Page 108 - Fiber Fracture

FRACTURE PROCESSES IN OXIDE CERAMIC FIBRES 93

Fig. 2. TEM bright field image of the Altex fibre after a heat treatment at 1130°C. Growth of mullite grains
surrounded by smaller grains of transitional forms of alumina.

450

9 250
4 200
150 Nex. 312 1
.-
0
s 100
50
oi I I I I
0 10 20 30
Silica Content (wt %)
Fig. 3. Evolution of elastic moduli of alumina-based fibres as a function of the silica content.

surface defects, as illustrated by Fig. 5, generated during the fabrication process or fibre
handling. Strength loss at high temperature occurs from 1000°C. Above 1200°C the
growth of mullite grains and large a-alumina grains renders the fibres extremely weaker.
Moreover, the presence of an amorphous silicate intergranular phase enhances creep
which begins from 900°C so that these fibres cannot be used for structural applications
above this temperature.