from their early role as primarily inexpensive commodity materials into the value-added, high-technology sector in which specific properties can be exploited for novel performance. Examples include piezoelectric polymers used for ultrasensitive sonar, high-temperature flame-retardant fabrics, nonstick surfaces, reusable pressure-sensitive adhesives, and encapsulants for drug delivery.

Structural polymers are already in widespread use and account for a major portion of the $100 billion global market for polymers. However, improvements in existing materials and development of new polymers with significantly enhanced properties are possible and will increase the range of structural applications and the size of the market. Progress to date is well illustrated by the remarkable strength-to-density ratio of aromatic polyamide polymers; development of other impressively strong polymers of lower cost can be expected. Polymer adhesives are already replacing rivets in many joining applications, and polymer-fiber composites are replacing metals in many

FIGURE 3.3 Fracture surface of silicon carbide-reinforced alumina. (Reprinted, by permission, from E.I.du Pont de Nemours & Co., Inc. Copyright © 1989 by E.I.du Pont de Nemours & Co., Inc.)