GRAIN-GROWTH AND STRENGTH DEGRADATION OF SIC MONOFILAMENTS AT HIGH-TEMPERATURES

The microstructural stability of SCS-6 SiC monofilaments was determine d by measuring the average grain size, tensile strength and critical l ength of the fibres as a function of various annealings in vacuum (app roximate to 0.1 Pa) at different temperatures (1400-1600 degrees C). T he average grain size, calculated from X-ray diffraction line broadeni ng, increased from 23 nm for the as-received fibres to 46 nm for fibre s annealed for 2 h at 1600 degrees C. The corresponding tensile streng th measured at room temperature dropped from 3.6 GPa for the as-receiv ed fibres to approximate to 2 GPa for the treated fibres. Simultaneous ly, the average critical lengths, measured using the glass-slide techn ique decreased from 0.37 mm for the as-received fibres to an average o f 0.23 mm for the heat-treated fibres. The degradation of the mechanic al properties was attributed to a combination of coarsening of the bet a-SiC grains as well as interactions with the annealing environment, n amely the vacuum hot-press chamber.