in the YO1.5-ZrO2 binary have been widely studied and are used in solid oxide fuel cells and thermal barrier coatings for gas turbines. Through addition of TiO2 a variety of other phases can be accessed that are all derivatives of the fluorite structure and exhibit a unique structural similarity. This makes the YO1.5-TiO 2-ZrO2 system ideal for fundamental studies of phase evolution during synthesis as well as for exploring the relationships between composition, phase constitution and properties.;Powder samples of a variety of compositions in the YO1.5-TiO 2-ZrO2 system were synthesized by liquid precursor routes. Selected samples were equilibrated at 1300°C and 1500°C, and the first comprehensive ternary phase diagrams were established for this system. A phase hierarchy map was constructed based on the improved understanding of the binary and ternary equilibria. Pyrolysis of many compositions led to an amorphous oxide that was subsequently heat treated in a stepwise manner at progressively higher temperatures to explore the changes in phase constitution. The defect-fluorite as well as the pyrochlore structure could be metastably extended well beyond the equilibrium solubility range.;The compositions Y2Ti0.8Zr1.2O 7 and Y2Ti1.2Zr0.8O7 go through a phase evolution sequence from amorphous to single-phase fluorite, single-phase pyrochlore, and pyrochlore+fluorite two-phase structure. The ionic conductivity of these phases was measured at fixed compositions by impedance spectroscopy, and was found to increase during crystallization. The analyses suggested that the specific grain interior conductivity increased upon ordering from fluorite to pyrochlore, which is counterintuitive and presumably implies an increased mobility of the oxygen ions in the Zr-doped pyrochlore structure.;The toughness of compacts of supersaturated tetragonal zirconia in the composition range of interest to thermal barrier coatings was studied. Additions of 7.6 and 15.2 mol% TiO2 to the standard 7.6YSZ material led to a more than two-fold increase in toughness. It was demonstrated that transformation toughening could not be responsible for the major part of this increase, and it was proposed that the gains in toughness were due to ferroelastic toughening. A clear correlation between the increasing toughness and the tetragonality of the unit cell due to addition of the smaller Ti4+ ions was observed and the magnitude of the increase showed reasonable agreement with calculations using a model for ferroelastic toughening. The superior toughness and phase stability of Ti-doped YSZ makes these compositions promising candidates for application in thermal barrier coatings with improved erosion resistance and cyclic life.