Papers by Author: Kim Vanmeensel

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: 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: Fully dense ZrO2-TiN composites containing 1.75-2 mol %Y2O3, 1 mol% Y2O3 and 1 mol% Nd2O3 stabilizers, small amounts of Al2O3, and electrical conductive TiN particles (40-70 vol%) have been produced by hot pressing and spark plasma sintering at 1550°C. Although the intrinsic hardness of TiN (1400 kg/mm²) is higher than that of t-ZrO2 (1200 kg/mm²), the decreasing hardness trend can be attributed to the larger TiN grain size with the higher TiN content. Since TiN is more brittle, the fracture toughness decreases with increasing TiN content. Transformation toughening has been attributed as the main toughening mechanism as a result of fracture toughness decreasing with the transformability. Spark Plasma sintering temperature was too high for mechanical properties and hydrothermal stability of the mixed stabilized composites. The transformability decreases so hydrothermal stability increases linearly with increasing TiN content as a result of smaller volume fraction of t-ZrO2 grains becoming susceptible to hydrothermal transformation due to the shielding effect of the present TiN grains.

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: 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: 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.

Abstract: In this study, the development of a functionally graded material (FGM) with hard outer surfaces and a tougher inner core was envisaged. The applicability of electrophoretic deposition (EPD) for the processing of FGM materials by continuously changing the suspension composition is shown. Optimisation of the colloidal processing technique was combined with hot pressing experiments on homogeneous composites in the Al2O3-ZrO2-Ti(C,N) system in order to create a very hard functionally graded material with beneficial residual stresses. Finally, the residual stress distribution was briefly discussed using an existing analytical model.