Page 106 - Fiber Fracture

FRACTURE PROCESSES IN OXIDE CERAMIC FIBRES 91

INTRODUCTION

If high performance fibres are to be exposed to oxidative atmospheres and temperatures
above 1200"C, they will have to be made from oxides with high melting points. a-alumina
is widely used for its refractory properties. Its complex crystal structure provides large
Burgers vectors so that high stresses are necessary to generate plasticity in monocrystals.
Monocrystalline a-alumina fibres showing no creep up to 1600°C can be obtained if the
fibre axis strictly corresponds to the [0001] axis (Gooch and Groves, 1973). However, no
viable processes exist at present to produce fine and flexible continuous monocrystalline
fibres. Therefore only polycrystalline fibres can be considered for the reinforcement of
ceramics. Various processing routes exist for making such fibres and these lead to a large
range of microstructures and fracture behaviours (Berger et al., 1999).

FABRICATION OF ALUMINA FIBRES
Precursors of alumina are viscous aqueous solutions of basic aluminium salts,
AlX,(OH)3-,, where X can be an inorganic ligand (Cl-, NO3-. . .) or an organic ligand
(HCOOH-. . .) (Taylor, 1999). Spinning of the precursor produces a gel fibre which is
then dried and heat-trcatcd. Decomposition of the precursor induces the precipitation of
aluminium hydroxides, such as boehmite AlO(OH), and the outgassing of a large volume
of residual compounds. The associated volume change and porosity at this step has to be
carefully controlled. It is also possible to directly spin aqueous sols based on aluminium
hydroxides. Dehydration between 300°C and 400°C yields amorphous aluminas and
leaves nanometric pores in its structure. Further heating to around 1100°C induces the
sequential development of transitional forms of alumina. These aluminas have spinnel
structures containing aluminium vacancies on the octahedral and tetrahedral sites. They
only differ by the degree of order in the distribution of these vacancies. At this stage the
fibre is composed of alumina grains of a few tens of nanometres, poorly sintered with a
finely divided porosity. Above 1100°C stable a-alumina nucleates and a rapid growth of
pm-sized grains occurs together with coalescence of pores. Porosity generated during
the first steps of the formation of metastable aluminas cannot be eliminated and is
increased by the higher density of a-alumina compared to the transitional forms. The
fibres become extremely brittle due the presence of large grains. Fracture initiated from
large grain boundaries emerging at the fibre surface and crack propagation is mainly in-
tergranular. Alumina fibres cannot be used in this form and the nucleation and growth of
a-alumina have to be controlled by adding either silica precursors or seeds for a-alumina
formation to the fibre precursors. This has led to two classes of alumina-based fibres with
different fracture behaviours which are transitional alumina fibres and a-alumina fibres.

TRANSITIONAL ALUMINA FIBRES

Alumina-silica fibres were the first ceramic fibres produced in the early 1970s, for
thermal insulation applications. Small amounts of silica, %3 wt% in the Saffil short