Effect of MnO on the microstructures, phase stability, and mechanical properties of ceria-partially-stabilized zirconia (Ce–TZP) and Ce–TZP–Al₂O₃ composites

Abstract

The effects of increasing amounts of MnO additions on the microstructures, phase stability, and mechanical properties of ZrO₂–12 mol % CeO₂ and ZrO₂–12 mol% CeO₂–10 wt.% Al₂O₃ were studied. MnO suppressed grain growth in ZrO₂–12 mol% CeO₂, while enhancing the mechanical properties significantly (strength = 557 MPa, fracture toughness = 9.3 MPa $\sqrt {\rm{m}}$ at 0.2 wt.% MnO). The enhanced mechanical properties were achieved despite an increased stability of the tetragonal phase, as evidenced by a lower burst transformation temperature (M_b) and a reduced volume fraction of the monoclinic phase on the fracture surface. In ZrO₂–12 mol% CeO₂–10 wt.% Al₂O₃, the addition of MnO suppressed the grain size of ZrO₂, while promoting grain growth and changing the morphology of Al₂O₃. More significantly, the stability of the tetragonal ZrO₂ phase decreased (high M_b temperature) with a concurrent increase in fracture toughness (13.2 MPa $\sqrt {\rm{m}}$ at 2 wt.% MnO) and transformation plasticity (1.2% in four-point bending). The widths of the transformation zones observed adjacent to the fracture surfaces showed a consistent inverse relation to the transformation yield stress, as would be expected from the mechanics of stress-induced phase transformation at crack tips. The improvements in mechanical properties obtained in the base Ce–TZP and the Ce–TZP–Al₂O₃ composite ceramics with the addition of MnO are critically examined in the context of transformation toughening and other possible mechanisms.