Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Abstract

Melt-spun/oxygen-cured polycarbosilazane monofilaments were pyrolysed under an argon or nitrogen atmosphere at pyrolysis temperatures, θ_p, up to 1600 °C. The organic/inorganic transition took place at 500 < θ_p < 850 °C with a first weight loss and diameter shrinkage (evolution of CH₄ and H₂). It yielded an amorphous Si-C-N-O material which underwent only minor changes (e.g. densification) when θ_p was raised to about 1400 °C. In contrast, beyond 1400 °C, major changes occurred which depended on the pyrolysis atmosphere. Under argon, a decomposition/crystallization process yielded a second weight loss and diameter shrinkage (evolution of SiO, CO, N₂) and resulted in a β-SiC + C mixture whose degree of crystallization increased when θ_p was raised. Under nitrogen, a decomposition/nitriding process yielded a material enriched in nitrogen and depleted in oxygen which remained partially amorphous up to 1600 °C. The tensile failure stress at room temperature increased sharply for 500 < θ_p < 800 °C and reached a maximum (close to 2000 MPa) for θ_p=1200 °C whereas the Young's modulus increased more smoothly with a maximum (close to 175 GPa) for θ_p≈1400 °C. Both oxygen and nitrogen have an impeding effect on the crystallization of β-SiC. Thus, the best mechanical properties were obtained for filaments pyrolysed under nitrogen. A semi-conducting/semi-metallic transition was observed for θ_p≈1400 °C related to the decomposition of the amorphous Si-C-N-O material and formation of free carbon. Ex-polycarbosilazane oxygen-cured filaments exhibited a thermal stability somewhat higher than their ex-polycarbosilazane counterparts.