Summary 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 β‐SiC grains 5–20 run in diameter and turbostratic aggregates of carbon 2–5 nm in diameter. The fibre is seen to be suffer 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.
SummaryThe new generation of silicon-carbide-based fibres made from organosilicon precursors, cross-linked by electron irradiation, have been compared with the earlier fibres which have undergone cross-linking in air. The latest fibres, known as Tyranno Lox-E and Hi-Nicalon, possess a lower oxygen content (Ϸ5 wt% and Ϸ0 : 5 wt%) whereas the NLM202 fibres contain 12 wt% and the Tyranno Lox-M 13 wt% of oxygen. The Tyranno fibres have been produced with a precursor similar to that used to produce the Nicalon fibres, but modified by the addition of titanium. All fibres possess a structure composed of b-SiC grains, free carbon aggregates, with no crystallized titanium compounds in the Tyranno fibres and an oxygen-rich intergranular phase, except in the Hi-Nicalon fibre. The Hi-Nicalon fibre has the largest grain size and its free carbon content is higher than in the NLM202 fibres. For all the fibres, the b-SiC grains grow when the temperature increases, whilst the strengths and Young's moduli decrease. The NLM202 shows the least change in grain size and tensile properties. The Hi-Nicalon is stiffer and stronger than the others at high temperature. TEM results show that grain growth is isotropic, even during creep tests. The growth depends on the nature and amount of the intergranular phase. Mechanical changes as a function of temperature can be explained by external oxidation during tensile tests in air and internal oxidation facilitated by the nanoporosity, which is greater in the Tyranno than in the Nicalon fibres. The presence of the oxygen-rich phase in the three fibres containing the most oxygen decreases the creep resistance. Titanium does not improve the mechanical properties and the creep resistance beyond 1523 K and does not have any positive influence in limiting the SiC grain growth. Tyranno fibres are less well stabilized than the Nicalon fibres. The Hi-Nicalon fibres have been shown to possess consistently better mechanical properties at all temperatures, including creep resistance, than the other fibres studied. All the fibres are sensitive to external oxidation at high temperature.
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