Page 110 - Fiber Fracture

FRACTURE PROCESSES IN OXIDE CERAMIC FIBRES 95

Fig. 6. TEM image of Fiber FP revealing a dense microstructure of a-alumina grains of 0.5 Fm.

of more than 40 wt% of a-alumina powder having a grain size of less than 0.5 km. The
use of a lower fraction of precursor reduces the porosity due to its decomposition and to
the dehydration of hydrous aluminas.
a-Alumina particles act as seeds for the growth of a-alumina and so remove the
problems associated with the delay of nucleation and rapid grain growth. In the case of
Fiber FP the grain size of the powder included in the precursor precluded the spinning
of fine filaments. The FP fibre had a diameter of 20 vm, this, added to the intrinsic
high stiffness of a-alumina (EFP = 410 GPa) and low strength (1.5 GPa at 25 mm)
due to its large grain size of 0.5 km as shown in Fig. 6, made the fibre unsuitable for
weaving. Flexible a-alumina fibres require diameters of around 10 km. This was first
achieved by Mitsui Mining by reducing the size of the a-alumina powder. In this way
the number of seeding sites could be maintained to a sufficient fraction of the volume
with a smaller amount of powder. However, this affected the control of porosity and the
resulting Almax fibre (Saitow et al., 1992) encloses a significant amount of pores inside
alumina grains which are of 0.5 km in size (Fig. 7) (Lavaste et al., 1995). Later, 3M
produced the Nextel 610 fibre (Wilson et al., 1993) which is a fully dense a-alumina
fibre of 10 km in diameter, with a grain size of 0.1 Lm as seen in Fig. 8, and possesses
the highest strength of the three a-alumina fibres described (2.4 GPa at gauge length of
5 I mm). This is achieved by the use of a ferric nitrate solution which produces 0.4 to 0.7
wt% of very fine seeds of a-Fe2O3, isomorphous to a-A1203. The ratio of nuclei sites
per volume is notably increased by this route and the addition of 0.2% of Si02 helps to
produce a dense sintered microstructure at 1300°C.
The observation of the room-temperature fracture morphologies of Fiber FP (Fig. 9)
and Almax fibre (Fig. 10) reveals more granular structures compared to the previous
alumina-silica fibres. For these pure a-alumina fibres the defect initiating the failure
cannot be seen. It is supposed that some larger and weaker grain boundaries reaching the
surface are responsible for crack initiation. Crack propagation was mixed inter- and intra-
granular for the FP fibre, whereas the presence of intragranular porosity weakened the
grains in the Almax fibre leading to a more marked intragranular crack propagation mode.