Composite Materials

It is a truism that technological development depends on advances in the field of materials. One does not have to be an expert to realize that a most advanced turbine or aircraft design is of no use if adequate materials to bear the service loads and conditions are not available. Whatever the field may be, the final limitation on advancement depends on materials. Composite materials in this regard represent nothing but a giant step in the ever constant endeavor of optimization in materials.

Reinforcements need not necessarily be in the form of long fibers. One can have them in the form of particles, flakes, whiskers, discontinuous fibers, continuous fibers, and sheets. It turns out that the great majority of materials is stronger and stiffer in the fibrous form than in any other form: thus the great attraction of fibrous reinforcements. Specifically, in this category, we are most interested in the so-called advanced fibers which possess very high strength and very high stiffness coupled with a very low density. The reader should realize that many naturally occurring fibers can be and are used in situations involving not very high stresses [1,2]. The great advantage in this case, of course, is that of low cost. The vegetable kingdom is, in fact, the largest source of fibrous materials. Cellulosic fibers in the form of cotton, flax, jute, hemp, sisal, and ramie, for example, have been used in the textile industry, while wood and straw have been used in the paper industry. Other natural fibers, such as hair, wool, and silk, consist of different forms of protein. Any discussions of such fibers are beyond the scope of this book. The interested reader, however, is directed to a good review article by Meredith [3].

A brief description of the various matrix materials, polymers, metals, and ceramics, is given in this chapter. We emphasize the characteristics that are relevant to composites. The reader should consult the references listed under Suggested Reading for greater details regarding any particular aspect of these materials.

We can define an interface between any two phases, say fiber and matrix, as a bounding surface where a discontinuity of some kind occurs. The discontinuity may be sharp or gradual. In general, the interface is an essentially bidimensional region through which material parameters, such as concentration of an element, crystal structure, atomic registry, elastic modulus, density, and coefficient of thermal expansion, change from one side to another. Clearly, a given interface may involve one or more of these items. Most of the physical, chemical, or mechanical discontinuities listed are self-explanatory. The concept of atomic registry perhaps needs some further elaboration. In terms of the atomic registry types, we can have a coherent, semicoherent, or incoherent interface. A coherent interfaces is one where atoms at the interface form part of both the crystal lattices; that is, there exists a one-to-one correspondence between atomic sites on the two sides of the interface. In general, a perfect atomic registry does not occur between unconstrained crystals. Rather, coherency at the interface invariably involves an elastic deformation of the crystals. A coherent interface, however, has a lower energy than an incoherent one. A classic example of coherent interface is the interface between G-P zones and the aluminum matrix.

Polymer matrix composites (PMCs) have established themselves as engineering structural materials, not just as laboratory curiosities or the cheap stuff for making chairs and tables. This came about not only because of the introduction of high-performance fibers such as carbon, boron, and Kevlar, but also because of some new and improved matrix materials (see Chap. 3). Nevertheless, glass fiber reinforced polymer composites represent the largest class among PMCs. We discuss the carbon fiber reinforced polymer composites separately, because of their great importance, in Chap. 8. In this chapter we discuss polymer composite systems containing glass, Kevlar, and boron.

The boron fiber reinforced 6061 aluminum matrix composite system was developed in the 1960s. Unidirectionally solidified eutectics with an aligned two-phase micro-structure were produced about the same time. Carbon fiber reinforced metallic composites were successfully made in the 1970s. With the availability of a wide variety of SiC and Al₂O₃ reinforcements, the research activity in the area of metal matrix composites increased tremendously the world over. Among the important MMC systems, we can include the following:

Ceramic materials in general have a very attractive package of properties: high strength and high stiffness at very high temperatures, chemical inertness, low density, and so on. This attractive package is marred by one deadly flaw, namely, an utter lack of toughness. They are prone to catastrophic failures in the presence of flaws (surface or internal). They are extremely susceptible to thermal shock and any damage done to them during fabrication and/or service. It is therefore understandable that on overriding consideration in ceramic matrix composites (CMCs) has been to toughen the ceramic matrices by incorporating fibers in them and thus exploit the attractive high-temperature strength and environmental resistance of ceramic materials without risking a catastrophic failure.

Carbon fiber composites started out in the 1950s and attained the status of a mature structural material in the 1980s. Not unexpectedly, the aerospace industry has been the biggest user of carbon fiber reinforced polymer matrix composites, followed by the sporting goods industry. The availability of a large variety of carbon fibers (Chap. 2) and an equally large variety of polymer matrix materials (Chap. 3) made it easier for carbon fiber reinforced polymer matrix composites to assume the important position that they have. This is the reason we devote a separate chapter to this class of composites. Epoxy is the most commonly used polymer matrix with carbon fibers. Polyester, polysulfone, polyimide, and thermoplastic resins are also used. Carbon fibers are the major load-bearing components in most such composites. There is, however, a class of carbon fiber composites wherein the excellent electrical conduction characteristics of carbon fibers are exploited; for example in situations where static electric charge accumulation occurs, parts made of thermoplastics containing short fibers are frequently used. As we did for other composite systems, we describe the fabrication, properties, interfaces, and applications of carbon fiber reinforced polymer matrix composites. A special emphasis is given to carbon/ carbon composites, an important subclass.

Multifilamentary composite superconductors started becoming available in the 1970s. These are niobium based (Nb-Ti and Nb₃Sn) superconductors. The record high temperature at which a material became superconductor was 23 K and was set in 1974. In 1987, there started appearing reports of superconductivity at temperatures up to 90 or 100 K in samples containing lanthanum, copper, oxygen, and barium or another Ha metal. These new ceramic superconductors have layered perovskite body-centered tetragonal structure and, not surprisingly, are very brittle. There remains an extremely large gap to be bridged between producing a small sample for testing in the laboratory and making a viable commerical product. The Nb-Ti system took 15–20 years between the discovery and the commercial availability. The new high-temperature oxide superconductors hold a great promise and it is quite likely eventually they will also be made into some kind of composite superconductors. Thus, it is quite instructive to review the composite materials aspects of niobium based superconductors. But first a short introduction to the subject of superconductivity is in order.

Laminated fibrous composites are made by bonding together two or more laminae. The individual unidirectional laminae or plies are oriented in such a manner that the resulting structural component has the desired mechanical and/or physical characteristics in different directions. Thus, one exploits the inherent anisotropy of fibrous composites to design a composite material having the appropriate properties.

We discussed in Chap. 10 the prediction of elastic and thermal properties, knowing the component properties. A particularly simple but crude form of this is the rule of mixtures, which works reasonably well for predicting the longitudinal elastic constants. Unfortunately, the same cannot be said for the strength of a fiber composite. It is instructive to examine why the rule of mixtures approach does not work for strength properties.