SI-C-N CERAMICS WITH A HIGH MICROSTRUCTURAL STABILITY ELABORATED FROMTHE PYROLYSIS OF NEW POLYCARBOSILAZANE PRECURSORS .2. EFFECT OF OXYGEN CURING ON PROPERTIES OF EX-PCSZ MONOFILAMENTS

Si-C-N-O monofilaments were spun from a molten new polycarbosilazane ( PCSZ) precursor, cured under oxygen within the temperature range 90-23 0-degrees-C and submitted to a pyrolysis treatment in argon up to 1600 -degrees-C. The curing step was studied by differential scanning calor imetry (DSC), thermogravimetric analysis (TGA)and infrared (i.r.) anal ysis. The effect of the curing conditions on the weight loss, composit ion, structure and tensile properties was characterized after pyrolysi s. The main reaction occurring during the curing step is the oxidation of Si-H bonds. The mechanical characteristics after pyrolysis have ma ximum values (sigma(R) = 1850 M Pa and E = 185 G Pa) for a curing temp erature theta(R) = 140-degrees-C, corresponding to an oxygen atomic co ncentration of 12%. The lowering of sigma(R) and E observed when theta (R) not-equal 140-degrees-C could be due to: (i) a tendency of the mon ofilaments to stick to one another during pyrolysis (theta(R) < 140-de grees-C) and (ii) the evolution of gaseous oxygen-containing species d uring pyrolysis as well as an increase in Si-O bonds (theta(R) > 140-d egrees-C). The Si-C-N-O monofilaments remain amorphous up to 1400-degr ees-C whatever their oxygen concentration is.