Papers by Author: Mathias Herrmann

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Authors: Jochen Schilm, Mathias Herrmann, G. Michael
Authors: Jochen Schilm, Mathias Herrmann, G. Michael
Abstract: The corrosion behavior of silicon nitride materials in aqueous media strongly depends on the composition and amount of the grain boundary. But there exist no systematic investigations of the relation between the corrosion behavior and the composition and amount of the grain-boundary phase. The subject of this study is based on the evaluation of the corrosion mechanisms of Si3N4 ceramics and YSiAlON glasses, both with defined and characterized compositions, in acidic and basic environments with different concentrations. Special effort has been made in describing the compositions of grain boundary phases in the sintered Si3N4 ceramics. A model to describe structural features of YSiAlON-glasses was developed which allows the correlation of the corrosion behavior of the ceramics and the glasses with their compositions. In combination with SEM and TEM investigations it became possible to give structural reasons for the different observed corrosion mechanisms.
Authors: S. Kurama, Mathias Herrmann
Abstract: At temperature above 1200°C, the thermal stability of α-SiAlON phases has been debated since 1992; however, it has been discussed if any α-SiAlON phase can be formed in Ce, La, Eu and Sr-doped SiAlON systems. In our previous studies it was shown that the use of Mg-Ce and Mg-Sr elements as dopants SiAlON compositions, in which all elements just have very low or no stability in the α-SiAlON structure, would promote the stability of Mg-Ce elements in the α- SiAlON phase [1, 2]. However, in Mg-Sr systems, it was obtained that Mg2+ is predominantly incorporated in α-SiAlON structure whereas Sr2+ mainly remains in the grain boundaries [2]. In this study, by applying spark plasma sintering (SPS) (at 1400-1700°C) and post-sintering thermal heat treatment (at 1500°C for 5 hrs and 1700°C for 2hrs) Mg or Mg-Sr doped SiAlON (50:50 mole ratios) ceramics were prepared. The results were compared with GPS sintered samples data. The effect of sintering temperature on densification process, phase transformation, microstructure and mechanical properties of samples were investigated. The results showed that by using SPS, Sr-Mg doped samples can be sintered at lower temperature (at 1600°C) than at GPS (at 1800°C) and it has no Sr-doped grain boundary phases.
Authors: A. Vorster, David S. McLachlan, G. Sauti, Mathias Herrmann
Authors: Mathias Herrmann, Stefan Holzer, Michael J. Hoffmann
Authors: W. Engel, H. Fietzek, Mathias Herrmann, P.B. Kempa
Authors: John H. Liversage, David S. McLachlan, Mathias Herrmann
Abstract: It is often the case that the low fracture toughness of polycrystalline ceramics limit the load-bearing capabilities of components made from such materials. In an effort to improve the fracture toughness of a SiC-TiC composite material, a novel stress-shielding approach was adopted so as to reduce the effective mode I stress intensity around a crack tip within the composite body. To this end, a series of functionally graded materials were produced, with samples having a preselected spatial SiC-TiC composition variation. The spatial composition variation within each sample was set up according to one of several exponential mathematical functions. In this way the resultant spatial variation in thermal expansion coefficients was also controlled through the thickness of the material, and a particular stress and strain distribution could be tailored within the body. Samples with tailored near-surface compressive stress states showed a significant improvement in c KI , as measured by means of a flexure test, in comparison with stress-free samples of a homogeneous composition corresponding to the composition found at the test-point within the gradient materials.
Authors: T.C. Shabalala, David S. McLachlan, I.J. Sigalas, Mathias Herrmann
Abstract: Boron Suboxide (B6O) powder was synthesized at temperatures of about 1400 oC from the reaction of boron and boric acid powders. The synthesized B6O powders were hot pressed at 1900 oC and at pressures of 50 MPa. Additionally to pure B6O materials, composites with Aluminum were prepared. The microstructure and properties of the sintered compacts were investigated. The addition of Aluminum in the composites results in the formation of an additional Aluminum Borate phase. The composites showed a similar hardness (~30GPa) as the pure B6O samples but an increased fracture toughness (~3.5MPa.m1/2).
Authors: H.S.L. Sithebe, David S. McLachlan, I.J. Sigalas, Mathias Herrmann
Abstract: Al-cBN cermets have been hot pressed at temperature between 800oC and 1100oC and pressure of 50MPa in vacuum. The effect of particle size of the starting powders as well as the effect of starting compositions and temperature was investigated. The materials could only densify up to 84-92% of theoretical density. After hot pressing at 800oC only Al and cBN could be observed by XRD, whereas higher hot pressing temperature result in the formation of AlN and AlB2 which retard the densification. The microstructure of the hot pressed material was studied using scanning electron microscopy. It was observed that oxide layers exist at the interface between Al and cBN phases. The presence of these oxide layers resulted in blocking the Al from spreading and therefore preventing full densification.
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