Chemically driven tetragonal-to-monoclinic polymorphic transformation in retrieved ZTA femoral heads from dual mobility hip implants.

Two short-term (two and nine months) retrieved zirconia-toughened alumina (ZTA) femoral heads and nine pristine femoral heads from the same manufacturer have been investigated with respect to their surface stability by means of confocal Raman spectroscopy. Quantitative estimations of monoclinic volume fraction have been carried out in both non-wear and main wear zones of the retrieved heads, which invariantly showed high volume fractions of monoclinic polymorph. In-depth (sub-surface) profiles, non-destructively collected in the main wear zones with the Raman probe in confocal configuration, indeed confirmed that polymorphic transformation was extended down to 100µm below the bearing surface of the femoral heads. Acceleration of tetragonal-to-monoclinic transformation rate leads to unexpectedly high fractions of monoclinic phase within very short-term in-vivo exposures. Phase transformation in-vivo is much more marked than what one could actually predict according to simply simulating a hydrothermal environment in-vitro and could not be simply ascribed to the mechanical stress fields generated during normal service at the bearing surface. Instead, the chemical consequences of metal contamination on the ZTA femoral head surface are shown to play the most detrimental role in phase destabilization.

Oxygen hole states in zirconia lattices: quantitative aspects of their cathodoluminescence emission.

Systematic assessments of cathodoluminescence (CL) spectroscopy, Raman spectroscopy (RS), and X-ray diffraction (XRD) are presented for pure zirconia and for a series of Y-doped zirconia powders (henceforth, simply referred to as undoped ZrO2 and YSZ powders, respectively) synthesized according to a coprecipitation method of Zr and Y chlorides. Emphasis is placed here on spectral emissions related to oxygen-vacancy sites (i.e., oxygen hole states) equally detected from undoped and Y-doped ZrO2 samples, either as intrinsic defects or, extrinsically induced, by means of cathodoluminescence. Most counterintuitively, the undoped ZrO2 sample (i.e., the one with presumably the lowest amount of oxygen vacancies) experienced the strongest CL emission. A progressive "quenching" effect on CL emission with increasing the fraction of Y(3+) dopant could also be observed because the intrinsic vacancies present in the undoped lattice are the most efficient since they can trap two electrons to gain electrical neutrality. However, as soon as Y(3+) ions are introduced in the system, those intrinsic vacancies migrate to Y-sites in next-nearest-neighbor locations, namely in a less efficient lattice location. This phenomenon is tentatively referred to as "delocalization" of vacancy sites. Moreover, the fact that Y-doped zirconia series presents quite similar CL spectra compared to the undoped zirconia could be an evidence that the radiative centers of undoped and Y-doped ZrO2 are basically the same. A fitting procedure has been made aiming to give a rational description of the variation of the spectra morphology, and a parameter able to describe the monoclinic to tetragonal phase transformation has been found. This parameter and the overall set of CL data enabled us to quantitatively assess polymorphic phase fractions by CL spectroscopy in the scanning electron microscope.