Papers by Author: Roland Weiss

Abstract: Ceramic Matrix Composites (CMC) have a wide interest for high temperature applications. The materials can be modified by the selection of the matrix precursor as well as of the reinforcing materials. C/C-composites can be easily modified by post-treatments with silicon in order to acquire different tribological properties from good sliding behaviour up to braking systems only depending on the manufacturing technique of these materials. It will be demonstrated during the presentation that the manufacturing depends on one hand side on the material which has to be manufactured and on the other side on the structural component and the number of parts which are required. Furthermore, it will also be shown, that silicon treatments can be performed up to a full conversion of C/C materials creating a new family of monolithic ceramic materials. Within the presentation detailed information will be given on possible processing routes as well as the resulting physical and mechanical properties of the materials.

Abstract: Combining sol-gel synthesis of 3/2 mullite through hydrolysis and condensation of tetraethoxysilane and aluminum-tri-sec-butylate with electrophoretic deposition (EPD) yields sufficiently thick and homogeneous layers which transform into mullite at T ≥ 1000 °C. The characterisation of the mullite precursor during synthesis was performed through electroacustic measurements. The protectiveness of the deposited mullite layers was tested in air in the temperature range 1200 °C ≤ T ≤ 1550 °C by means of isothermal thermogravimetric analysis for up to 200 hours. Comparing the oxidation rate of mullite coated C/C-Si-SiC samples to that of uncoated reference samples clearly demonstrated that mullite offers a significant improvement to the oxidation resistance of the uncoated material. At temperatures above 1600 °C the protectiveness of the deposited layer is reduced due to the existence of a liquid phase and the formation of CO bubbles above the cracks in the SiC layer. In order to prolong the protectiveness of our mullite layers at higher temperatures we deposited an additional layer from a suspension of mullite precursor with 5 wt. % of Al2O3 powder. The protectiveness of so obtained mullite and mullite/ Al2O3 layers was also tested under cyclic conditions at 1500 °C and 1550 °C. These experiments clearly demonstrated that all samples withstood at least for 4-10 cycles which were performed subsequently in different time intervals (from 2-3 days to 1 h).

Abstract: Creep tests have been performed for carbon fibre bundles (Toray, HTA5131) in a temperature range from 1500 °C to 1800 °C and at various stress levels in a tensile testing machine in vacuum (pressure was kept below 10-3 mbar). Creep parameters have been obtained from the diagrams elongation versus time. X-rays (SAXS and WAXD) have been used for accompanying structural investigations. An increase in the alignment of the graphene sheets with respect to the fibre (and thus the loading) axis was observed with increasing test temperature. For fibres stabilised by heat treatment (2 hours, 2100 °C, without load) no creep and no structural change could be observed.