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

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steps demonstrates that silicon present in the gas phase during the
fiber-coating process could react with the excess carbon in the fiber to
form silicon carbide (Table 10.4).
The reaction disturbs the chemistry
and morphology of the fiber, which results in mechanical property
degradation.
10.4.2 Elemental silicon
The presence of the 0.17-pm silicon coating was verified using
X-ray line scan data obtained using a JEOL 733 electron microprobe
(Figure 10.8).
The flexure specimens exhibited extremely low strength
and brittle fracture and no evidence of fiber pull-out or reinforcement.
This behavior is displayed in the fracture surfaces and in the loaddisplacement
curve in Figure 10.9.
Indentation measurements demonstrated that the silicon layer
produced extremely high interfacial bonding in the sample. A load of
3 N was required to displace the fibers, and in most tests, the loads
heavily damaged the fiber and the surrounding matrix. Extensive cracking
of the matrix and crushing of the fibers suggests that the interfacial
stresses were approaching the useful limits of the indentation method
(Figure 10.10).
As with the previous specimens, thermochemical examination of the
system suggests the silicon can diffuse into the fiber during interlayer
deposition or during further processing. The silicon can react with the
free carbon to form Sic, which can damage the integrity of the
reinforcement. Tensile testing to determine the strength of the siliconcoated
fiber tows was impossible because the specimens failed during heat
treatment.