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Unformatted text preview: ﬁbers.
Boron ﬁbers are used for stiffening aluminum matrices. Since boron is inherently
brittle, it is chemically deposited on a tungsten (W) wire or a carbon-coated glass ﬁber.
The ﬁbers containing the W ﬁlament are expensive, but have superior properties compared with glass-ﬁlament ﬁbers. Since the chemical vapor deposition process (discussed
in Chapter 16) often results in surface defects, the surface of boron ﬁbers is polished to
remove defects and is also coated with a thin layer of SiC to produce compressive surface
stresses and pacify the surface, as shown in Figure 14.3–4. The surface treatments considerably enhance the fracture strength of B ﬁbers; however, they also add to their cost.
Properties of boron ﬁbers are given in Table 14.3–1.
Carbon ﬁbers have a highly distorted and defective graphitic-like structure. The atoms
within the basal planes are covalently bonded. Parallel basal planes are joined together by | v v 586 iq | e-Text Main Menu | Textbook Table of Contents pg587 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 Composite Materials σθ
(+) 0.7 ... 0.8 kN-mm–2 r
(–) 0.3 ... 0.5 kN-mm–2
SiC jacket (–) 1.0 ... 1.4 kN-mm–2 Tungsten core σθ Tungsten-boride
(a) σr Mantle (b) FIGURE 14.3–4 Schematic cross sections of a boron ﬁber. (a) The ﬁber is a composite consisting of a series of
concentric layers. (b) The residual stress pattern across a section of the boron ﬁber. In this view the intermetallic layer
and SiC jacket are not shown. (Source: K. K. Chawla, Composites Material Science and Engineering, 1987, SpringerVerlag, New York, Reprinted with permission of Springer-Verlag, New York Publishers.) | v v weak van der Waals bonds. The bonds perpendicular to the basal planes are secondary
bonds. Carbon ﬁbers are produced so that the basal plane lies along the ﬁber axis.
Therefore, the elastic modulus along the ﬁber axis can be as high as 1000 GPa, as shown
in Figure 14.3–3b. In the transverse direction (i.e., the direction perpendicular to the ﬁber
axis), the modulus can be as low as 35 GPa.
The mechanical, chemical, electrical, and other physical properties of carbon ﬁbers
vary widely depending on the raw material and the processing treatment. The raw material
for carbon ﬁber is called a precursor and consists of a ﬁber itself. While practically any
ﬁber can be transformed with heat into carbon ﬁbers, only a few precursors are commercially viable. Polyacrylonitrile may be used to make high-modulus, high-strength ﬁbers
using pyrolysis, which is described in Chapter 16. In addition, carbon ﬁbers are made on
a commercial scale using pitch. Pitch is a product of oil reﬁning (asphalt pitch) or of
combining coal with appropriate ﬂuids (coal tar pitch). The pitch is converted into a liquid
crystalline material, melt-spun into ﬁbers, and then pyrolyzed.
The ﬁrst synthetic polymer reinforcing ﬁbers were nylon and polyester. These ﬁbers
have reasonable strength and modulus, giving good toughness. They are...
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