Page 104 - Fiber Fracture

Fiber Fracture
M. Elices and J. Llorca (Editors)
0 2002 Elsevier Science Ltd. All rights reserved

FRACTURE PROCESSES IN OXIDE

CERAMIC FIBRES

Marie-HCKne Berger

Ecvle des Mines de Paris, Centre des Matkriaux, B.P. 87, 91003 Evry Cedex, France

Introduction ..................................... 91
Fabrication of Alumina Fibres ........................... 9 1
Transitional Alumina Fibres ............................ 9 I
a-Alumina Fibres .................................. 94
Single Phase a-Alumina Fibres ........................ 94
a-Alumina/Zirconia Fibres .......................... 99
a-Alumina/Mullite Fibres ........................... 101
Other Oxide Fibres ................................. 102
Conclusion ..................................... 103
References ...................................... 104

Abstract

Fine oxide ceramic fibres are mainly based on alumina in one of its forms, often
combined with silica or other phases such as zirconia or mullite. All such fibres are
brittle and linearly elastic at room temperature and failure is most usually initiated by
process flaws and surface defects and less by the fibre microstructure. Fibres containing
a few percent of amorphous silica exhibit fracture morphologies which resemble those
of glass fibres because they enclose transition alumina grains of the order of only 10
nm. Fracture morphologies of pure a-alumina fibres are typical of granular structures
made of grains of 0.5 Fm. Crack propagation is both inter- and intra-granular at room
temperature and evolves toward an intergranular mode above 1OOO"C. Failure in creep is
caused by damage accumulation throughout the fibre. The coalescence of intergranular
microcracks leads to a non-planar intergranular fracture. Creep resistance of a-alumina
fibres can be improved by the inclusion of a second phase such as zirconia or mullite,
the latter giving the lowest creep rates. Failure at high temperatures can however be