ALBERT

Description / Table of Contents: Carbon Fibre Reinforced Al2O3 and MulliteThe hydrolysis process from metalalkoxides has been investigated to prepare carbon fibre reinforced Al2O3 and mullite composites. Carbon fibres (SIGRAFIL HF with 10,000 monofilaments) were im pregnated with metalalkoxides (Al-tri-sec-butylate to form Al(OH)3, mixtures of tetramethoxy-silan and Al-tri-sec-butylate to form mullite), hydrated in air and hot-pressed in a graphite mold at about 1700 °C, 10 min. Beside this also the preparation of composites with chopped fibres is described. The mechanical properties were measured in a three point bending test. Composite material with more than 10 vol% fibres show plastic deformation with matrix cracking during testing. For Al2O3 an additional SiC-interlayer (0.5-1 μm SiC by a CVD-process from CH3SiCl3 and H2 at 1280 °C) is necessary to obtain a bonding between fibre and matrix. Only in this case with fibre content higher 10% it is possible to increase the fexural strength of Al2O3. The strength values lie on a line with 60% of the fibre strength. The rule of mixture cannot be applied for Al2O3 as matrix. Low fibre contents decrease the flexural strength. It appeares that in this case the Al2O3 matrix is damaged before testing. But with mullite as matrix the measured values of flexural strength lie on a line which is 65% of the rule of mixture. This is caused by a relative good bonding between fibre and matrix and is independend of an additional SiC-interlayer. The Young-modulus of Al2O3 is drastically decreased with 10% carbon fibre content (for mullite matrix: 28 vol%). It was possible to calculate the beginning of the formation of cracks in the Al2O3 matrix caused by the mismatch of the components. This value does agree with the measured decrease of Young-modulus. The toughness of both materials measured by the critical intensity factor KICV (for a composite material) with notched speciments (0.1 mm notch width) is increased with fibre content. (These KICV-values are a function of notch length and width in a range of the probable error and therefore the uncorrected values are considered). For Al2O3 matrix the KICV-values increase only if SiC-scoated fibres are used. For mullite as a matrix KICV increases with fibre content for both of the SiC coated and uncoated fibre composites. With higher fibre volume than 50% the values decrease because the interspace between the fibres is not filled with matrix. The increased KICV for these composites compared to that of the matrix alone can be explaned by energy absorption at the fibre-matrix interface. In the case of mullite matrix this mechanical behaviour is caused by a good bonding between the components. Therefore the fracture energy of these mullite composites is drastically increased with fibre content. This means that a strong mullite material with good toughness is formed. These properties are a function of the preparation method. The hydrolysis process formes a very fine grain starting material, which formes a relative dense mullite by reaction sintering between the carbon fibre monofilaments. During this reaction also a good bonding between the fibres and the matrix is formed.