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

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reinforcement w Lll be lost. 'The ideal situation is one in which Aa = 0
or is slightly positive, therefore ensuring interfacial friction and
efficient load transfer (Figure 5.3).
For Aa > 0, the expansion of the
fibers must be less than in the matrix (crf < a,), but this condition will
place the matrix in tension. Recalling that the thermally induced strain
cannot exceed the ultimate strain of the matrix, the minimum value of af
is restricted to
where emu is the failure strain of the matrix. Therefore, the thermal
expansion coefficient of the fibers must be in the range
to produce a useful Composite material.
5.4 Thermochemical Considerations
Chemical reactions at the fiber-matrix interface have been shown to
play a major role in reducing the strength of a composite. The formation
of brittle zones in ceramic fiber-reinforced metal-matrix composites
generally decreases the strength of the material. Oxidation
embrittlement has been noted in glass-matrix composites containing
untreated Sic fibers (19,20,63,64).
Short-term exposure of ceramic
fiber-reinforced glasses to elevated temperatures in air resulted in
increased fiber-matrix bonding and, ultimately, brittle behavi-or upon
testing at room temperature.