Papers by Author: Michael J. Hoffmann

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.

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.

Abstract: In this work, SiC ceramics were prepared by liquid phase sintering using 10 wt.% of AlN and Y2O3 mixtures in the proportion of 4:1 and 2:3 as additives Sintering was done in a graphite resistance heated furnace at 2080oC under 0.2 MPa N2 atmosphere for 1h. Part of the samples was further heat-treated at 2000oC for 4h to allow grain growth. The microstructures of the sintered samples were analyzed using polished and plasma etched surfaces. Furthermore, relative densities, weight loss during sintering and SiC-polytype distributions are reported.

Abstract: Reaction bonded MgSiN2 (RBMSN) was prepared by direct nitridation of a Si/Mg2Si/Mg/Si3N4 powder compact in a temperature range of 1350-1550°C. The oxygen content of MgSiN2 was in the range of 0.4 – 0.6 wt%. A thermal stability examination showed that MgSiN2 is stable up to 1400°C at 0.1 MPa N2 pressure. The activation energy of decomposition calculated from the temperature dependence of weight loss is H = 383 kJ⋅mol-1. The time dependence and nitrogen pressure dependence of MgSiN2 decomposition was also investigated at constant temperature. MgSiN2 is stable at 1560°C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs free energy of formation of MgSiN2 was calculated in a temperature range 300-2500 K. Dense MgSiN2 ceramics or MgSiN2/Si3N4 composites with fluorine-based additives were prepared by hot pressing. The composite materials had a 4-point bending strength of 427 MPa and Vickers hardness (HV1) of 20.8 GPa, respectively. The indentation fracture toughness was 5.3 MPa.m1/2, due to the presence of elongated β-Si3N4 grains. The dielectric constant of dense reaction bonded MgSiN2 at 100 kHz was 9.5-10, while that of MgSiN2/Si3N4 composite in a wide range 50 – 6000, depending on composition and heat treatment.

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.