Papers by Author: Rainer Oberacker

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Authors: Jan Tabellion, Rolf Clasen, J. Reinshagen, Rainer Oberacker, Michael J. Hoffmann
Authors: Stefan Holzer, Jan Patrick Häntsche, Ulrich Spicher, Bernd Huchler, Alwin Nagel, Rainer Oberacker, Dirk Badenheim, Michael J. Hoffmann
Abstract: Silicon nitride based ceramics have been investigated since more than forty years. Nevertheless, a good understanding of the microstructure-properties relationships on the level of small samples is not enough to bring ceramic parts into the industrial market. More convincing is a demonstration of the excellent potential in specific applications where the requirements of a more complex system are considered. In this work, sialons are investigated for a use in lubricated wear applications. The target system is a fuel injection pump for pressures up to 30 MPa. It requires very small friction coefficients and wear rates. Isooctane was used as lubricating liquid. A friction coefficient of 0.3 has been achieved with a piston-on-plate configuration for two sialon ceramics with different a/b-ratios. The sample surfaces were in a ground condition. For comparison, tribological testing was also performed with commercial alumina and silicon nitride materials which exhibited higher friction coefficients. In order to be closer to the real conditions a test rig has been designed and constructed with a piston running in a cylinder that actually pumps isooctane at the desired pressures of up to 30 MPa. With it, the high potential of sialon ceramics for lubricated sliding applications was proven. In the fields of microstructural design neodymia and ytterbia containing sialons with varying a/b-sialon ratios and different amounts of additives have been investigated. These parameters have a strong effect on the achieved aspect ratios of the sialon grains which can be correlated to the amount of liquid phase during sintering, kinetic considerations and the additives' cationic radii. Finally, the effect of these varying grain shapes on the mechanical properties hardness and fracture toughness has been determined.
Authors: M. Dröschel, Michael J. Hoffmann, Rainer Oberacker, H.v. Both, W. Schaller, Yu Ying Yang, D. Munz
Authors: M. Dröschel, Rainer Oberacker, Michael J. Hoffmann, W. Schaller, Yu Ying Yang, D. Munz
Authors: Marco Riva, Rainer Oberacker, Michael J. Hoffmann, Theo Fett
Abstract: Environmental-assisted subcritical crack growth on two different /-sialon ceramics is determined in static fatigue tests. The goal of these investigations was to determine the influence of delivering medium of high pressure pumps on their durability (life cycle). Therefore 4-point-bending bars were statically loaded in distilled water and subcritical crack growth was analyzed. The large number of spontaneous failure and survivals made the development of a modified testing procedure necessary in order to obtain a significant database.
Authors: Marco Riva, Rainer Oberacker, Michael J. Hoffmann, Carlos Ziebert
Abstract: Effects of plasma etching on mixed /-sialon ceramics with different /-ratiowere investigated. A mix of CF4/O2 in a ratio of 2:1 was chosen as reactive gas. Parameters such as etching time and the material composition were examined. It was shown that -grains exhibit a larger etching rate than the grain boundary glassy phase or the -grains, generating pockets in a range of few µm. The so created surfaces were characterized both by scanning electron microscopy (SEM) and by atomic force microscopy (AFM) in contact mode. These complementary techniques also enabled the determination of the bearing ratio.
Authors: Andreas Mattern, Rainer Oberacker, Michael J. Hoffmann
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