PROCESSING AND CHARACTERIZATION OF AN AMORPHOUS SI N (O) FIBER

Si-N-(O) fibres were grown according to a high temperature vapour-soli d process involving the reaction between SiO and NH3 on a substrate. T he oxygen concentration of the fibres is related to the partial pressu res of SiO and NH3 during fibre growth, depending respectively, on the processing temperature and the ammonia flow. The fibres consist of am orphous silicon oxynitride of composition SiO2xN4(1-x)/3 (0.1 < x < 0. 2). They exhibit a large spread in tensile strength. The lowest values (about 1 GPa) correspond to large surface defects caused by intergrow th while the highest values reach 5 GPa for perfect fibres. The fibres are stable in nitrogen up to 1450 degrees C (10 h) in terms of compos ition, structure and mechanical behaviour owing to their high processi ng temperature (1450 degrees C) and the nitrogen pressure preventing d ecomposition. A superficial crystallization into Si3N4 is only observe d at 1500 degrees C inducing a moderate decrease of strength. In argon , decomposition starts at 1400 degrees C yielding gaseous species (SiO and N-2), crystalline Si3N4 and free silicon beyond 1400 degrees C an d induce a catastrophic drop of strength. Annealing in oxygen results in a growth of a protective SiO2 scale, amorphous or partially crystal line at 1400 degrees C. (C) 1998 Chapman & Hall.