Deceleration of hydrothermal degradation of 3Y-TZP by alumina and lanthana co-doping

Zirconia has been used as an orthopaedic material since 1985 and is increasingly used in dental applications. One major concern with the use of zirconia is the significant loss in mechanical properties through hydrothermal degradation, with the uncontrolled transformation of tetragonal to monoclinic (t→m) zirconia. We report on the addition of alumina and lanthana as dopants to an yttria-stabilized tetragonal zirconia polycrystal ceramic as an effective strategy to significantly decelerate the hydrothermal degradation kinetics, without any loss of mechanical properties, in particular, fracture toughness. Hydrothermal degradation was studied on the exposed surface as well as in the sub-surface region using Raman microspectroscopy, atomic force microscopy and cross-sectional transmission electron microscopy, providing a comprehensive insight into the mechanism of propagation of the t→m transformation. The addition of dopants resulted in the reduction of monoclinic zirconia nucleation rate at the surface and a substantial deceleration of the overall transformation kinetics, in particular a greatly reduced propagation of the transformation into the bulk and decreased grain boundary microcracking. High-resolution transmission electron microscopy analysis showed that the co-dopant cations segregate to the grain boundaries where they play a key role in the stabilization of the zirconia tetragonal phase.