Papers by Author: Omer Van der Biest

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: 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).

Abstract: From an environmental, safety and economic perspective water should be the solvent of choice for electrophoretic deposition under industrial circumstances. However, because of the electrolytic decomposition of water under the influence of direct current, the majority of EPD is carried out in non-aqueous solvents. In this work, experiments prove that deposits can be obtained from aqueous alumina suspensions while avoiding electrolysis of the medium by using unbalanced alternating current fields [1]. In addition it is shown that the formed deposits have a green density which is intrinsically higher than those formed by traditional DC EPD from ethanol based suspensions. A theoretical basis for both electrophoretic deposition by means of unbalanced alternating fields and the higher density of deposits formed by application of such fields is provided.

Abstract: Deposition experiments in a Hull cell showed that high conductivity suspensions yield uniform deposits while low conductivity suspensions result in non-uniform deposits. This difference in deposition behavior is related to the resistance increase of the deposit during EPD. Impedance measurements during EPD showed that the ratio of the deposit resistance to the suspension resistance increases much more for high than for the low conductivity suspensions. They also showed that the total resistance of the EPD cell dropped almost to the suspension resistance after the electric field was turned off. This means that the deposit has no inherent resistance, but that its resistance during polarization is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. The change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.

Abstract: Electrophoretic deposition (EPD) allows the fabrication of ceramic coatings at lower cost and higher speed than most other deposition techniques. The processing consists of powder deposition from a suspension under the influence of an electric field and subsequent consolidation of the coating by sintering. Adherent zirconia coatings with coating thicknesses up to 0.1 mm were obtained from different suspensions, one methyl-ethyl-ketone and the other ethanol based. The standard sintering temperature is 1200°C, which easily may damage or change the substrate and also means high production costs. In order to reduce the sintering temperature, suspensions with the addition of ZrN were investigated. Due to reaction bonding, sintering in air at a remarkable low temperature of 1000°C was successful. The elastic modulus of the EPD coatings has been derived from impulse excitation experiments and the thermal conductivity from laser flash analysis. The elastic modulus was about 22 GPa and the thermal conductivity between 0.4 and 0.6 W/(m•K) at room temperature, both decreasing slightly with temperature. Especially the exceptionally low thermal conductivity makes EPD coatings a promising candidate for thermal barrier coatings.

Abstract: Graded Al2O3/Y-ZrO2 ceramics are developed to receive a construction material combining favourable properties of both constituent components, alumina (low wear rate, high hardness) and zirconia (high strength and toughness). The high performance of this material can be reached by optimising the internal residual stress distribution resulting mainly from phase specific stresses after cooling from the sintering temperature. For this purpose, non-destructive neutron diffraction mapping of residual stresses has been employed. However, the application of the conventional method does not provide straightforward results on macroscopic residual stresses. This experimental technique uses the crystal lattice plane as a built in microscopic strain gauge and the measured quantities are then lattice strains detected in each constituent phase separately. Based on these experimental resources, the paper proposes a procedure of separation of the residual macroscopic stress from phase specific stresses. The application of the presented method is demonstrated on functionally graded materials (FGM) prepared by electrophoretic deposition (EPD).

Abstract: At present, cobalt is the most commonly used binder material in tungsten carbide based hardmetals. Current research on sliding wear performance of these cemented carbides, however, reveals promising results for nickel binder as well. Test samples of WC-Co and WC-Ni hardmetals have been machined and surface finished by wire-EDM and grinding. From comparative dry sliding pin-on-plate experiments on wire-EDM’ed, ground and polished grades, correlations are derived between wear volume loss and friction on the one hand and contact pressure, sliding distance, binder phase and microstructure on the other hand. The lowest wear levels are encountered with polished cemented carbides. The EDM induced surface modification turns out to deteriorate wear resistance, especially during the running-in stage of sliding. These findings are in agreement with Xray diffraction measurements of the residual stress level in the WC phase.

Abstract: WC-Co based cemented carbides are widely used in technical applications, in which they are exposed to complex thermo-mechanical loadings. Previous research work has demonstrated that these materials exhibit a lifetime-limiting fatigue sensitivity at room temperature. This investigation is focused on the influence of heating as a result of friction of pin-on-plate tribocouples. WC-Co samples have been manufactured and surface finished by wire-EDM and grinding. Reciprocative dry friction experiments are performed under various loading conditions and sliding velocities. Correlations are established between the bulk temperature rise of the test samples and the imposed test conditions, material properties, surface conditions and coefficient of friction. Topographies and cross-section views of the tested samples were examined by SEM, revealing temperature dependent wear mechanisms, including binder phase modification.

Abstract: Zirconia-based ceramic composites such as ZrO2-WC, ZrO2-TiCN and ZrO2-TiN, are suitable for wire-EDM, due to their sufficiently electro-conductive secondary phases inside. Thus, the material removal technique of EDM to shape complex geometry materials economically and with high accuracy, irrespective of mechanical properties, could be successfully employed on these ceramics. Samples of these ZrO2-based ceramics were developed in laboratory and manufactured and surface finished by wire-EDM. Reciprocative dry sliding pin-on-plate experiments revealed that the ZrO2-WC composite exhibits better tribological characteristics in comparison with the composites of ZrO2-TiCN and ZrO2-TiN. Furthermore, topographies and cross-sectional views of worn surfaces were analyzed by SEM, revealing that the secondary phase inside the investigated composites governs the wear mechanism.