Characterization and control of the fiber-matrix interface in ceramic ...

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The resultant fibers are composed of approximately 55 wt %
silicon,
30 wt % carbon, and 15 wt % oxygen. These elements are present as
60 wt % /?-Sic, 30 wt % amorphous Si02, and 10 wt % amorphous free carbon,
and the fibers have a measured density of 2.55 g/cm3. The oxygen content
is a consequence of the curing cycle and is partially responsible for the
thermal instability of the Nicalon Si-C-0 fibers.
The mechanisms of fiber degradation have been a topic of debate
for a number of years (119-135).
The manufacturer quotes a strength of
2500 MPa for the material; however, extensive evaluation has shown the
ultimate strength of fiber tows to be about 2100 MPa (see Table 10.3).
Heating the fibers in any atmosphere to temperatures above 1075 K results
in a reduction in fiber strength. This is the major disadvantage of the
Nicalon fibers, and this degradation of properties has been attributed to
factors such as grain growth, carbothermal reduction of Si02 to form Sic
and CO gas, composition changes, etc.
The composition and structure of the fibers have been analyzed using
many analytical techniques (119-134). From the investigations, the
fibers appear to be composed of ultrafine 1.7-nm grains of /?-Sic bonded
together by an amorphous silica matrix. Nuclear magnetic resonance has
revealed the presence of bonding that is characteristic of silicon
oxycarbide, suggesting that this material acts as a transition layer at
the surface of the grains. Others (135) have also hypothesized that the
Sic particles are surrounded by a carbon layer. Regardless, the Nicalon
fibers are damaged by heating to elevated temperatures, and many factors
exacerbate the problem. The strength of Nicalon fibers subjected to a
variety of treatments was given in Table 10.3.