Kinetics and the role of off-stoichiometry in the environmentally driven phase transformation of commercially available zirconia femoral heads

The low-temperature polymorphic transformation behavior of two types of commercially available femoral head, both made of 3 mol.% Y2O3-stabilized tetragonal ZrO2 polycrystals (3Y-TZP), was examined by in vitro experiments. Both materials contained a small amount (0.25 wt.%) of Al2O3, but they differed slightly in their SiO2 impurity content, in the morphology and crystallinity of the dispersed Al2O3 phase, and in grain size. In vitro experiments were conducted in a water-vapor environment at temperatures in the range 90–134 °C and for periods of time up to 500 h. Despite the materials having the same nominal composition, quite different behaviors were found in the hydrothermal environment for the two types of femoral head investigated. A phenomenological description of the kinetics of monoclinic nuclei formation/growth led to the experimental determination of activation energy values for the environmentally driven polymorphic transformation. From the material physics viewpoint, cathodoluminescence spectroscopy enabled us to rationalize the role of surface stoichiometry on the mechanisms leading to polymorphic transformation. Spectroscopic experiments unveiled some new relevant aspects of surface off-stoichiometry, which lie behind the different phase transformation kinetics experienced by the investigated femoral heads.