Papers by Author: Jef Vleugels

Abstract: In this study, 2 mol% Y2O3 (2Y), 1 mol% Y2O3+6 mol% CeO2 (1Y6Ce) and 12 mol% CeO2 (12Ce)-doped tetragonal ZrO2 ceramics were made by spark plasma sintering (SPS) for 2 min at 1450°C under a pressure of 60 MPa. The influence of stabilizers on microstructure, phase and mechanical properties of the ZrO2 ceramics was investigated. After sintering, the 2Y and 1Y6Ce were intact, containing full tetragonal ZrO2 phase on the polished cross-sectioned surface, whereas the 12Ce exhibited macro-cracks, corresponding to a large amount of monoclinic ZrO2 phase. Graded microstructure and mechanical properties were observed in the 1Y6Ce, showing a gradually decreased fracture toughness from sample edge towards centre, together with the slight decreased hardness. The 2Y had a uniform microstructure and mechanical properties. The formation of the graded structure and toughness profile was explained in terms of the gradual CeO2 reduction to Ce2O3 in the Y2O3+CeO2 doped ZrO2 ceramics.

Abstract: Electrophoresis is the effect that when an electric field is applied to a suspension of a powder in a liquid, the powder particles move under influence of this field. Frequently the powder particles also deposit at one of the electrodes. The form of the electrode determines the form of the deposit, hence shaping is possible. The current insights into the science and technology of electrophoretic deposition (EPD) will be summarized. EPD is well suited for shaping layered microstructures (laminates), by simply changing repeatedly between two or more suspensions during deposition. Tubular laminates consisting of silicon carbide layers and crack deflecting graphite interlayers have been produced. These tubes demonstrate an enhanced fracture energy and a gradual mode of failure. Another area of advanced ceramics where the use of EPD makes sense are functionally graded materials (FGM) in which one tries to combine in one component high hardness and high toughness. EPD allows the formation of FGM by depositing from a powder suspension to which a second suspension is continuously added during the process. An example will be shown of a graded WC-Co hardmetal.

Abstract: The ceramic cutting tool requirements for metal machining are reviewed, taking into account the trends in industry towards dry high-speed cutting and the need for tools with complex geometry. The emphasis will be on bulk materials rather than on coatings and most attention will be devoted to machining of iron-based alloys (steels). In the development of new tool materials, special attention should be given to the chemical matching of tool and workpiece material at elevated temperatures. The chemical compatibility of the tool-workpiece combination can be investigated by means of static interaction couples at elevated temperatures, which have been exposed for times long enough to be able to characterise the interaction layer. Complementary to the experimental investigation, the chemical compatibility of tool and workpiece materials can be predicted from thermodynamic equilibrium solubility calculations of tool materials in a selected workpiece material. Taking into account the mechanical properties needed, new ZrO2-based composites were defined based on the thermodynamically estimated chemical stability. The selection, development and proof testing of the new ZrO2-based composites is highlighted and their wear behaviour and performance in high speed dry turning of construction steel is compared with that of state-of-the-art cutting tool materials.

Abstract: Electrophoresis is the effect that when an electric field is applied to a suspension of a powder in a liquid, the powder particles move under influence of this field. Frequently the powder particles also deposit at one of the electrodes. The form of the electrode determines the form of the deposit, hence shaping is possible. The current insights into the science and technology of electrophoretic deposition (EPD) will be summarized. EPD is well suited for shaping layered microstructures (laminates), by simply changing repeatedly between two or more suspensions during deposition. Tubular laminates consisting of silicon carbide layers and crack deflecting graphite interlayers have been produced. These tubes demonstrate an enhanced fracture energy and a gradual mode of failure. Another area of advanced ceramics where the use of EPD makes sense are functionally graded materials (FGM) in which one tries to combine in one component high hardness and high toughness. EPD allows the formation of FGM by depositing from a powder suspension to which a second suspension is continuously added during the process. An example will be shown of a graded WC-Co hardmetal.

Abstract: Due to their excellent properties of high strength, biocompatibility and stability in physiological environments, ceramics are investigated as bone substitute materials. In this way ceramic components have been used for total hip replacement components since the early 1970s. Alumina and zirconia monoliths are mainly used for these components. However, zirconia can undergo low temperature degradation in aqueous environment and alumina is brittle. To increase the strength, functionally graded Al2O3/ZrO2 ball-heads and acetabular cup inserts were made in this work 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 thermal residual stresses at the surface after sintering. Experimental work revealed that a sequence of counter-electrodes was necessary to EPD complex shaped functionally graded material (FGM) components. To obtain deposits with a shape different from the deposition electrode, the deposit was grown up to the counter-electrode and the design of the counter-electrodes was supported by electrical field calculations to generate a constant electric field at the surface of the deposition electrode

Abstract: The deposit can induce an extra potential drop near the electrode, depending on the suspension composition. This can result in a levelling off of the deposition rate in a constant-voltage deposition process. The magnitude of the extra voltage drop determines the uniformity of the deposit as function of the uniformity of the electric field present at the deposition electrode. It was experimentally proven that a uniform Al2O3 coating thickness was obtained in a non-homogeneous electrical field in ethanol with addition of HNO3, while the coating thickness varied uniformly with the E-field strength for a MEK with n-butylamine based suspension. The uniformity of the coating deposited from these suspensions was related to the measured potential drop over the deposit during electrophoretic deposition.

Abstract: A model was developed to explain the magnitude of the potential drop over the deposit for non-conductive powders during electrophoretic deposition (EPD). The magnitude of the potential drop over the deposit is explained in terms of a reduced ion transport through the deposit, as controlled by the pore potential that is related to the thickness of the electrostatic double layer relative to the pore radius and the magnitude of the surface potential of the powder particles. This model was validated for EPD of Al2O3 powder from ethanol-based suspensions with HNO3 addition. The specific resistivity of the deposit could be related to the calculated potential in the pores of the deposit.

Abstract: The electrical field drop over a deposit during electrophoretic deposition (EPD) determines the deposition rate and the uniformity of the deposit when a non-uniform electrical field is present. Due to the large practical consequences of a potential drop over the deposit, a procedure was developed to calculate the electrical field strength at the deposition front from currentconductivity measurements during EPD. The evolution of the electrical field strength during EPD was calculated for MEK and ethanol based suspensions. It was found that the suspension composition determines whether a potential drop over the deposit is present or not. EPD experiments on a membrane revealed that the extra potential drop is over the deposit and not caused by electrode polarization for the ethanol-acid based suspensions.

Abstract: Melt-spun flakes and air atomised powder of a multi-component Al-Si-Fe-X alloy were consolidated by field assisted (FAST) or spark plasma sintering (SPS) in vacuum using steel dies and punches. Experiments were carried out at 350, 400, 450 and 500°C under applied loads ranging from 81 to 283 MPa. The resulting compacts were microstructurally and mechanically characterized. Ultimate strength values up to 1000 MPa and plastic strains up to 20% were observed during compression tests. The effect of the powder shape on the sintering behaviour is compared. The effect of process parameters such as temperature and applied load on the densification and mechanical properties is discussed. It was found that compacts sintered from melt-spun flakes resulted in a higher strength and ductility than compacts produced from air-atomized powder, sintered under identical conditions.

Abstract: Future materials for wear resistant components require a combination of excellent mechanical properties such as hardness and toughness, short processing times and good electrical conductivity to facilitate shaping by electro discharge machining (EDM). In this work, the hardness and fracture toughness of t-ZrO2 based electro conductive composites was optimised, while short processing times below 20 minutes using spark plasma sintering were sufficient to obtain near fully dense materials. The influence of powder processing technique using TiC0.5N0.5 as the starting powder and yttria as a stabiliser on the mechanical properties of ZrO2-TiC0.5N0.5-Al2O3 based composites was investigated. Fully dense Y-TZP based composites possessed an excellent toughness of 9.2 MPa.m1/2 and an increased Vickers hardness of 1397 kg/mm².