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Recent studies on zirconia-toughened alumina (ZTA) evidenced that in vivo aged implants display a much higher monoclinic zirconia content than expected from in vitro simulations by autoclaving. At the moment, there is no agreement on the source of this discrepancy: Some research groups ascribe it to the effect of mechanical impact shocks, which are generally not implemented in standard in vitro aging or hip walking simulators. Others invoke the effect of metal transfer, which should trigger an autocatalytic reaction in the body fluid environment, accelerating the kinetics of tetragonal-to-monoclinic transformation in vivo. Extrapolations of the aging kinetics from high (autoclave) to in vivo temperature are also often disputed. Last, Raman spectroscopy is by far the preferred method to quantify the amount of monoclinically transformed zirconia. There are, however, many sources of errors that may negatively affect Raman results, meaning that the final interpretation might be flawed. In this work, we applied Raman spectroscopy to determine the monoclinic content in as-received and in vitro aged ZTA hip joint implants, and in one long-term retrieval study. We calculated the monoclinic content with the most used equations in the literature and compared it with the results of X-ray diffraction obtained on a similar probe depth. Our results show, contrary to many previous studies, that the long-term surface stability of ZTA ceramics is preserved. This suggests that the Raman technique does not offer consistent and unique results for the analysis of surface degradation. Moreover, we discuss here that tetragonal-to-monoclinic transformation is also necessary to limit contact damage and wear stripe extension. Thus, the surface metastability of zirconia-containing ceramics may be a non-issue. [ABSTRACT FROM AUTHOR]

This study is aimed at establishing a method of polarized/confocal Raman spectroscopy capable of quantitatively assessing crystallographic orientation in wurtzitic GaN with a micron-scale resolution. First, Raman selection rules are explicitly put forward from a theoretical viewpoint in their complete form; then, experimentally retrieved intensities of the Raman signal as a function of Euler angles are fitted to the obtained theoretical dependencies in order to quantify a set of Raman tensor elements using experiments on known crystallographic planes of a wurtzitic GaN single-crystal. According to the above two procedures, a spectroscopic algorithm, incorporating the use of Raman tensor elements and Euler angles in tandem, becomes available for estimating unknown crystallographic orientations. As an application of the developed method, a confocal Raman probe was used to non-destructively unfold the relative orientation of a wurtzitic GaN epilayer with respect to (0001)-oriented sapphire substrate. The microscopic distribution of tilt/twist angles in space for the GaN lattice could be measured in a bulk region of the sample with no cut or other sample manipulations being necessary. Polarized/confocal Raman spectroscopy provides a practical and convenient tool for characterizing on the micron scale the interfacial disorder that affects energy transport phenomena at GaN/substrate interfaces. [ABSTRACT FROM AUTHOR]

The fracture behavior of a zirconia-toughened alumina-matrix composite (added with small amounts of mixed oxides) for ceramic hip joint prostheses has been evaluated with emphasis placed on the effect of environmental surface degradation in moist environment. Accelerated aging tests were performed up to 300 h in an autoclave operating at 121°C (under 0.1 MPa pressure) in vapor environment, which represents a quite severe environmental testing condition. Besides conventional fracture mechanics characterizations, including different types of fracture toughness test, microscopic insight into the effect of environmental surface degradation on toughness could be obtained according to Raman and fluorescence microprobe spectroscopy. The main outcomes of this study were as follows: (i) after 10-h autoclaving (according to ISO standard recommendation) no significant change of monoclinic volume fraction and fracture toughness could be detected; (ii) after very long exposure time (300 h) the monoclinic phase content increased and the surface fracture toughness decreased by approximately 30%, although it was still above the toughness level of pure alumina; and (iii) the bulk toughness was unaffected by autoclave exposure, independent of exposure time elongation. [ABSTRACT FROM AUTHOR]