MICROSTRUCTURE AND THERMOMECHANICAL STABILITY OF A LOW-OXYGEN NICALON FIBER

A new Nicalon SiC-based fibre, characterized by a low oxygen content (0.5% wt) has been studied. The absence in this fibre of a continuous Si-C-O phase, which characterized the previous NLM 202 series of fibres, induces larger mean sizes for the constituents: the fibre is composed of beta-SiC grains 520 nm in diameter and turbostratic aggregates of carbon 25nm in diameter. The fibre is seen to be stiffer at room temperature (E = 300 GPa) and stronger due to a reduction in critical defects thanks to improvements in processing conditions. The Young's modulus remains almost stable up to 1473 K in air and above this temperature the core of the fibre exhibits continuous grain growth up to 1773 K, but without the degradation that occurred in the previous generation of fibres. Fibre strength was seen to be lowered when compared to room temperature values even when exposed in air to temperatures of 1073 K. A comparable fall is not seen with the NLM 202 fibres until 1273 K and this difference is attributed to the oxidation of the carbon-rich surface of the new fibre. SiC is oxidized at higher temperatures, inducing, above 1473 K, the growth of a silica layer on the surface, with defects at the glass/ceramic interface. The large discrepancies between the good thermo-mechanical characteristics in inert atmosphere and the behaviour in air may be reduced if a coating resistant to oxidation could be applied to the fibre.