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

5. COMPOSITE DESIGN
5.1 Introduction
Composites are produced when two materials are joined together to
give a combination of properties that cannnot be attained in the original
materials.
Composite materials are designed to give beneficial combina-
tions of strength, stiffness, high-temperature performance, weight,
thermal conductivity, or corrosion resistance. The properties of the
product are entirely dependent on the constituents and the physical,
chemical, and mechanical interactions between the phases. Fiber-matrix
combinations are chosen to provide the properties and stability defined
by the application; however, a variety of factors from processing to
chemical and mechanical compatibility must be considered in the design
of a composite system.
A summary of the mechanical and physical
properties of selected fibers, whiskers, and matrix materials is given
in Table 5.1 (3,20,45-48).
For a particular ceramic fiber and ceramic matrix combination to
perform well as a composite, several compatibility requirements must be
satisfied. The thermal expansion mismatch between the fibers and matrix
cannot be too
great because extensive cracking and damage will occur
upon temperature cycling.
The relative moduli of the components
determine the extent to which cracking can be hindered.
Generally, the
fibers are stronger than the matrix materials and the goal in the
fabrication of the composite is to have the fiber bear a greater portion
of the load. Load distribution is a function of the components, and thus
must be considered in the choke of matrix and reinforcement.
Finally,
the constituents must be chemically compatible with each other and their
external environment at temperatures of fabrication and use.
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