Page 94 - Fiber Fracture

FRACTURE PROCESSES IN FINE SILICON CARBIDE FIBRES 79

Fig. 2. Nicalon NL 200 room temperature fracture morphology.

Fig. 3. Dark field image of a Nicalon NL 200 fibre. The white spots indicate Sic grains of approximately 2
nm in size.

The microstructure of the Nicalon NL 200 fibre is composed of 56 wt% P-SiC grains,
of around 2 nm, 40% of an intergranular phase which has been calculated to give a mean
composition of SiO1.15Co.85 and 5% of randomly oriented free carbon aggregates, less
than 1 nm in diameter. Coo2 lattice fringes images showed small stacks of two fringes
of 0.7 or 0.8 nm suggesting that for the basic structural units (BSUs) a face-to-face
association of aromatic ring structures, called dicoronenes, the sizes of which are given
as 0.74 nm and in which the hydrogen-to-carbon atomic ratio was 0.5. If a theoretical
density is calculated from this composition, the fibre must contain 2% of porosity
to fit with the measured density. Some authors consider that the composition of the