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Unformatted text preview: generally used
in soft matrices, such as reinforcement of rubbers in tires, belts, and hoses. About two
decades ago polymer ﬁbers with much higher molecular orientation and without chain
folds were developed. These materials represented a signiﬁcant increase in strength and
modulus over conventional nylons and polyesters, revolutionizing the development of
stiff, strong, lightweight (all polymer) composites. Examples of these polymers are Kevlar
and Spectra. The high molecular orientation of Kevlar and other similar aramids (short
for aromatic polyamides) is achieved by dissolving the inherently stiff polymer in a solvent
so that a liquid crystalline solution is obtained. The solution is spun into a ﬁber with
extremely high axial molecular alignment. Spectra is formed by dissolving ultrahigh–
molecular weight ﬂexible polyethylene chains in a solvent to form a gel. The gel is spun
into a ﬁber and collapsed by multiple drawings at high temperatures. In both of these | e-Text Main Menu | Textbook Table of Contents 12.01.98 plm QC2 rps MP 587 pg588 [V] G2 7-27060 / IRWIN / Schaffer Part III 12.01.98 plm QC2 rps MP Properties processes, which are described in more detail in Chapter 16, chain folding is almost completely absent and molecular orientation is high. While these ﬁbers have excellent tensile
properties at room temperature, their use temperature is limited to a few hundred degrees
C or less. In addition, their axial and lateral compressive properties are extremely poor.
The most common ceramic reinforcing materials include ﬁbers of Al2 O3 and SiC as
well as SiC whiskers and particles. Other ceramic ﬁbers with good properties include
silicon nitride, boron carbide, and boron nitride. SiC ﬁbers are made using a chemical
vapor deposition process or by controlled pyrolysis of organosilane precursors, while
whiskers are obtained by vapor phase growth. These processes are discussed in Chapter 16. Much like carbon ﬁbers, the properties of ceramic ﬁbers depend on the processing
method used to fabricate them. Because ceramic ﬁbers are brittle, it is important to
produce them in small diameters to achieve ﬂexibility and good strength.
A variety of metals can be used to draw high-strength wires, which can serve as metal
ﬁbers. The most prominent metal ﬁbers include beryllium alloys—which have high
strength, high modulus, and low density—steel, and tungsten. The strengths of metal
ﬁbers are consistent and reproducible. The properties of typical carbon, polymer, ceramic, and metal ﬁbers are given in Table 14.3–1.
In summary, the ideal ﬁber material for strengthing and stiffening a matrix requires the
following attributes: low density, high tensile strength, high modulus of elasticity, high
ﬂexibility, and the ability to form a strong interface with the matrix. 14.3.3 Characteristics of Matrix Materials
Like ﬁbers, matrix materials can be polymers, ceramics, or metals. Carbon is also used
as a matrix material with carb...
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