Materials Science Forum Vols. 492-493

Abstract: This study aimed to show that the polymerization contraction of dental light cured resin-based materials, when it was used as adhesives on hard substrate, produces voids at the material/substrate interface. Two liner/base systems, one glass-ionomer, one restorative composite resins and two compomers were cured using mirror-like glass slide as a compliance-free reference substrate. The adhesive surface was analyzed by atomic force microscopy, and the polymerization contraction of bulk material was tested by laser beam scanning method. Nanoperiodic structure of 3D images, section analysis and roughness characteristics (Ra and Rz)) indicated that polymerization contraction produced voids at the interface. In visible light-cured (VLC) materials, the interface porosity decreased when an irradiation pause was carried out during gelation.

Abstract: Practical applications of metal/ceramic joints can be found in the biomedical field regarding the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or in the production of drug delivery systems, biomedical sensors and electrodes. Most of metal/ceramic joints are produced by the active metal brazing technique, which originates a multi-layered interface which should be able of accommodating the abrupt electronic, crystallographic, chemical, mechanical and thermo-mechanical discontinuity that characterize these systems. Additionally, when considering biomedical applications, corrosion resistance becomes of prime importance. In this work, the corrosion resistance of Ti/glass-ceramic interfaces obtained by active metal brazing was evaluated by electrochemical impedance spectroscopy (EIS) tests. The electrochemical behaviour of the interface was monitored, as a function of time, in a simulated physiological solution at room temperature. In order to evaluate the contribution of each layer and galvanic interactions between them, to the degradation mechanism of the interface, individual samples, representative of reaction layers present at the interface, were fabricated and electrochemically tested. Results show that the corrosion behaviour, of the whole interface was strongly influenced by the chemical composition of its constitutive layers. Thus, layers containing high contents of both titanium and silver showed a polarisation resistance increase with the immersion time, as a result of the formation of a thermodynamically stable passive film. On the other hand, the copper rich layer, appears to be the main responsible for the interface degradation. In fact, for high immersion times, an instable passive film is formed and, as a consequence, large amounts of copper are released. Galvanic interactions between the copper and the silver rich layers where also identified.

Abstract: To develop a successful bone fixation device that can also release therapeutic agents such as antibiotics one has to consider mechanical, drug release, and biocompatibility properties. We have used bioabsorbable polymers (PLGA 80/20 or PLDLA 70/30) as the matrix and ciprofloxacin (CF) as antibiotic to develop such an implant. Initial shear strengths of the studied ciprofloxacinreleasing screws were 152 MPa (P/L/DL)LA) and 172 MPa (PLGA). Studied screws retained their mechanical properties for least 12 weeks (P(L/DL)LA) and 9 weeks (PLGA) in vitro at the level that ensures their fixation properties. Pull-out tests indicated that the early version of screws have lower values as compared to controls. CF was found to be released after 44 weeks (P/L/DL)LA) and 23 weeks (PLGA) in vitro. It remained in the range of 0.06 – 8.7 µg/ml/day (for P(L/DL)LA) and 0.6 - 11.6 µg/ml/day (for PLGA) after the jump start. Release CF was demonstrated to significantly inhibit S. epidermides growth, attachment and biofilm formation different than controls. Histology showed no difference from plain polymer screws, except for increased giant cells at the implantation site. Accordingly, SR-P(L/DL)LA and SR-PLGA MF implants were considered appropriate to proceed to pilot clinical application.

Abstract: New functionally graded Al2O3/Y-ZrO2 ceramics are developed as a new material for production of hip prosthesis, especially all-ceramic hip bearings. The used concept of graded ceramics is expected to provide better long-term performance based on improved mechanical properties and lower clinical risks. Mechanical properties are strongly dependent on the distribution of residual stresses resulting mainly from phase specific stresses after cooling from the sintering temperature. However, these stresses could be optimized by an appropriate selection of a concentration profile of constituent phases. The non-destructive neutron diffraction mapping of residual stresses has been used to investigate this problem experimentally. In addition, the smallangle neutron scattering technique was applied to study the porosity in the ceramics as function of the production parameters.

Abstract: The preparation of functionally graded materials requires specially adapted shaping techniques and special attention need to be paid to changes in the amount of shrinkage through the cross-section of the graded material during sintering, to avoid high residual stresses leading to crack formation. In this work we report on the results of investigations of FGMs of Al2O3, ZrO2 and ZTA and their densification as a function of the starting suspensions’ properties (alumina/zirconia ratio, solids content and dispersion state), with the aim of compensating for the different amount of sintering shrinkage of the outer and inner materials in step-graded Al2O3-ZTA composites. The sintering studies revealed that the green density and hence the densification of the slip-cast ceramic parts are almost independent on the solids content, while significant effect was observed for dispersion state of the suspensions. Hence, different amounts of total sintering shrinkage in stepgraded Al2O3-ZTA may be achieved by modification of the zeta-potential by adjusting the pH of well-dispersed suspensions.

Abstract: Complex shaped functionally graded alumina and zirconia based femoral ball-heads for biomedical applications were shaped by electrophoretic deposition (EPD). A composition gradient in alumina and zirconia was engineered to obtain a pure alumina surface region and a homogeneous alumina/zirconia core with intermediate continuously graded regions to generate appropriate thermal residual stresses after sintering. The gradient profiles were designed to obtain maximum compressive surface stresses and minimal tensile stresses in the core of the component to increase the strength and wear properties when compared to pure alumina components.

Abstract: When a surface of a titanium disk was melted in an atmosphere of pure nitrogen using a 3D Micro Welder which was designed by the present authors, the surface was nitrided to a depth of 90 to 260μm depending on the arc current of 6 to 24 A. The concentration of nitrogen in the nitrided layer was approximately 50 mol% at the surface, and the concentration decreased as the distance from the surface increased. A TiN layer was formed at the surface, and beneath the TiN layer, a dual phase layer of TiN and α-Ti was formed. Vickers hardness was approximately 1800 in the TiN layer and it varied from 900 to 200 in the dual phase layer as the distance from the surface increased.

Abstract: Microstructural and stress-corrosion cracking characterization of two glass-based coatings on Ti6Al4V with different SiO2 content (61% and 64% of SiO2) have been investigated in this work. These coatings belong to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system and were obtained using a simple enamelling technique. They will be used as the first layer of the bioactive FGM coating. Microstructural characterization performed in the coatings by SEM shows the separation of the sintered glass particles after acid etching. The XRD integration method shows that the percentage of the crystalline phase (2.4CaO•0.6Na2O•P2O5) due to the partial devitrification is between 3-16 % vol. Mechanical characterization was made using Vickers and Hertzian indentation. Both coatings were sensitive to Vickers indentation subcritical crack growth with longer crack lengths for the smaller SiO2 content. This coating was also more sensitive to stress-corrosion “ring” cracking by Hertzian indentation. These two results are related with the larger residual stresses due to the thermal expansion mismatch.

Abstract: Diopside CaMgSi2O6 is an attractive material because of its multi applications. It was prepared by sintering the mixture at different temperatures (900°C-1300°C). Starting materials were pure SiO2 and dolomite raw materials. In order to improve the properties of sintered samples, such as the density egg white (ovalbemin) has been added into diopside. Furthermore, the effect of P2O5 addition on the sintering of the prepared diopside in the range of 0.5 to 5.0 wt% was studied as a function of the sintering temperature. A density of 96.5% of theoretical has been achieved when 5.0 wt% P2O5 was added, at a sintering temperature of 1225°C, whereas the density of diopside samples, without P2O5 addition was lower than 83.0% of theoretical.