Papers by Author: Giuseppe Pezzotti

Abstract: With elongation of average human life, problem such as bone embrittlement and osteoporosis call for quick solution and the expectation for artificial biomaterials heightens. Many ceramics widely used as artificial biomaterials are limited by their poor reliability characteristics. A CeO2 stabilized tetragonal zirconia polycrystalline (Ce-TZP) matrix incorporating nanometer sized Al2O3 particles within the zirconia grains (Ce-TZP/Al2O3) was recently developed. This material experienced significant improvements in both fracture toughness and strength above the standard mechanical performance of monolithic zirconia. In this paper, we performed a macro/microscopic fracture mechanics assessment of this developed Ce-TZP/Al2O3 nanocomposite, in comparison with a 3 mol% Y-TZP according to advanced in situ confocal Raman spectroscopy techniques.

Abstract: Alumina matrix composite (AMC) has been widely used for artificial hip and knee joints because of its phase stability in human body and its excellent wear resistance. The excellent mechanical properties of strength and fracture toughness of zirconia materials are well known to be closely related to stress-induced transformation from the tetragonal to the monoclinic phase, which is accompanied with 4% volume increase of the zirconia crystal cell. However, it is also to be considered that the material is prone to low temperature aging degradation (LTAD) under hydrothermal environment, like in the human body. This LTAD is influenced by the tetragonal to the monoclinic (t-m) phase transformation. T-m transformation also induces the formation of microcracks at the material surface, and an increase in surface. Microcracking leads to a decrease of mechanical properties, and could explain the failure of implants after some years in vivo [1, 2] .Therefore, it is very important to study how to prevent phase transformation in zirconia components. Transformed monoclinic zirconia percentage can be experimentally measured by Raman spectroscopy and the residual stress distribution, which is related to phase transformation, can be determined by a non-destructive piezo-spectroscopic analysis. In this paper, we attempted to evaluate it from both stress and mechanical properties points of view by confocal Raman and fluorescence spectroscopy.

Abstract: As for zirconia toughened alumina (ZTA) with various ratios of alumina/zirconia, crystal and micro structures, fracture toughness and phase stability were evaluated by X-ray diffraction, Raman spectroscopy, and aging test in hydrothermal environment. The grain size and monoclinic fraction of zirconia phase and residual stress in alumina matrix changed as a function of zirconia content. The ZTA showed higher fracture toughness than conventional alumina. The fracture toughness of ZTA was highest at which the content of tetragonal zirconia was maximum. The monoclinic fraction of ZTA did not increase even after aging test at 121°C for 150 hr. This study indicates that the optimization of tetragonal zirconia content is essential for achieving higher fracture toughness of ZTA. The ZTA with phase stability as well as with fracture toughness is expected as bearing materials which could extend lifetime of artificial joints in clinical use.

Abstract: Phase stability of 3 mol% yttria stabilized polycrystalline zirconia ceramics (3Y-TZP) was evaluated by aging test in water vapor environment and Raman spectroscopic technique. In our previous study, it has been confirmed that phase stability was improved by controlling sintering temperature. In this study, we pointed our attention and evaluated the influence of surface conditions related to machining and to heat treatment, thus monitoring the changes in phase transformation fraction and residual stress on the material surface. From the results of aging test, an increase in monoclinic fraction was observed for grinded surfaces as compared with polished surfaces. Samples subjected to heat treatment after machining showed improved phase stability. A Vickers indentation print was also introduced on a 3Y-TZP surface, and the relationship between surface condition and low temperature aging was evaluated in the neighborhood of the print. We found that the residual stress fields induced in phase-transformed areas were enhanced during low temperature aging, and that phase stability was improved by heat treatment.

Abstract: Electron-stimulated (cathodo)luminescence spectroscopy is quantitatively used for obtaining information about the residual stress fields piled up in semiconductor materials and devices during manufacturing. This additional micromechanical output can be added to the microscopic structural/chemical information available in the scanning electron microscope (SEM) by other conventional techniques. Independent of the physical mechanisms behind the luminescence emission, the spectral position of selected cathodoluminescence (CL) bands, observed in both crystalline and glassy materials, is shown to possess high stress sensitivity and thus suitability as a sensor for residual stress assessments. In this paper, we first quantitatively characterize the stress dependence of typical CL bands observed in ceramic and glassy semiconductor materials. Then, based on this knowledge, applications are shown of two-dimensional residual stress mapping on the nano-meter scale in electronic devices involving atomically sharp interfaces, according to an optimized automatic procedure performed into a field-emission gun (FEG-)SEM.

Abstract: To improve mechanical properties of mullite, a mullite-Al2O3\mullite laminate composite was prepared. Lamination generates residual stresses within the structure, measured by piezospectroscopy. A preliminary and complete piezo-spectroscopic characterization of the Al2O3\mullite system was carried out. A method to determine the concentration of Al2O3 in the composite by Raman spectrum was proposed and used to assess the composition of the laminated structure along the cross section. The experimental results evidenced a gradual change of composition and residual stress state between the two layer.

Abstract: The piezo-spectroscopic (PS) effect, which may be defined as the shift in wavelength of a spectroscopic transition in a solid in response to an applied strain or stress, may occur both in crystalline and in amorphous structures, regardless of the particular spectroscopic transition involved (e.g., luminescence or Raman spectrum), and independent of the specific mechanism of luminescence emission (i.e., including spectra generated from substitutional impurities, optically active point defects, etc.). The PS effect can be monitored on electro-stimulated spectra when the scale on which the needed characterization lie is of a nanometer length. This effect, being a physical property of the studied material, should be calibrated case by case. The high scanning speed (and computer control) of the electron beam, which can be easily obtained with scan coils, is unsurpassed. Since the most recently developed optoelectronic devices have active areas of submicron dimensions and many of them less than 100 nm, no obvious choice is left but urgently developing an electro-stimulated probe for nano-scale residual stress assessments. In this paper, we show the feasibility of nano-scale stress assessments in the neighborhood of the tip of a crack propagating in GaN, selected as a paradigm semiconductor material.

Abstract: Zirconia ceramics have been widely used for new generation of bearing materials in biomedical applications. In this context, a zirconia-matrix, stabilized with cerium oxide and dispersed with fine alumina particles (Ce-TZP/Al2O3 nanocomposite) was recently developed and this material experienced significant improvements in both fracture toughness and strength above the standard mechanical performance of monolithic zirconia. In this paper, we used confocal Raman spectroscopy to provide quantitative assessments with high spatial resolution of phase structure and residual stress fields developed in the Ce-TZP/Al2O3 nanocomposite. According to confocal Raman spectroscopy, we have directly visualized patterns of phase transformation and residual stress fields stored on the very surface of the material around an indentation print. These spectroscopic assessments may open a perspective in understanding the micromechanical behavior of the Ce-TZP/Al2O3 nanocomposite when subjected to local surface impingement and shocks.

Abstract: The kinetics of tetragonal-to-monoclinic phase transformation (t→m transformation) in the earlier generation zirconia femoral heads was evaluated by X-ray diffractometry, laser microscopy and Raman microprobe spectroscopy. From previous results of hip-simulator study, it was confirmed that phase transformation on the surface of zirconia femoral heads had little influence on wear rate of UHMWPE sockets, and in some zirconia femoral heads, only a slight increase in monoclinic fraction was observed during hip-simulator test. In this study, we suggest that the models of phase transformation progress during tests in hip-simulator and aging tests are different based on both laser microscopic and Raman/fluorescence spectroscopic observation. Besides this finding, this study shows that Raman spectroscopy is a useful technique for the evaluation of the kinetics of phase transformation in femoral heads after both in vitro and in vivo environmental exposure.

Abstract: Zirconia ceramics were introduced in the seventhies for use as structural biomaterials after laboratory tests and simulator studies. However, nowadays concerns remain about their reliability in vivo, despite published clinical studies have already established the safety and the good tribological performance of these materials. It is still unclear what level of reliability can be achieved in ceramic biomaterials and how much their toughness level can be enhanced by microstructural design. The polycrystalline nature of ceramic materials may make both the observed properties and performance very scattered. In particular, the grain size and other microstructural features likely play a fundamental role in the mechanical behavior of the material. In this paper, we propose a set of fracture mechanics assessments, aimed to establish the quantitative amount of toughness achievable in a zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite), and in situ confocal Raman spectroscopy to visualize toughening mechanisms, including polymorph transformation and residual stress fields stored around the crack path.