Page 107 - Fiber Fracture

92 M.-H. Berger

Fig. I. TEM dark field image of the as received Altex fibre composed of y-alumina grains of about 20 nm
in an amorphous silicate intergranular phase.

fibres from IC1 (Birchall, 1983), 15 wt% in the Altex fibre from Sumitomo (Abe et al.,
1982) allow the sintering of the transitional forms of alumina of less than 50 nm, as
shown in Fig. 1, in a silicate intergranular phase and produce above 1100°C the crys-
tallisation of mullite grains, as illustrated in Fig. 2, with a composition ranging between
2A1203.Si02 and 3A1203.2Si02. This delays the nucleation of a-alumina to 1300"C,
the growth of which is then restricted by the presence of the mullite intergranular phase.
The a-alumina formation can be totally suppressed if enough silica is added to
consume the metastable alumina by mullite formation. 3M produces the Nextel series
of fibres having the composition of mullite. Boria addition lowers the temperature
of mullite formation, helps sintering and increases the fibre strength. Various degrees
of crystallinity can be obtained according to the amount of boria and the pyrolysis
temperature. Nextel 312 with 14% B203 is a quasi amorphous fibre (Johnson, 1981),
the high-temperature properties of which are limited by the volatilisation of boron
compounds from 1100°C. Nextel 440 contains 2% B2O3 and is composed of y-
alumina in amorphous silica. The same fibre composition, heated above the mullitisation
temperature yields fully dense crystallised mullite with 50 to 100 nm grain sizes
(Johnson et al., 1987). However, the good high-temperature creep resistance which
could be expected from the complex mullite structure is not obtained due to the presence
of an amorphous boro-silicate intergranular phase.
The effect of silica on the room-temperature properties of alumina fibres is to reduce
their overall stiffness (Esio2 x 70 GPa, E*1203 x 400 GPa) as can be seen in Fig. 3,
and to increase their room-temperature strength by avoiding the formation of large
grains (Fig. 4). This results in flexible fibres which can be used in the form of bricks
or woven cloths for thermal insulation. All these fibres have an external appearance
similar to that of glass fibres and their fracture is brittle and most often initiated from