Synthesis and Electrophoretic Deposition of Mullite Precursor Sols for the Oxidation Protection of Carbon Fibre Reinforced Composites
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.
K. Moritz et al., "Synthesis and Electrophoretic Deposition of Mullite Precursor Sols for the Oxidation Protection of Carbon Fibre Reinforced Composites ", Advances in Science and Technology, Vol. 45, pp. 1281-1286, 2006