Key Engineering Materials Vols. 361-363

Abstract: The aim of the examinations was to produce various dispersion ceramics in the range of 0 -100% Al2O3, and to determine the strengths and selected characteristics such as density, fracture toughness KIC, thermal conductivity, elastic modulus and hardness. Furthermore, the phase composition of all samples was examined by means of X-ray diffraction following Rietveld refinement. The examinations showed that the material parameters can be specifically influenced by the corresponding adjustment of the ZrO2 portion. The utilisation of the effects of the dispersion and transformation reinforcement can be exploited particularly with the mixture 20 % Al2O3-80 ZrO2(Y-TZP). Significant differences between the two forming procedures – slip casting and pressing – could not be established.

Abstract: Alumina and zirconia ceramics have been successfully used as materials for joint endoprostheses. The present study has been aimed at combining the advantages of the two singlephase materials, Al2O3 and ZrO2 while avoiding their disadvantages, by producing a composite ceramic. Within the scope of a study of an ATZ ceramic we tested the influence of aging by a hydrothermal treatment on strength, phase composition and surface properties. We found that the hydrothermal treatment of the ATZ ceramic did not significantly influence these parameters, unlike those of a single-phase ZrO2 ceramic.

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: Wear of hip implants is a significant problem for the life expectancy of artificial joints. By using alumina ceramic on ceramic couplings the wear can be decreased. But for further improvement of the safety of THR the aim is the development of new ceramic materials. For orthopaedic applications an Alumina Toughened Zirconia Ceramic ATZ (80% ZrO2-20%Al2O3) and a Zirconia Toughened Alumina ZTA (25% ZrO2-75%Al2O3) were tested regarding their tribological behaviour by means of hip simulator testing after hydrothermal treatment. The absolute wear amount for the aged samples after 5 million cycles is slightly increased on a very low level, but even less wear than for common alumina pairings. In consideration of these excellent results both dispersion ceramics are highly suitable for long term applications.

Abstract: Carbon nanotubes could avoid the crack propagation and enhance the toughness of the ceramic material used for prostheses applications. So nanozirconia partially coated carbon nanotubes have been obtained via hydrothermal synthesis of zirconia nanoparticles in presence of multiwall carbon nanotubes. The as covered nanotubes should have a better wettability in the ceramic matrix and improve the dispersion of the CNTs in the nanocomposite, which results in a new ceramic biomaterial with a longer lifetime and better reliability. The obtained product has been structurally characterized by several techniques such as FTIR, XRD, SEM, AFM, EELS, XPS and TGA. The citotoxicity of the sintered product was studied by the change in the pH and ICP-AES in in-vitro biocompatibility tests.

Abstract: The effect of sintering condition, sandblasting and heat treatment on biaxial flexure strengths of the zirconia/ alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite, referred to NANOZR) was evaluated in comparison to that of yttria stabilized tetragonal zirconia polycrystals (Y-TZP). The disc-shaped specimens of NANOZR and Y-TZP were cut from the full-sintered or middle-sintered ones. The discs cut from the middle-sintered ones were finally sintered at the same temperature for the full-sintered one. These four kinds of disc were treated in various conditions combined with the sandblasting, the heat treatment, and the storage. The biaxial flexure strength of both middle- and full-sintered Y-TZP decreased with the autoclaving, whereas those of both NANOZR did not change with it. The monoclinic content of both the materials increased with the sandblasting and decreased with the heat treatment. Regardless of the sintering condition, the monoclinic content of the Y-TZP dramatically increased with the autoclaving and those of NANOZR remarkably increased with the sandblasting. Regardless of the different surface roughness, the biaxial flexure strengths of both materials strongly depended on the content of monoclinic ZrO2 on the surface.

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: The reactivity of CaTi4(PO4)6 (CTP) with alumina and yttria-stabilized zirconia (Y-TZP) ceramics was studied. CTP powder was synthesized and composites with commercial alumina or zirconia matrices containing 10 wt% of CTP were prepared. They were sintered at different temperatures and characterized using XRD, SEM, and EDX analyses. The results showed that the alumina/CTP and Y-TZP/CTP composites start to react below 1000 °C. In the alumina/CTP composite the first reaction product, detected at 970 °C, was AlPO4. At temperatures above 1280 °C TiO2 and CaTiO3 were also formed and no CTP peaks could be detected using XRD analysis. The composite sintered at 1500 °C consisted of Al2O3 matrix, AlPO4, TiO2, CaTiO3 and Al2TiO5. The reaction products formed in the Y-TZP/CTP composite at 970 °C were TiO2 and Ca2Zr7O16. At higher sintering temperatures, 1280 °C and above, CTP was no longer present, Ca2Zr7O16 decomposed, forming CaO2 and ZrO2, and Y2O3 was consumed to form YPO4. Consequently, upon cooling to room temperature the matrix phase transformed to monoclinic ZrO2. Based on these results it can be concluded that CTP is not a suitable bioactive second phase for the fabrication of CTP composites with alumina or zirconia matrices.

Abstract: Ceramics have been increasingly used in orthopaedics during the last 30 years. Their biological inertness, high hardness and good mechanical strength make them excellent candidates for components such as femoral heads and acetabular cup in Total Hip Replacement prostheses. Currently used bio-inert ceramics – alumina and zirconia – give good clinical results, especially compared to metal – polymer couplings. However, they are subjected to severe biological, tribological and mechanical solicitations during more than fifteen years for the most successful prostheses. They answer these solicitations by presenting specific degradation mechanisms. We will thus examine the phenomena that can account for the long-term behaviour of zirconia components (heads and cups) in THR prostheses.

Abstract: Fully dense ZrO2-TiN composites containing 1.75-2 mol %Y2O3, 1 mol% Y2O3 and 1 mol% Nd2O3 stabilizers, small amounts of Al2O3, and electrical conductive TiN particles (40-70 vol%) have been produced by hot pressing and spark plasma sintering at 1550°C. Although the intrinsic hardness of TiN (1400 kg/mm²) is higher than that of t-ZrO2 (1200 kg/mm²), the decreasing hardness trend can be attributed to the larger TiN grain size with the higher TiN content. Since TiN is more brittle, the fracture toughness decreases with increasing TiN content. Transformation toughening has been attributed as the main toughening mechanism as a result of fracture toughness decreasing with the transformability. Spark Plasma sintering temperature was too high for mechanical properties and hydrothermal stability of the mixed stabilized composites. The transformability decreases so hydrothermal stability increases linearly with increasing TiN content as a result of smaller volume fraction of t-ZrO2 grains becoming susceptible to hydrothermal transformation due to the shielding effect of the present TiN grains.