Papers by Author: Jef Vleugels

Abstract: Magnesium and magnesium alloys are gaining considerable attention for use in biomedical applications due to their capability to completely resorb in the human body without noticeable side effects. For structural biomedical applications however, the resorption rate is too large. In order to decrease this rate researchers are investigating magnesium alloys with an increased corrosion resistance and/or biodegradable coatings, such as dense protein layers, which retard the resorption.In this work, we demonstrate the electrophoretic deposition of Bovine Serum Albumin (BSA) directly onto pure magnesium substrates using unbalanced alternating fields (AC-EPD). The effect of the obtained coatings on the corrosion behavior of the substrates was evaluated by potentiodynamic polarization. The results show that an albumin layer deposited by AC-EPD from a 50/50 ethanol/H₂O medium significantly reduces the corrosion rate.

Abstract: A new powder metallurgical processing route for porous Ti coatings on Ti-6Al-4V substrates based on the electrophoretic deposition (EPD) of TiH₂ suspensions is presented. After dehydrogenation and sintering in vacuum, coatings with a fully interconnected porosity (up to 51%, interconnective pore channels (IPC) of 2-50 µm) and high adhesion strength (up to 47 MPa) are obtained. Further evaluation of these coatings for potential use in biomedical implants shows that EPD Ti coatings are significantly less prone to bacterial adhesion compared to state-of-the-art vacuum plasma sprayed (VPS) coatings, while still allowing substantial bone ingrowth. Using EPD, the coating process can easily be transferred to complex-shaped implant components.

Abstract: Various solid state or ‘meltless’ recycling techniques have recently been developed for light metal scrap in form of chips. Main objective of all approaches is to bypass the need for remelting in order to reduce the overall required energy, and to avoid the material losses that occur during this step. Within this paper, the use of Spark Plasma Sintering (SPS) is proposed as a novel solid state recycling/welding technique for sheet metal scrap. Aluminium 5182 alloy scrap, derived from sheet metal, was successfully consolidated into a fully dense billet via SPS. The use of pulsed electric current heating, in temperatures well below the alloy melting point, combined with mechanical pressure, enchased the densification process resulting into a void-less material. The recycled SPS sample was fully densified and microstructural investigation has been performed in order to confirm effective oxide film breakage. The results illustrate the effectiveness of SPS in aluminium scrap consolidation, also in form of sheet scrap, providing additional means in solid state recycling. The involved mechanisms that contribute to oxide film fracture and scrap consolidation in SPS are being discussed.Keywords: Aluminium, recycling, spark plasma sintering (SPS)

Abstract: The influence of the addition of 0.25, 2 and 5 wt.% alumina on the mechanical properties and low temperature degradation (LTD) behaviour of 3 mol% yttria-coated ZrO₂ powder based Y-TZP ceramics was investigated, and compared to commercial powder based co-precipitated 3Y-TZPs with 0-0.25 wt.% Al₂O₃ addition. The ceramics were subjected to accelerated hydrothermal degradation in an autoclave in H₂O at 134°C up to 40 hrs. X-ray diffraction and Raman spectroscopy were used to assess the LTD behaviour. Incorporating the Y₂O₃ stabilizer by means of a coating method resulted in a higher LTD resistance without compromising the higher fracture toughness, compared to the co-precipitation method. Alumina addition did not significantly influence the mechanical properties of all Y-TZPs but significantly increased the LTD resistance of the Y-TZP ceramics. The LTD resistance of 0.25 wt% Al₂O₃ doped TZPs was substantially higher than that of ceramics containing 2 or 5 wt.% Al₂O₃, which had a comparable susceptibility. The highest LTD resistance for the 0.25 wt.% alumina doped ceramics could be correlated to the solubility limit of alumina in zirconia.

Abstract: Electrophoretic deposition is a promising method for the near net shaping of ceramics if deposit damage during removal from the electrode can be prevented. The latter can be achieved by providing a lubricated interface between electrode and deposit. During application of such a lubricant care must be taken that none of the electrode surface details are lost. Hence thins layers which closely represent the original electrode surface are needed. In the present work electrophoretic deposition of alumina powder on a thin layer of ionic liquid applied on polymer electrodes is described. After deposition this ionic liquid layers serves as a shear plane during the deposit removal. The resulting deposits exhibit a smooth surface quality and high green density. Furthermore experiments show that the ionic liquid can be used as a means for producing electrodes with areas at which deposition is locally prevented.

Abstract: The surface chemistry of a suspended particle greatly affects it behavior during electrophoretic deposition. The type and amount of surface groups determines whether the particles can be charged by interaction with the solvent. Furthermore, it is suspected that the surface chemistry plays a prominent role in the mechanisms governing the actual deposition of the particles. In the present work the surface chemistry of as-received and surface modified alumina powder is characterized by means of contact angle measurements and Diffuse Reflectance Infrared Fourier Transform spectroscopy. The wetting is measured using a modified Washburn method which yields quantitative contact angle values. The acid-base and dispersive surface energy components are calculated from these values using the surface tension component theory. Infrared spectroscopy was used to compare the surface groups of the treated and untreated powders and confirm the trends in surface properties as calculated from the contact angles.

Abstract: The recent development of the alternating current electrophoretic deposition (AC-EPD) technique has rendered it possible to deposit material from aqueous suspensions while preventing the electrochemical reactions associated with the application of high voltages on such systems. This does not only allow for more economical and ecological processes but also opens up electrophoretic deposition as a processing technique to a whole range of materials sensitive to either electrochemical reactions or non-aqueous solvents. Living cells can be considered as one class of such materials. In this paper the deposition of two types of bacteria, the Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli, and one type of yeast cells, Saccharomyces cerevisiae, is demonstrated.

Abstract: Pulsed electric current sintering allows densifying most ceramics at high heating and cooling rates within very short times at elevated temperature, allowing to minimise grain growth. In order to fully explore the PECS potential, it is beneficial to flow the current through the powder compact by either using conductive powder or a powder compact that becomes conductive during densification. Although in-situ Joule heating of the powder compact allows very fast heating rates, it does not necessarily result in a homogeneous temperature distribution. The influence of the current flow on densification and the impact of electrical conductivity on the temperature distribution during PECS are illustrated. The PECS technology at present is limited to the fabrication of simple geometrical shapes. Electrical Discharge Machining (EDM) on the contrary allows production of complex shapes, providing the ceramic has a minimum electrical conductivity. Although EDM has no mechanical impact, the thermal impact is high and the EDM parameters should be carefully selected in order to optimise surface quality and component strength. During wire-EDM, the fast and rough initial cut has to be followed by a sequence of lower energy finishing cuts to optimise the surface quality. The case studies presented are B4C-TiB2 ceramics and ZrO2-based composites with electrically conductive phase addition.

Abstract: Recent developments demonstrated that liquid templates in the form of solid particles stabilized emulsions can be used to produce porous materials. The use of such emulsions offers the possibility to control the porous properties over a wide range of pore sizes and porosities for a variety of materials. In addition, the liquid nature of the template enables the formed products to be sintered without a low temperature debinding step. In this work, the electrophoretic deposition (EPD) of these liquid templates for the production of porous alumina is reported. The experimental parameters needed to obtain stable emulsions, their influence on the final porous properties, as well as the influence of the deposition parameters are discussed.

Abstract: The development of texture was studied during electrophoretic deposition in alumina suspensions containing plate shaped alumina particles. The mechanism of platelet orientation during EPD was examined with respect to the influence of the electric field, gravity and hydrodynamic forces. This was realized by using two different deposition cells, with vertically or horizontally positioned deposition electrode. The texture of the green deposit was further enhanced during sintering by templated grain growth in which the platelet shaped alumina particles were growing at the expense of the fine grained matrix. The sharp ‘fiber texture’ obtained after templated grain growth during sintering of the deposit was characterized by means of x-ray diffraction and Electron Backscatter Diffraction (EBSD).