Papers by Author: R. Herbig

Abstract: The preparation of mullite coatings for the oxidation protection of carbon fibre reinforced composites using a combination of sol-gel synthesis and electrophoretic deposition (EPD) has been investigated. Mullite precursor sols were synthesised by controlled hydrolysis and condensation reactions of the metal alkoxides TEOS (tetraethoxysilane) and Al(OBus)3 (aluminiumtri-sec-butylate). The structure and properties of the mullite precursor were strongly influenced by the synthesis parameters, especially by the water to TEOS ratio (rw/Si) and the pH value of the water. A variety of synthesis conditions was tested for optimising the mullite precursor sols regarding their suitability for the electrophoretic deposition. The electrokinetic behaviour of the sols and the charging of the sol particles which is necessary for a successful EPD were investigated by measurements of the Electrokinetic Sonic Amplitude (ESA signal). 29Si CP/MAS NMR measurements were used to get information about the coordination of the silicon and the homogeneity of the Al/Si distribution in the precursors. Heat-treated samples were characterised by X-ray diffraction for investigating the mullite formation. Coatings prepared by EPD and sintering at 1300 °C in Ar enabled an effective oxidation protection in the temperature range 1200 °C ≤ T ≤ 1550 °C.

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).