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

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predicted weight loss was 3%.
The flexure results confirmed this
suspicion.
Only the flexure bars cut from the edge closest to the
original cut surface exhibited the expected low strength.
The oxidation
temperature was kept low to minimize possible matrix damage.
This did
not allow for the total removal of the carbon layer even though pure
oxygen was used to increase the oxidation rate.
However, the few specimens did yield useful results. The flexure
strengths of the treated and unoxidized specimens are listed in
Table 10.1.
The oxidized specimens possessed lower matrix fracture
strengths and decreased ultimate strength compared with the untreated
specimens from the composite sample.
Representative flexure curves and
photographs of fracture surfaces for oxidized and untreated specimens are
shown in Figures 10.16 and 10.17.
10.4.5 Boron coatine;
The presence of a boron coating was verified using X-ray line scan
data (Figure 10.8).
The composite exhibited low flexure strength and
little strain-to-failure. This is depicted in the fracture surfaces and
the load-displacement curve in Figure 10.18(a).
A small degree of fiber
pull-out is evident in the SEM photographs, and a closer examination of
the fracture surfaces showed the fiber pull-out grooves to be rough and
have crystalline appearance (Figure 10.19). A number of fibers also
possessed a reaction zone noted by discoloration and an inner boundary
line.
Thermodynamic evaluation of the coating processes and the stability
of the boron in contact with the fibers or matrix predicts that a series
of reactions may develop at the interfaces (Table 10.7). Some of these