Key Engineering Materials Vols. 309-311

Abstract: A new test method for the determination of the static burst strength of ceramic hip joint heads is described. The common test method is the test with an axial direction of the compressive force. In the following publication a finite element analysis of the stress distribution in ceramic heads under angular loading is shown. The results concerning the location of the highest hoop and tensile stresses are compared with the results of experimental tests for the determination of the static burst load of ceramic heads under a load direction of 25°.

Abstract: The wettability behavior of orthopaedic materials influences the fluid film layer that affects both the friction and wear of the articulating surfaces in total joint arthroplasty. This study examined the wettability of various orthopaedic bearing materials such as alumina, zirconia, cobalt chrome (CoCr), and oxidized zirconium (OxZr). Diamond-like carbon (DLC) coating on CoCr was also examined. Additionally, the effect of radius of curvature was examined using OxZr femoral heads of various diameters. The contact angle of the liquid droplet on the surface of the material was measured using a optical contact angle method. Both water and bovine serum with 20 g/L protein concentration were used during testing, with a droplet size of 0.25 -L. The droplet was dispensed from an automated syringe and brought into contact with the sample surface. The contact angle was then measured by fitting polynomial curves to the sample surface and drop geometry. Ten individual drops were analyzed on each test component, with at least three test components for each material. There were no differences in contact angles with changing head size or when using serum compared to water. The alumina, OxZr, and zirconia femoral heads all exhibited a similar contact angle, while CoCr and DLC showed significantly greater contact angles. The smaller contact angles for the oxide ceramic surfaces indicate that they tend to be more wettable than the metals, which may help explain their lower friction and superior adhesive wear performance.

Abstract: A non-destructive assessment of phase transformation and residual stress is presented for a 3 mol.% Y2O3 added ZrO2 ceramic using Raman microprobe spectroscopy. Low CIP pressure has been selected in the sample procedure to increase a potential to transform ZrO2. Aging tests were made and the transformation depth and residual stresses caused by transformation were evaluated by Raman spectroscopy A Raman microprobe technique using a visible wavelength laser coupled with a confocal optical device may enable one to retrieve spatially resolved information along the material subsurface. To demonstrate the potentiality of the confocal technique, aging of a ZrO2 sample has been made in autoclave and phase transformation gradually promoted from the surface towards the sub-surface of the sample (up to ～60 µm, in a sample autoclaved 168 h). Then, a quantitative spatially resolved assessment was attempted on these samples from their surface. The confocal information from the subsurface was compared with results of Raman spectroscopy collected from a cross-section. Accordingly, a quantitative equation was proposed, which allows the quantitative assessment of the thickness of the surface layer, which underwent phase transformation in ZrO2 ceramics, according to in-depth non-destructive assessments.

Abstract: Structural reliability, biocompatibility and bioinertness are fundamental prerequisites for bioceramics used in artificial hip and knee joints. Among structural properties, superior fracture toughness is necessary for guaranteeing high reliability during implantation lifetime. Bioinert ceramics employed in artificial joints are mainly limited to alumina and zirconia materials. In this paper, the critical crack-tip stress intensity factor, KI0, and the tetragonal-to-monoclinic phase-transformation behavior of a 3 mol % Y2O3-doped tetragonal ZrO2 polycrystals (3Y-TZP) were studied as a function of grain size. 3Y-TZP’s with four different grain sizes were prepared and the size and morphology of the monoclinic transformation zone generated around the tip of an indentation crack were analyzed by quantitative Raman microprobe spectroscopy. The stress intensity factor, KI0, was evaluated by the crack opening displacement (COD) method using a recently proposed equation for calculating the compliance of an indentation crack.

Abstract: A survey of confocal Raman/fluorescence microprobe spectroscopic techniques is presented with emphasis placed on surface analysis of artificial hip joints. Suitable instrumental configurations are first explained in some details in order to describe the versatility of the spectroscopic microprobes to biomedical materials analyses. Then, these notions, which represent the foundation for structural and mechanical analyses of joint surfaces, are applied to selected cases of paramount importance in hip arthroplasty.

Abstract: The electro-stimulated luminescence spectrum of a rare-earth ion added to zirconia (ZrO2) lattice was investigated with the aim of using it as a sensor for nano-scale stress (fluorescence piezo-spectroscopy) and phase transformation assessments in a field emission scanning electron microscope (FE-SEM). In this paper, the selected rare-earth fluorescent ion Eu, added to ZrO2 as a raw oxide powder (Eu2O3) before sintering (in the amount of 1.0 wt. %). Spectroscopic results indicated that the spectral shift of some fluorescent band of the selected rare-earth ion was sensitive to residual stress and that the electron-stimulated spectra of Eu2O3-doped ZrO2 in its tetragonal and monoclinic polymorphs were different to each other. Based on these findings, the luminescent substance can be useful as a “stress and phase transformation sensor”, in order to clarify the elementary mechanisms behind synthetic ZrO2.

Abstract: Yttria stabilized tetragonal zirconia polycrystals (Y-TZP) have been applied to dental crown and bridges. Whereas, to further improve its mechanical strength, the zirconia/alumina nano-composite stabilized with cerium oxide (Ce-TZP/Al2O3 nano-composite) was developed. In the present study, biaxial flexure strength, fracture toughness and hardness were determined before and after soaking in water-based conditions and the possibility of application to all ceramic dental restorations was discussed. In comparison to Y-TZP, Ce-TZP/Al2O3 nano-composite has quite high flexure strength and fracture toughness along with satisfied durability for LTAD in various water-based conditions encountered in dentistry. Therefore, it is concluded that the nano-composite can be safely applied to dental restoratives such as all-ceramic bridges.

Abstract: A hip-joint femoral head, made of alumina matrix composite, has been evaluated with respect to its surface degradation upon increasing elongation time in water vapor environment. A microscopic evaluation of surface degradation phenomena was obtained according to laser microprobe Raman and fluorescence spectroscopies. According to a confocal configuration of the optical probe, the spectroscopic assessments were performed in volumes limited to the very neighborhood of the material surface, thus minimizing the effect of the sub-surface, which was less affected by environmental degradation. Two main features were observed: (i) significant transformation of zirconia occurred in moist environment from the tetragonal to the monoclinic polymorph; such an environmentally induced phase transformation, conspicuously increased the fraction of monoclinic polymorph (i.e., ≅ 18 vol.% in average) already present in the as-received femoral head; (ii) the equilibrium residual stress field stored at the joint surface changed from a tensile field in the as-received material to a slightly compressive stress field after several hours aging in moist atmosphere and, after exposures >50 h to an increasingly tensile stress state. A residual stress field of tensile nature in the joint surface may trigger wear degradation in the femoral head in presence of microscopic (local) weight impingements due to micro-separation and third-body wear phenomena.

Abstract: Phase stability of 3Y-TZP was evaluated by acceleration test in water vapor environment and Raman spectroscopy. We found that the phase stability was affected not only by grain size but also by residual stress field, which could be quantitatively measured by Raman piezo-spectroscopy and controlled by sintering temperature. This study shows that Raman spectroscopy is a very useful technique for non-destructive analysis of 3Y-TZP.

Abstract: The effect of cation dopant on tetragonal to monoclinic (t-m) phase transformation of zirconia is examined, which causes the degradation of zirconia in vivo. The transformation behavior is much different depending of the dopant. The transformation is promoted when the pentavalent element is added. In contrast, the transformation is suppressed when the bivalent element is added. In case the tetravalent element is added, the transformation is promoted if the ionic radius of the element is larger than that of zirconium, and that is suppressed if the element with smaller ionic radius is added. The valence and the ionic radius of dopant must be important factors for the phase stability of zirconia.