Page 100 - Mechanical Behavior of Materials

Section 3.7 Composite Materials 101

Table 3.12 Representative Types and Examples of Composite Materials

Reinforcing Type Matrix Type Example Typical Use
(a) Particulate composites
Ductile polymer Brittle polymer Rubber in Toys, cameras
or elastomer polystyrene
Ceramic Ductile metal WC with Co metal Cutting tools
binder
Ceramic Ceramic Granite, stone, and Bridges, buildings
silica sand in
Portland cement

(b) Short-fiber, whisker composites
Strong fiber Thermosetting Chopped glass in Auto body panels
plastic polyester resin
Ceramic Ductile metal SiC whiskers in Al alloy Aircraft structural
panels

(c) Continuous-fiber composites
Ceramic Thermosetting Graphite in epoxy Aircraft wing flaps
plastic
Ceramic Ductile metal Boron in Al alloy Aircraft structure
Ceramic Ceramic SiC in Si 3 N 4 Engine parts
(d) Laminated composites
Stiff sheet Foamed polymer PVC and ABS sheets Canoes
over ABS foam core
Composite Metal Kevlar in epoxy Aircraft structure
between Al alloy
layers (ARALL)

microstructure of which has already been illustrated in Fig. 3.19. Another ductile particle composite
made from two polymers is shown broken open in Fig. 3.24.
Particles of a hard and stiff (high E) material added to a ductile matrix increase its strength and
stiffness. An example is carbon black added to rubber. As might be expected, hard particles generally
decrease the fracture toughness of a ductile matrix, and this limits the usefulness of some composites
of this type. However, the composite may still be useful if it has other desirable properties that
outweigh the disadvantages of limited toughness, such as the high hardness and wear resistance of
cemented carbides.
If the hard particles in a ductile matrix are quite small and limited in quantity, the reduction
in toughness is modest. In a metal matrix, a desirable strengthening effect similar to that of
precipitation hardening can be achieved by sintering the metal in powder form with ceramic particles
of size on the order of 0.1 μm. This is called dispersion hardening. The volume fraction of particles
seldom exceeds 15%, and the amount may be as small as 1%. Aluminum reinforced in this manner
with Al 2 O 3 has improved creep resistance. Tungsten is similarly dispersion hardened with small