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

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upon matrix failure.
This is the condition under which catastrophic
failure OCCUTS. These values were then corrected for porosity. In the
extreme case, it was assumed that the fibers were completely separated
from the matrix and, therefore, did not contribute to the composite
properties and that the fiber cavities acted as voids.
analogous to the behavior of CVD Sic containing 60 vol %
The behavior is
porosity.
The data in Table 11.3 and the fracture surface photographs in
Section 10 show that only the composites fabricated from carbon-coated
preforms possessed high strengths and exhibited some degree of fiber
pull-out.
The measured strengths for the majority of these samples
approached the predicted strength of 500 MPa, and the difference in the
values results from fibers that were not aligned because of the rotating
orientation of the cloth layers.
The highest strengths were recorded for
the sample with the intermediate interfacial frictional stress.
Carbon has proven to be a useful interfacial layer since the
earliest ceramic composite experiments.
Carbon coatings were applied to
fibers to prevent attack of the fibers by silicon during liquid-metal
infiltration in the fabrication of carbon-fiber-reinforced silicon
Composites.
Many of the other CVI processes require the preforms to be
rigidized prior to infiltration.
This is accomplished by impregnating
the fibrous structure with resin, which is pyrolyzed to form carbon.
Carbon is relatively stable with respect to the fibers and the
matrix.
The carbon reacts with the silica layer at the fiber surface to
form Sic and probably does not diffuse into the fibers.
The carbon
appears to bond to the fibers and not to the matrix.
The thermochemical