Fracture processes in fine silicon carbide fibres

Abstract

The fracture morphologies of the fine silicon carbide fibers made from polymer precursors reveal the nature of the microstructures of the fibers. The need to reduce the oxygen content of the fibers led to the modification of the cross-linking process. Cross-linking of polymers can be induced by irradiation by high-energy elementary particles or electromagnetic radiation. The side groupings on the macromolecules are cut by this technique, which permits bonds between the macromolecules to be created without the introduction of oxygen into the polymer. The cross-linking step is followed by a heat treatment at 300 °C for a short time to reduce the number of free radicals that are trapped in the irradiated polycarbosilane (PCS) fiber. Successive generations of fibers have been created with reduced oxygen content, thereby producing better controlled microstructures and eliminating the presence of amorphous intergranular phases, which limit mechanical behavior at high temperatures. As a consequence the SiC grains have become larger and the Young moduli have increased. The latest, near-stoichiometric fibers have moduli of up to 400 GPa and fail with a granular fracture morphology reflecting the large SiC grains of their structures, whereas the earliest SiC fibers had Young moduli of around 200 GPa and showed glassy fracture morphologies because of their nanometric grain structure.