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

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similar samples show the film on both the fiber surface and the pull-out
grooves in the matrix.
This layer appears as a glassy translucent film
and can be seen in the micrographs in Figure 10.6.
The flat smooth fracture surfaces suggest that the silica layer
bonds the fibers and matrix too strongly. Indentation measurements
confirmed the high interfacial frictional stresses for samples CVI-178
and CVI-173. In some cases, the loads caused splitting of the fiber and
cracking of the surrounding matrix [Figure 10.7(a) and ( b)]. The cracks
generated during indentation propagated through the matrix to neighboring
fibers. There was no evidence of crack deflection at these fiber-matrix
interfaces (i.e., cracks passed undisturbed across the boundary).
Although the interfacial shear stress for sample 176 [Figure 10.7(c)]
was much lower than the measured values for the previously described
composites, the flexure strengths for the sample showed no improvement.
The silicon carbide intermediate coating was deposited at a lower
temperature, which may have resulted in a very-low-density coating. If
this is true, the coating could enhance debonding at the interface. The
fibers, however, provided no reinforcement, as demonstrated by the low
strengths.
The loss of fiber strength was evident in that continuous
fibers coated with silicon carbide from methylsilane could not be tensile
tested because the coated tows failed during heat treatment.
The intense bonding and fiber-property degradation are a result of
the chemical reactions that occur within the body of the fiber and at the
fiber-matrix interface.
The vapor-deposited Sic coating understandably
bonds strongly to the underlying silica layer.
Thermochemical analysis
of the interactions that would likely occur during the various processing