SYNTHESIS AND CHARACTERIZATION OF SILAZANE-BASED POLYMERS AS PRECURSORS FOR CERAMIC-MATRIX COMPOSITES

The goal of this investigation was to optimize the synthesis of silaza ne-based polymers for processing fibre-reinforced ceramic matrix compo sites (CMCs). Liquid oligomeric silazanes were synthesized by ammonoly sis of chlorosilanes and characterized spectroscopically (FTIR, NMR) a s well as by elemental analysis. The silazanes were obtained in high y ield and purity. Different functional groups (system S1: Si-H, Si-CH3, Si-CH = CH2) and different degrees of branching in the Si-N backbone [system S2; Si(NH)(3), Si(NH)(2)] were realized in order to study the properties of the silazanes that are dependent on the molecular struct ure. For processing ceramics via pyrolysis of pre-ceramic oligomers, m olecular weight, rheological behaviour, thermosetting and ceramic yiel d were investigated systematically and correlated with the molecular s tructure of the silazanes. Low molecular weights (500-1000 g mol(-1)) as well as low viscosity values (0.1-20 Pa s) enable processing of the silazanes in the liquid phase without any solvent. Due to the latent reactivity of the functional groups, curing of the polymers via hydros ilylation is achieved. Structural changes and weight loss during polym er curing as well as the organic/inorganic transition were monitored b y FTIR spectroscopy and differential thermogravimetric analysis. With increasing temperature (room temperature to 800 degrees C) the hydroge n content decreases from 7 to < 0.5 wt% due to the formation of gaseou s molecules (NH3, CH4, H-2). High ceramic yields up to 80% were reache d by branching the oligomers, thus reducing the amount of volatile pre cursor fragments. Up to 1300 degrees C, ceramic materials remained amo rphous to X-rays. At higher temperatures (1400-1800 degrees C) either SiC or SiC/Si3N4 composites were selectively crystallized, depending o n the pyrolysis conditions. The utility of the optimized precursors fo r CMCs has been demonstrated by infiltration of fibre preforms and sub sequent pyrolysis. (C) 1997 by John Whey & Sons, Ltd.