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

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was derived from extensive testing of alumina specimens with known levels
of porosity.
In the equation, p is the volume fraction of porosity and n
is a geometrical factor between 4 and 7.
The effect of voids on the
Young's modulus of ceramics has been evaluated in similar experiments.
The voids are assumed to possess zero Young's modulus, thereby altering
the elastic properties of a material.
The influence of closed porosity
in a continuous rinatrix is described by the expression
E, = Eo(l - 1.9p + 0.9p 2 ) ,
(9)
where, again, p is the volume fraction of porosity,
E,
is the porosity
corrected modulus, and Eo is the modulus for a fully dense material.
5.3 Lhg.rmomechanica1 Compatibility
Because ceramics are inherently brittle and thus lack ductility,
efforts iiiust be maintained to minimize residual thermal stresses in a
ceramic composite by appropriately matching fiber and matrix thermal
expansions.
Differences irn thermal expansion between fiber and matrix
can cause severe damage, and this factor has rendered a nuinbc1r of
potential composite systems unacceptable for high-temperature use.
Thermal expansion mismatches result in very large stresses in the fiber
and matrix, and thermal cycling can lead to fiber or matrix failure.
Thermal expansion eEfects can lead to liigh stress levels at the
fiber-matrix interface (5,57-59).
The effect is dependent on the degree
of bornding that is present at the fiber-matrix interface.
If the bond is
strong, the stresses can fracture the fibers or matrix; if the bonding is
weak, separation of the fibers and matrix will occur, resulting in a loss