SI-C-N CERAMICS WITH A HIGH MICROSTRUCTURAL STABILITY ELABORATED FROM THE 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 90-230-degrees-C and submitted to a pyrolysis treatment in argon up to 1600-degrees-C. The curing step was studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)and infrared (i.r.) analysis. The effect of the curing conditions on the weight loss, composition, structure and tensile properties was characterized after pyrolysis. The main reaction occurring during the curing step is the oxidation of Si-H bonds. The mechanical characteristics after pyrolysis have maximum values (sigma(R) = 1850 M Pa and E = 185 G Pa) for a curing temperature theta(R) = 140-degrees-C, corresponding to an oxygen atomic concentration 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 monofilaments to stick to one another during pyrolysis (theta(R) < 140-degrees-C) and (ii) the evolution of gaseous oxygen-containing species during pyrolysis as well as an increase in Si-O bonds (theta(R) > 140-degrees-C). The Si-C-N-O monofilaments remain amorphous up to 1400-degrees-C whatever their oxygen concentration is.