The Structure of Materials
1.4 STRUCTURE OF COMPOSITES
1.4.5 The Composite Reinforcement
In Section 1.4.2, we described several classification schemes for composites, including one that is based upon the distribution of the constituents. For reinforced composites, this scheme is quite useful, as shown in Figure 1.75. In reinforced composites, the reinforcement is the structural constituent and determines the internal structure of the composite. The reinforcement may take on the form of particulates, flakes, lamina, or fibers or may be generally referred to as “filler.” Fibers are the most common type of reinforcement, resulting in fiber-matrix composites (FMCs). Let us examine some of these reinforcement constituents in more detail.
1.4.5.1 Fiber-Matrix Composites. As shown in Figure 1.75, there are two main classifications of FMCs: those with continuous fiber reinforcement and those with discontinuous fiber reinforcement. Continuous-fiber-reinforced composites are made from fiber rovings (bundles of twisted filaments) that have been woven into two- dimensional sheets resembling a cloth fabric. These sheets can be cut and formed to a desired shape, orpreform, that is then incorporated into a composite matrix, typically a thermosetting resin such as epoxy. Metallic, ceramic, and polymeric fibers of specific compositions can all be produced in continuous fashions, and the properties of the
Table 1.30 Common Ceramics for Ceramic-Matrix Composites
Matrix Material
Density (g/cm3)
Use Temperature (◦C)
Alumina, Al2O3 4.0 ∼1000
Glass ceramics 2.7 900
Si3N4 3.1 ∼1300
SiC 3.2 ∼1300
Large- particle
Dispersion- strengthened
Particle-reinforced Fiber-reinforced Structural Composites
Aligned Randomly oriented Continuous
(aligned)
Discontinuous (short)
Laminates Sandwich panels
Figure 1.75 Types of reinforced composites.
resulting composite are highly dependent not only on the type of fiber and matrix used, but also on the processing techniques with which they are formed. We will discuss continuous-fiber-reinforced composites in more detail in Sections 5.4 and 7.4.
Discontinuous-fiber-reinforced composites are much more widely used, and there are some general underlying principles that affect their overall properties. Four main factors contribute to the performance level of a fiber in discontinuously reinforced FMCs. The first factor is fiber orientation. As shown in Figure 1.76, there are several ways that short fibers can be oriented within the matrix. One-dimensional reinforce- mentoccurs when the fibers are oriented along their primary axis. This offers maximum mechanical strength along the orientation axis, but results in anisotropic composites; that is, the mechanical and physical properties are not the same in all directions. Pla- nar reinforcement occurs with two-dimensional orienting of the fibers, as often occurs with woven fabrics. The fabric, as is common in woven glass fibers, is produced in sheets, and it is laid down (much like a laminate) to produce a two-dimensional reinforcement structure.Three-dimensional reinforcement results from the random ori- entation of the fibers. This creates an isotropic composite, in which the properties are the same in all directions, but the reinforcing value is often decreased compared to the aligned fibers.
The second factor that affects performance in discontinuously reinforced FMCs is fiber length. This has an effect primarily on the ease with which the composite can be manufactured. Very long fibers can create difficulties with methods used to create discontinuously reinforced FMCs and can result in nonuniform mechanical properties.
The third factor is also related to fiber geometry, namely, the fiber shape. Recall that the
(a) (b)
(c)
Figure 1.76 Types of fiber reinforcement orientation (a) one-dimensional, (b) two-dimensional, and (c) three-dimensional.
STRUCTURE OF COMPOSITES 107
definition of a fiber is a particulate with length greater than 100µm and an aspect ratio greater than 10:1. This definition allows for a great deal of flexibility in the geometry of the fiber. For example, aspect ratio can vary widely; many reinforcement filaments have aspect ratios much larger than 10:1. It is also not necessary that the fiber cross section be exactly circular. Hexagonal, ellipsoidal, and even annular (hollow fiber) cross sections are quite common. Finally, it is also not necessary that the fiber, even those with circular cross sections, be exactly cylindrical. “Dumbbell”-shaped fibers are very common, and even preferable, since mechanical stresses tend to concentrate at the fiber ends. The dumbbell shape helps distribute these stresses.
The final factor affecting the reinforcement performance is its composition. Chem- istry affects properties, and strength is usually the most important property of a reinforcing fiber. Though we will concentrate on mechanical properties of materials in Chapter 5, it is useful at this point to familiarize yourself with some of the common fiber reinforcements, as summarized in Table 1.31. It is worth noting that oftentimes the important design consideration for reinforcement materials (or the matrix, for that matter) is not the absolute value of a particular design criterion, such as tensile strength
Table 1.31 Selected Properties of Some Common Reinforcing Fibers
Fiber
Density (g/cm3)
Melting Point (◦C)
Specific Modulus (GPa·cm3/g)
Specific Strength (MPa·cm3/g)
Aluminum 2.70 660 27 230
Steel 7.87 1621 25 530
Tantalum 16.6 2996 12 37
Titanium 4.72 1668 24 410
Tungsten 19.3 3410 21 220
Boron 2.30 2100 192 1500
Beryllium 1.84 1284 165 710
Molybdenum 10.2 2610 35 140
Aluminum oxide 3.97 2082 132 170
Aluminosilicate 3.90 1816 26 1060
Asbestos 2.50 1521 60 550
Beryllium carbide 2.44 2093 127 420
Beryllium oxide 3.03 2566 116 170
Carbon 1.76 3700 114 1570
Graphite 1.50 3650 230 1840
E-glass 2.54 1316 28 1360
S-glass 2.49 1650 34 1940
Quartz 2.20 1927 32 407
SiC (on tungsten) 3.21 2316 140 1000
Si3N4 2.50 1900 100 1344
BN 1.91 3000 (decomp.) 47 722
ZrO2 4.84 2760 71 427
Wood 0.4 – 0.8 — 17 —
Polyamide (Kevlar) 1.14 249 2.5 730
Polyester 1.40 249 2.9 490
Polypropylene 0.9 154 1.8 77
or modulus, but the value per unit weight, such asspecific strength orspecific modu- lus. These values are listed in Table 1.31 rather than absolute values to illustrate this point. This fact is extremely important for many applications, such as automotive and aerospace composites, for which weight savings is paramount. Note also that reinforc- ing fibers come from all materials classes. Let us examine some of the more common fibers in more detail.
Organic fibers generally have very low specific gravities, so they are attractive for applications where strength/weight ratio is important. They are also very common for textile applications, since some organic fibers are easily woven. Carbon fibers offer excellent thermal shock resistance and a very high strength-to-weight ratio. There are two general types of carbon fibers, PAN-based carbon fibers and pitch-based carbon fibers. PAN-based carbon fibers are manufactured by the pyrolysis ofpolyacrylonitrile (PAN), as illustrated in Figure 1.77. The PAN is polymerized and fibers spun from the pre-polymer. The PAN fibers are then pyrolyzed to remove the hydrogens and form benzene-ring structures.Pitch-based fibers are produced in a similar pyrolysis process of a precursor fiber, except that the precursor in this case is pitch. Pitch is actually a liquid crystalline material, often calledmesophase pitch, and is composed of a complex mixture of thousands of different species of hydrocarbons and heterocyclic molecules.
It is the residual product of petroleum refining operations.
Wood fibers are technically organic, and though we do not discuss wood as a separate materials class in this text, it is an important structural material. Wood fibers, often in the form of wood flour, possess a variety of properties depending upon the type of tree from which they are derived, but are used extensively in low-cost composites. Wood fiber has a good strength/weight ratio and provides a use for recycled paper products.
Turning to synthetic organic fibers, we see that polyamide fibers, such as Kevlar, offer excellent specific mechanical properties. Kevlar is used in many applications where high toughness is required, such as ropes and ballistic cloths. In addition to polyamides, polyesters, and polypropylene fibers listed in Table 1.31, nylon and polyethylene are other common polymeric fibers used for composites. In all cases, an added attraction of synthetic fibers is their chemical inertness in most matrix materials.
Glass fibers are the most common reinforcing fiber due to their excellent combi- nation of mechanical properties, dielectric properties, thermal stability and relatively low cost. As a result, there are many different types of silicate glass fibers, all with varying properties designed for various applications (see Table 1.32). The majority
C CH
CH2 CH C N
CH2 N
C CH
CH2 CH C N
CH2 CH N
C
C C
C C C N
C C C N
C C C N
Figure 1.77 Pyrolysis of polyacrylonitrile (PAN) to form carbon fibers.
STRUCTURE OF COMPOSITES 109
Table 1.32 Composition of Commercial Glass Fibers
Composition (wt%)
SiO2 Al2O3 Fe2O3 B2O3 ZrO2 MgO CaO Na2O K2O Li2O TiO2 F2
A-glass (typical)
73 1 0.1 4 8 13 0.5
E-glass (range)
52–56 12–16 0–0.5 8–13 0–6 16–25 ←<1 total→ 0 0–1.5
AR glass (range)
60–70 0–5 15–20 0–10 10–15 0–5
C-glass (range)
59–64 3.5–5.5 0.1–0.3 6.5–7 2.5–3.5 13.5–14.5 8.5–10.5 0.4–0.7 S and R
glasses (range)
50–85 10–35 ←4–25 total→ 0
Table 1.33 Some Properties of Commercial Glass Fibers
Working Coefficient
Liquidus Temperature of Thermal Young’s
Strength(MN·m−2) Temperature (η=100 Pa s) Density Expansion Refractive Modulus Undamaged Strand from
(◦C) (◦C) (g cm−3) (◦C−1) Index (GN·m−2) Filament Roving
A-glass 1140 1220 2.46 7.8×10−6 1.52 72 3500
E-glass 1400 1210 2.54 4.9×10−6 1.55 72 3600 1700–2700
AR glass 1180–1200 1280–1320 2.7 7.5×10−6 1.56 70–75 3600 1500–1900
C-glass ∼2.5
S- and R-glasses ∼2.5 ∼85 ∼4500 2000–3000
component in all of these glass fibers is SiO2, with various amounts of intermedi- ates and modifiers added to improve strength, chemical resistance and temperature resistance. Some properties of these glass fibers are listed in Table 1.33. The two most common types of silicate glass fibers are E-glass (for “electronic” glass) and S-glass (for “strength” glass). S-glass was developed to provide improved strength in comparison to E-glass, while maintaining most of the same properties as E-glass.
Both glass filaments are widely used in polymer–matrix composites, especially with epoxy-based matrixes. The result is a glass-fiber-reinforced (GFR) composite that is used extensively in automotive, aerospace, marine, electronics, and consumer prod- uct industries.
As a class, ceramic fibers offer better thermal resistance than glass fibers, and they are the preferred reinforcement in high-temperature structural composites. There are a number of commercial oxide-based fibers available, such as Saffil (SiO2/Al2O3), Nextel (SiO2/B2O3/Al2O3), Fiberfrax (SiO2/Al2O3), and Kaowool (SiO2/Al2O3), as well as nearly pure single-component metal oxide fibers of Al2O3 and ZrO2. There are also a number of slag-based fibers of varying composition, based upon SiO2, Al2O3, MgO, and CaO, which are recovered from smelting operations. The more refractory fibers consist of nitrides and carbides, such as Si3N4 and SiC. These fibers are usually produced by more exotic techniques. For example, Si3N4fibers are produced from polymeric precursors such as polysilazanes (Si–N), polycarbosilanes (Si–CH2), or
polysilanes (Si–Si). Silicon carbide fibers are currently substrate-based and are formed by the deposition of SiC on a metallic filament such as tungsten or carbon. Silicon carbide whiskers can also be produced by the pyrolysis of polycarbosilane precursors.
Finally, metallic fibers find some limited applications as reinforcement in com- posites. They are generally not desirable due to their inherently high densities and because they present difficulties in coupling to the matrix. Nonetheless, tungsten fibers are used in metal-matrix composites, as are steel fibers in cement composites. There is increasing interest inshape memory alloy filaments, such as Ti–Ni (Nitanol) for use in piezoelectric composites. We will discuss shape-memory alloys and nonstructural composites in later chapters of the text.
1.4.5.2 Particulate Composites. Particulate composites encompass a wide range of materials, from cement reinforced with rock aggregates (concrete) to mixtures of ceramic particles in metals, calledcermets. In all cases, however, the particulate com- posite consists of a reinforcement that has similar dimensions in all directions (roughly spherical), and all phases in the composite bear a proportion of an applied load. The percentage of particulates in this class of composites range from a few percent to 70%.
One important class of particulate composites isdispersion-hardened alloys. These composites consist of a hard particle constituent in a softer metal matrix. The particle constituent seldom exceeds 3% by volume, and the particles are very small, below micrometer sizes. The characteristics of the particles largely control the property of the alloy, and a spacing of 0.2–0.3µm between particles usually helps optimize properties.
As particle size increases, less material is required to achieve the desired interparticle spacing. Refractory oxide particles are often used, although intermetallics such as AlFe3 also find use. Dispersion-hardened composites are formed in several ways, including surface oxidation of ultrafine metal powders, resulting in trapped metal oxide particles within the metal matrix. Metals of commercial interest for dispersion-hardened alloys include aluminum, nickel, and tungsten.
A cermet is a particulate composite similar to a dispersion-hardened alloy, but consists of larger ceramic grains (cer-) held in a metal matrix (-met) (see Figure 1.78).
The refractory particulates can be from the oxide category, such as alumina (Al2O3),
Refractory Particle
(a) Cermet (b) Dispersion hardened matrix Matrix
Figure 1.78 Comparison of (a) cermet and (b) dispersion-hardened alloy. Reprinted, by per- mission, from M. Schwartz,Composite Materials Handbook, 2nd ed., p. 1.32. Copyright1992 by McGraw-Hill.
STRUCTURE OF COMPOSITES 111
magnesia (MgO), or thoria (ThO2), or from the carbide category, like tungsten carbide (WC), chromium carbide (Cr3C2) or titanium carbide (TiC). Cermets are formed by traditional powder-metallurgical techniques, although bonding agents must sometimes be added to improve bonding between the ceramic particulate and the metallic matrix.
Cermets are used in a number of applications, including (a) fuel elements and control rods in the nuclear industry, (b) pulse magnetrons, and (c) cutting tools.
Finally, metal- and resin-bonded composites are also classified as particulate com- posites.Metal-bonded compositesincluded structural parts, electrical contact materials, metal-cutting tools, and magnet materials and are formed by incorporating metallic or ceramic particulates such as WC, TiC, W, or Mo in metal matrixes through traditional powder metallurgical or casting techniques. Resin-bonded composites are composed of particulate fillers such as silica flour, wood flour, mica, or glass spheres in phenol- formaldehyde (Bakelite), epoxy, polyester, or thermoplastic matrixes.