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

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calculations of the matrix deposition onto the carbon layer reveals the
increased presence of carbon at the interface, thus supporting the
observation.
The remainder of the samples exhibited brittle fracture with little
evidence of fiber pull-out or strength after matrix failure.
The
interfacial coatings reacted chemically with the fibers and/or matrix to
either degrade fiber strength, enhance fiber-matrix bonding, or both.
The samples having high interfacial frictional stress had the lowest
strengths, and fracture surfaces were flat and smooth. Although the
other samples had lower measured interfacial stresses, the strengths of
these specimens were relatively low and little fiber pull-out was
observed.
The strengths of the specimens were within a few percent of
the predicted strength of an 80% dense composite reinforced with
extremely weak fibers (uf = 0).
One must conclude that the interfacial coatings reacted with the
fibers and decreased their strength. The various elements of the
coatings must diffuse into the fibers and disrupt composition and
structure within the bulk of the material. The fibers consist of p-Sic
crystallites, amorphous Si02, and free carbon. The diffusing elements
could react with these compounds to form new compounds or to increase (or
decrease) the concentration of these compounds. The diffusion rates of
relevant elements in silica and silicon carbide are given in Figure 11.5
(144-149). These kinetic factors can be combined with thermochemical
calculations to determine the possible chemical reactions within the
Nicalon fibers .