Page 242 - Engineered Interfaces in Fiber Reinforced Composites

molten alumina. The low viscosity of molten alumina and its high melting
temperature (-2070°C) preclude the melt spinning process so that slurry and sol-gel
spinning processes have been developed to avoid the melting step. A particular
advantage of the sol gel spinning process is the ability to control the fiber diameter
in the range of 1-7 pm.
Scanning electron microphotographs and surface roughness profiles of three
alumina fibers, PRD-166, Nextel 610 and Saphikon fibers, are shown in Fig. 5.38. It
is noted that the surface of the PRD-166 fiber is significantly rougher than the other
two fibers, which is attributed to its relatively large grain size (4.5 pm). The Nextel
610 fiber, although polycrystalline, is very smooth because of its extremely fine grain
size. It contains 0.4-0.7 wt% Fe202 and about 0.5 wt% SO2, the latter is to reduce
the final grain size. The roughness of the fiber and the relative magnitude of the
thermal expansion coefficient between fiber and matrix are the predominant factors
determining the fracture behavior of the composite involving interface debonding
and subsequent fiber pull-out. Representative properties of some alumina fibers are
given in Table 5.16.
5.5.6.2. RMC t iori hcrrric.r COLI t iiigs on A 1203 fibers
Most a-alumina fibers are not readily wetted by most metals, due to their low
surface energy, particularly if the fibers are in the form of short whiskers. The
wettability of these fibers and whiskers can be improved by a CVD process of a thin
metallic coating, such as Ni (Sutton, 1966) or Ni alloys containing active metals like
Ti (Noone et al., 1969) for a molten silver matrix. A duplex Ti-Ni coating further
promotes the wetting and improves significantly the bonding, as revealed by the
improvement in composite tensile strength. The fracture mode changes from
interfacial failure to matrix shear failure with the coated fibers. The Ti-Ni coatings
are also found to be effective for other matrices like A1 and Ni-Cr alloy (Nicholas,
1968).

Fig. 5.38. Scanning electron microphotographs of (a) PRD-166, (b) Nextel 610 and (c) Saphilkon A1203
fibers, showing different surface roughness profiles. After Chawla (199% Fig. 9.25. p. 330. Reproduced
by permission of Chapman & Hall.