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. - Graphical abstract: Metal-enhanced alterations of the oxygen sublattice in both Al₂O₃ and ZrO₂ of the ZTA composite play a decisive role in the polymorphic transformation. According to spectroscopic evidence obtained through nanometer-scale analyses, enhanced annihilation of oxygen vacancies in Al₂O₃ and ZrO₂ occurs near the metal stain, while the hydrothermal environment pushes the overall Al₂O₃ lattice to dehydroxylate and form new oxygen vacancies. A mechanism for this chemically driven "reactive metastability" is suggested which results in accelerating the polymorphic transformation. - Highlights: • The presence of transition metal stains causes oxygen vacancy annihilation and destabilizes ZTA. • New algorithms were introduced for assessing surface off-stoichiometry in ZTA. • Details of surface chemistry are mandatory in assessing ceramic bearing biomaterials.