Microstructure and thermo‐mechanical stability of a low‐oxygen Nicalon fibre

Abstract

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.

Non-destructive mechanical characterization of SiC fibers by Raman spectroscopy

“…2.74 [4,33,72] 2.77 [5] 2.75 [21] SiC NLM Wavenumbers / cm [21] (black circles) with the Full Width at Half Height (FWHH) of their "sp 3 -carbon" stretching mode on surface recorded Raman spectra (white circles) ; (b) wavenumber and FWHH of "sp 3 -carbon" mode plotted as a function of tensile strengths found in Berger et al [21] or Kumagawa et al [22] for heat treated fibers.…”

“…±700 [3] 2970 [33] 1.4 [22,72] 1.2 ±0.09 [3] Hi 1.39 [21] 1.41 [5] 1.55 [81] 1.35 [82] 0.5 [21,33,72] e-5 [5] 5.4 [32] 4.5 [21] 2-15 [5,7] 5-20 $ [4] 3-5 [82] 1-30…”

“…This synthesis route leads to a high matter homogeneity and very smooth surfaces, the lack of defects explaining tensile strengths σ r as high as 3 GPa. SiC fibers have been widely studied, especially their aging [2][3][4][5][6][7] which evidenced that densification is linked to compositional changes (for instance the loss of some residual hydrogen [8] ). A peculiarity of ceramics issued from polymeric precursors is no impurities are concentrated at the grains boundaries.…”

“…±600 [3] 1 [72] 0.95 [4] 0.9 ±0.03 [3] Hi-S [32] 1.05 0.2 e-10.9 / / 3.1 420 ≥2500 0.6 SA * 1.08 [22] ≈ 0.3 Ox 38 [22] 200 [21] 100-300 [22] # 3.02 [21,22] 420 [22] 2800-3000 [22] 0.7 [22] Sylramic [33] < cited references give a size range based on electronic microscopy observations ; ♦ 0.2 wt% H [72] ; ∅ 2-3 layers of 10 "C 6 -rings" [7] ; $ mostly 5-7.5 nm ; & from Selected-Area Diffraction Pattern ; layers ; * ≤1wt% Al ; #At triple points [21] ; ♠ B = sintering aid ⇒ 3wt% TiB 2 (0.5 µm)+1wt% B 4 C(0.1 µm)…”

Analysis of Strain and Stress in Ceramic, Polymer and Metal Matrix Composites by Raman Spectroscopy

“…Prior to using the fibres as a probe for the study of composite micromechanics, one must characterise their Raman spectrum evolution as a function of thermal treatments, to mimic the evolution in the composite. [15] A correlation has been found between the mechanical tensile strength of Hi TM [59] and NLM TM [60] Nicalon fibres and the Raman vibrational data (Full Width at Half Height, wavenumber). [61,62] …”