Chromaticity, hydrothermal stability, and mechanical properties of t-ZrO2/Al2O3 composites doped with yttrium, niobium, and ferric oxides

Tetragonal (t) zirconia–alumina composites were prepared by adding Al2O3 into t-ZrO2 solid solutions doped with Y2O3 and Nb2O5 ((Y,Nb)-TZP/Al2O3). The effect of Fe2O3 addition on chromaticity, hydrothermal stability, and mechanical properties of the composites was investigated by adding Fe2O3 in 0.1 mol% intervals in the range of 0.1–0.5 mol%. Although monolithic (Y,Nb,Fe)-TZPs containing more than 0.2 mol% Fe2O3 showed t→m phase transformation after autoclaving for 20 h at 180°C under 3.5 MPa water vapor pressure, no m-ZrO2 was observed for the aged (Y,Nb,Fe)-TZP/Al2O3 composites containing 0–0.5 mol% Fe2O3, probably due to the preferential solid solubility of Fe2O3 in Al2O3, the presence of the rigid Al2O3 particles, and the inherent phase stability of (Y,Nb)-TZP. The optimized strength and the fracture toughness of (Y,Nb,Fe)-TZP/Al2O3 composites were 700 MPa and 8.5 MPa m1/2, respectively, when 0.1 mol% of Fe2O3 was added. Both monoliths and composites showed a gradual color change from a slight white ivory to a pale yellowish brown with increasing the Fe2O3 concentration.