KMg2AlSi4O12 phyllosiloxide as potential interphase material for ceramic matrix composites .2. Coated fibres and model composites

Nicalon fibres were coated with a sol-gel precursor of the KMg2AlSi4O12 phyllosiloxide, by dip-coating. However, both the 1 mu m gel coating and the fibres were severely damaged during the phyllosiloxide crystallization under supercritical conditions (700 degrees C, 50 MPa, 48 h) in glycol monomethyl ether. A 3-5 mu m thick coating of KMg2AlSi4O12 was successfully applied from a slurry on SiC chemical vapour deposited- or alpha-alumina single crystal-monofilaments, in which the phyllosiloxide platelets were oriented parallel to the fibre surface in a first approximation. Small one-dimensional model composites with a borosilicate matrix were prepared from these coated monofilaments, by hot pressing (900 degrees C, 30 MPa, 10 min). Evidence of fibre pull-out in bending failure tests suggesting a weak fibre matrix bonding, was observed. It is not known whether this weak bonding is related to the layered crystal structure or to the porosity of the phyllosiloxide interphase.