Page 114 - Fiber Fracture

FRACTURE PROCESSES IN OXIDE CERAMIC FIBRES 99

Fig. 13. External surface of the FP fibre broken at 1300°C exhibiting grain boundary decohesions and
intergranular microcracks.

required, as in thermal insulation. They were not developed to work in the conditions
for which their poor creep resistances have been demonstrated. However, the creep
mechanisms which have been revealed have allowed the microstructure which would
improve the high-temperature behaviour of a-alumina-based fibres to be better defined.
These fibres must have fine grains, as large grains are detrimental for the fibre strength,
but grain sliding has to be inhibited. Inclusions of second phases and fine but elongated
oriented grains have been considered as possible solutions to achieve these goals.

a-AluminalZirconia Fibres

The dispersion of small particles of tetragonal zirconia between a-alumina grains
was first employed by Du Pont with the aim of producing a modified FP fibre with
improved flexibility. This fibre, called PRD-166 (Romine, 1987), was obtained by the
addition of zirconium acetate and yttrium chloride to the blend of the alumina precursor
and a-alumina powder. The fibre had a diameter of 20 pm and contained 20 wt% of
yttrium-stabilised tetragonal zirconia in the form of grains of 0.1 Fm as can be seen in
Fig. 14, which restricted the growth of a-alumina grains to 0.3 pm, on average (Lavaste
et al., 1995). Young’s modulus was lowered to 370 GPa because of the lower stiffness of
zirconia (Ez,.o* % 200 GPa). Tetragonal to monoclinic transforniation of zirconia around
the crack tip at room temperature (Fig. 15) toughened the fibre and a higher strength
was obtained (1.8 GPa at 25 mm). However, this was not sufficient to ensure flexibility
and the production did not progress beyond the pilot stage.
The effect of the addition of zirconia on the high-temperature mechanical behaviour
is to delay the onset of plasticity to 1100°C and to decrease the strain rates in creep
(Pysher and Tressler, 1992; Lavaste et al., 1995). The mechanisms proposed have been
the pinning of the grain boundaries by the intergranular zirconia particles and more
recently the modification of the AI3+ diffusion rates at the alumina/alumina grain
boundaries by the presence of ZF+ and Y3+ ions. However, these mechanisms are less