Page 118 - Fiber Fracture

FRACTURE PROCESSES IN OXIDE CERAMIC FIBRES 103

Fig. 18. Evolution of the microstructure after a creep test at 1400°C lasting 14 h.

(BeA1204) (Whalen et al., 1991), mullite (Sayir and Farmer, 1995), or of yttrium-
aluminium-garnet bulk samples (Y3Al5OI2) (Corman, 1993) have shown the excellent
creep resistance of these systems.
However, the fabrication of monocrystalline fibres by solidification from the melt
does not permit to produce continuous filaments with the required flexibility, that is with
diameters of the order of 10 km. Fine fibres have been experimentally produced from
these systems by sol-gel techniques which then gave rise to polycrystalline structures,
(Morscher and Chen, 1994; Lewis et al., 2000). Such single phase fibres exhibited
excellent behaviours during bend stress relaxation tests (BSR tests) (Morscher and
DiCarlo, 1992), which were an indication of potential good creep resistance. However,
the development of large grains during pyrolysis, larger than the critical defect size,
was responsible for the low strengths of these fibres and made creep test results in pure
tension difficult to obtain. The control of grain size can be achieved by the inclusion of
second phases acting as grain growth inhibitors. This has been accomplished in mullite
fibres by the inclusion of zirconia particles (Lewis et al., 2000).

CONCLUSION

The requirement to have reinforcing fibres capable of operating at very high tempera-
tures and under corrosive environments demands the development of oxide fibres having
high strength and creep resistance.
Their room-temperature fracture behaviour depends on the size of the grains. Those