Page 93 - Fiber Fracture

78 A.R. Bunsell

Pyrolysis

The cured PCS or PTC fibres are pyrolysed in a nitrogen or argon atmosphere.
The pyrolysis of the Nicalon NL-200 fibres is stopped around 1200"C, the temperature
at which their tensile strengths reach a maximum, whereas, for the same reason, the
LOX-M is most probably pyrolysed at around 1300°C. It has been reported that the
Hi-Nicalon is pyrolysed at 1500°C (Shibuya and Yamamura, 1996) and the LOX-E fibre
at 1300°C. Modifications of the microstructures of all the fibres have been detected
after post-heat treatments of 1 h at their pyrolysis temperatures. This indicates that
the durations of the pyrolysis that the fibres experienced were too short to allow the
microstructures to be entirely stabilised.

Near-Stoichiometric Sic Fibres

Nippon Carbon has obtained a near-stoichiometric fibre, the Hi-Nicalon Type S, from
a PCS cured, in a hydrogen rich environment, by electron irradiation using a 'modified
Hi-Nicalon process'. It is claimed that the excess carbon is reduced from C/Si = 1.39
for the Hi-Nicalon to I .05 for the Hi-Nicalon Type S.
Dow Coming has produced near-stoichiometric Sic fibres from a PCS containing a
small amount of titanium, similar to the route taken for most of the Tyranno fibres. The
precursor fibres are cured by oxidation and doped with boron. In this way degradation
of the oxicarbide phase at high temperature is controlled and catastrophic grain growth
and associated porosity as occurred with the previous oxygen-rich fibres, is avoided.
The precursor fibre can then be heated to around 1400°C so that the excess carbon and
oxygen are lost as volatile species and then sintered at a higher temperature to yield a
polycrystalline, near-stoichiometric, SIC fibre called SYLRAMIC fibre.
Ube Industries has announced the development of a near-stoichiometric fibre made
from a polycarbosilane cross-linked by an aluminium compound. The precursor fibre is
cured by oxidation, pyrolysed in two steps first to allow the outgassing of CO and then
sintered at a temperature up to around 1800°C.
A comprehensive description of Sic fibre manufacture and behaviour is given by
Bunsell and Berger (1999).

FRACTURE MORPHOLOGIES OF SUCCESSIVE GENERATIONS OF SIC
FIBRES

All the Sic fibres are circular in cross-section with diameters of 15 pm or less. The
earliest Sic fibres, the Nicalon 100 series, showed all the fracture characteristics of a
glassy structure, although TEM studies showed the presence of very small Sic grains
of less than 2 nm as well as even smaller free carbon particles. The Nicalon 200 series
became the standard fibre used for most ceramic matrix composites. The fibre has a
diameter of 15 pm and also shows a glassy fracture morphology, as Fig. 2 reveals.
This fracture morphology accurately reflects the microstructure of the fibre. Fig. 3
shows a dark field image of the NL 200 fibre.