Page 95 - Fiber Fracture

80 A.R. Bunsell

intergranular phase must be written as SiOxC1-x,2 instead of SiOxC2-x (Le Coustumer
et al., 1993). This expression permits the composition to vary continuously from Sic to
Si02 by increasing the percentage of oxygen. The fibre is composed by weight of 56%
Sic, 10% C and 34% SiO1.12Co.M.
Porosity in the NL 200 as in other polymer-derived fibres has never been imaged in
transmission electron microscopy but was theoretically assessed before it was shown
experimentally by X-ray scattering techniques. Outgassing of hundreds of volumes of
gas formed per volume of fibre has to take place during pyrolysis for the progressive
transformation from an organic to a mineral structure to occur. Diffusion through a
solid phase would be too slow to allow all this gas to escape during the pyrolysis.
Nanochannels must be present at this stage of transformation which then undergo
viscous collapse on further heating above the gas evolution temperature. A porosity of
6.2% has been measured by X-ray scattering with the Nicalon NL 100 fibre pyrolysed at
1400°C.
The Tyranno LOX-M fibre has a diameter of 8 pm and contains approximately 13
wt% of oxygen as the use of M, which is the thirteenth letter in the alphabet, indicates.
The structure has been described as being composed of P-SiC grains of 2 nm and small
aggregates of free carbon of less than 2 nm. Although these fibres are known to contain
titanium, no titanium compounds are found in the fibre. It is assumed that the titanium
is incorporated into a Si-0-Ti-C amorphous continuum. The density of the fibre is less
than that of the Nicalon NL 200 although their compositions and grain sizes are very
close indicating a greater amount of porosity.
The Hi-Nicalon represents a departure in the original manufacturing process as
cross-linking takes place by electron irradiation. The result is a large reduction in
oxygen, to about 0.5 wt%, in the fibre and as a consequence the elimination of the
amorphous intergranular phase. Transmission electron microscopy reveals that the fibre
contains larger Sic grains of around 5 nm for the majority of the grains but the biggest
crystallites can reach 20 nm (Berger et al., 1995). This does not lead to any marked
difference in the fracture morphology of the as received Hi-Nicalon fibre as seen in the
SEM. Fig. 4 shows that a large fraction of the Si and C atoms is not entirely crystallised
in the form of P-Sic and the atoms are present between the fi-SiC grains in a less dense
arrangement. Free carbon is found with a more organised structure than in the NLM-202
fibre, composed of 4 to 10 distorted layers with an interfringe distance of around 0.36
nm over a length of 2 nm on average with some aggregates measuring up 5 nm.
By analogy with the structure of the NL 200 the small amount of oxygen in the
fibre is presumed to be present at the SiC/SiC boundaries. It is supposed that oxygen
substitutes for one carbon atom to form Sic30 tetrahedra.
The structure of the Hi-Nicalon fibre can be represented as being composed of
about 59% of B-Sic by weight, 11% of free carbon and the 26% of non-crystallised
Si-C combined with the 4% of Si-C-O phase described above, giving 30% of an
intergranular phase composed in majority of Sic with some substitution of carbon by
oxygen. As with the NL 200 fibre the Hi-Nicalon contains a small amount of nanometric
porosity.
The Tyranno LOX-E fibre was produced by Ube Industries by the electron radiation
method adopted by Nippon Carbon for the Hi-Nicalon fibre. However, the introduction