Composite Materials, 5

Structural materials can be divided into four basic categories as metals,
polymers,
ceramics, and composites. Composites, which consist of two or
more separate materials combined in a structural unit, are typically made
from various combinations of the other three materials. In the early days of
modern man-made composite materials, the constituents were typically
macroscopic. As composites technology advanced over the last few decades,
the constituent materials, particularly the reinforcement materials, steadily
decreased in size. Most recently, there has been considerable interest in
“nanocomposites” having nanometer-sized reinforcements such as carbon
nanoparticles, nanofibers, and nanotubes, because of the extraordinary
properties of these materials.
The relative importance of the four basic materials in a historical context
has been presented by Ashby that clearly shows the steadily increasing importance of polymers, composites,
and ceramics and the decreasing role of metals. Composites are generally
used because they have desirable properties that cannot be achieved by any
of the constituent materials acting alone. The most common example is the
fibrous composite consisting of reinforcing fibers embedded in a binder or
matrix material. Particle or flake reinforcements are also used, but they are
generally not as effective as fibers.