CONTINUOUS SIC-BASED MODEL MONOFILAMENTS WITH A LOW FREE CARBON CONTENT .1. FROM THE PYROLYSIS OF A POLYCARBOSILANE PRECURSOR UNDER AN ATMOSPHERE OF HYDROGEN

Pyrolysis of a Yajima-type polycarbosilane (PCS) has been performed un der an atmosphere of hydrogen on both bulk samples and model monofilam ents up to 1000 degrees C, in order to reduce the free carbon content of the resulting ceramics. The organic/inorganic transition occurs wit h in the 400-800 degrees C temperature range, with mainly an evolution of CH4. At 1000 degrees C, it yields an hydrogenated amorphous cerami c with a C/Si atomic ratio and a free carbon content significantly low er than for its counterpart obtained under inert atmosphere (namely, 1 .18 and 9 at% versus 1.72 and 27 at%). Hydrogen is thought to favour t he release of the pendent methyl groups of the PCS via demethanation r adical reactions. Continuous model filaments were produced via the mel t spinning of the PCS, electron beam curing, pyrolysis under hydrogen up to 1000 degrees C, and a final heat treatment under argon up to 160 0 degrees C. The ceramic fibres exhibit a C/Si atomic ratio of 1.10, a free carbon content of approximate to 8 at%, a Young's modulus of 260 -300 GPa and a tensile failure stress of 2100 MPa. Their thermal stabi lity is limited to 1400 degrees C due to some oxygen contamination dur ing the process.