Page 92 - Fiber Fracture

FRACTURE PROCESSES IN FINE SILICON CARBIDE FIBRES 77

inorganic Sic based compounds (Yajima et al., 1981). This new precursor was obtained
by the grafting of a titanium alkoxide Ti(OR)4R=CnHh+l onto the PCS chains. This
grafting linked the molecular chains together, increasing the molecular weight of the
polymer and hence its spinnability. In 1987 Ube industries announced the development
of the Tyranno fibres from this precursor and reported that they had better thermal and
chemical stability compared to the then existing Nicalon fibres. The Tyranno fibres could
be produced with a diameter half that of the diameters of the Nicalon fibres.

Cross-Linking of the Precursor Fibre

The conversion of PCS or PTC filaments into ceramic fibres is achieved by pyrolysis
above 1000°C. Before this step the precursor fibre has to be cured to induce cross-
linking between the molecular chains in order to avoid the softening of the fibre during
this heating process and to maintain a fibrous form.

Oxidation Curing

The process of cross-linking employed for the first generation of Sic-based fibres
was by oxidation in air at around 2OO0C, in which the Si-H and C-H bonds were
oxidised and mainly Si-0-Si but also some Si-0-C bonds were created, linking the
molecular chains. This is directly analogous to the manufacture of carbon fibres from
polyacrylonitrile precursors which are cross-linked in air (Fitzer and Heine, 1988). The
curing process, in both types of fibres, introduces oxygen in their molecular structures.
In the case of carbon fibre manufacture, this oxygen is eliminated during pyrolysis.
The oxygen in the precursor fibres used for the manufacture of the Sic-based fibres is
not, however, totally eliminated during pyrolysis and its presence limits the chemical
and mechanical stability of the fibres at high temperature (Simon and Bunsell, 1984).
Oxidation curing has been employed for the fabrication of the different grades of the
of NL series of Nicalon fibres, as well as the Tyranno A to G and Tyranno S, Tyranno
LOX-M fibres and the near-stoichiometric Sylramic fibre produced by Dow Corning.

Radiation Curing

The need to reduce the oxygen content of the fibres led to the modification of the cross-
linking process. Cross-linking of polymers can be induced by irradiation by high-energy
elementary particles or electromagnetic radiation. The side groupings on the macro-
molecules are cut by this technique which permits bonds between the macromolecules to
be created without the introduction of oxygen into the polymer. The technique adopted
by Nippon Carbon for the fabrication of the Hi-Nicalon in 1992 was the irradiation of
PCS, in helium, by an electron beam of 2 MeV with a dose rate of 2-5 kGy/s up to a dose
of 15 MGy (1 gray (Gy) = 1 joule/kg) (Takeda et al., 1991). Si-H and C-H bonds are
broken and Si-Si or Si-C bonds are formed. The cross-linking step is followed by a heat
treatment at 300°C for a short time in order to reduce the number of free radicals which
are trapped in the irradiated PCS fibre. Simultaneously Ube Chemicals, using a similar
curing technology, produced the Tyranno LOX-E fibre (Yamamura, 1993).