Page 312 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Composites and Fillers 275

In comparison to polymer-intense composites, MMCs offer
No moisture absorption
Greater fi re resistance
Higher use temperatures
Greater radiation resistance
Greater stiffness and strength, and
Higher thermal and electrical conductivities

MMCs also have some disadvantages in comparison to polymer-matrix composites. These include

Higher cost
Newer and less-developed technology and scientifi c understanding
Less known long-term information
Generally more complex fabrication, and
Greater weight

As noted above, the range of fibers employed does not precisely overlap with those employed for

organic composites. Because the formation of the MMCs generally requires melting of the metal
matrix the fibers need to have some stability to relatively high temperatures. Such fi bers include

graphite, silicon carbide, boron, alumina-silica, and alumina fibers. Most of these are available as

continuous and discontinuous fibers. It also includes a number of thin metal wires made from tung-
sten, titanium, molybdenum, and beryllium.
As with organic-matrix composites, the orientation of the reinforcing material determines
whether the properties will be isotropic or oriented in a preferential direction so that the strength
and stiffness are greater in the direction of the fi ber orientation.
There are a number of differences between organic and MMCs. These include the following:
First, the metal matrix can also have considerable strength themselves so their contribution to the
overall strength is more important than for organic-matrix composites. A second difference more
often encountered for MMCs is a greater difference between the coefficient of expansions between

the reinforcing material and metal matrix. Because of the greater use temperature differences often
required for MMCs, these differences become more important. Such differences can result in large
residual stresses in MMCs that may result in yielding. A third difference is related to the relative lower

flexibility of MMCs. This leads to a greater need to be concerned with the marrying or joining of such
composite parts. Many methods of joining these composite parts have been developed. A fourth dif-
ference is the possible greater reactivity between the matrix and fiber for MMCs. This limits combina-

tions but has been overcome in many situations. One major approach is to place a barrier coating onto
the reinforcement. For example, boron carbide is applied as a barrier coating to boron fi bers, allowing
their use to reinforce titanium. Because these coatings can be “rubbed” away from usage, composites
made from these coated reinforcements should be monitored more closely and more often.
Some effort has gone into working with copper wire containing niobium and tin looking for how
to make niobium-tin alloys which is one of the better low-temperature superconductors. Such alloys
are brittle so copper was added in the hopes of creating a less-brittle material. It was noticed that the
copper-niobium-tin, and also simply copper-niobium combination gave a material with greater than
expected strength. Because copper and niobium are not miscible at low niobium concentrations, it
formed dendrites within the copper matrix allowing strong, ductile wires and rods to be drawn. As

the size of the rods became smaller, niobium filaments, about only 10 nm, about 30 atoms wide,
across formed. If cast, the thickness is 5 microns thick, much thicker than the niobium fi laments
formed from the drawing down of the copper-niobium rod. This is then not only a MMC, but a
metal–metal composite.

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