Reconciling in vivo and in vitro kinetics of the polymorphic transformation in zirconia-toughened alumina for hip joints: III. Molecular scale mechanisms.

Understanding the intrinsic reason(s) for the enhanced tetragonal to monoclinic (t→m) polymorphic phase transformation observed on metal-stained surfaces of zirconia-toughened alumina (ZTA) requires detailed knowledge of off-stoichiometry reactions at the molecular scale. In this context, knowledge of the mechanism(s) for oxygen vacancy creation or annihilation at the material surface is a necessary prerequisite. The crucial aspect of the surface destabilization phenomenon, namely the availability of electrons and holes that allow for vacancy creation/annihilation, is elucidated in this paper. Metal-enhanced alterations of the oxygen sublattice in both Al2O3 and ZrO2 of the ZTA composite play a decisive role in accelerating the polymorphic transformation. According to spectroscopic evidences obtained through nanometer-scale analyses, enhanced annihilation of oxygen vacancies triggers polymorphic transformation in ZrO2 near the metal stain, while the overall Al2O3 lattice tends to dehydroxylate by forming oxygen vacancies. A mechanism for chemically driven "reactive metastability" is suggested, which results in accelerating the polymorphic transformation. The Al2O3 matrix is found to play a key-role in the ZrO2 transformation process, with unambiguous confirmation of oxygen and hydrogen transport at the material surface. It is postulated that this transport is mediated by migration of dissociated O and H elements at the surface of the stained transition metal as they become readily available by the thermally activated surrounding.