The development of many technologies that make our existence so comfortable has been intimately associated with the accessibility of suitable materials. Anadvancement in the understanding of a material type is often the forerunner to the stepwise progression of a technology. For example, automobiles would not have been possible withoutthe availability of inexpensive steel or some other comparable substitute. In our contemporary era, sophisticated electronic devices rely on components that are made from differentmaterials.
Solid materials have been conveniently grouped into three basic classifications: metals, ceramics, and polymers. This scheme is based primarily on chemical makeup andatomic structure, and most materials fall into one distinct grouping or another, although there are some intermediates. In addition, there are three other groups of importantengineering materials—composites, semiconductors, and biomaterials.
Composites consist of combinations of two or more different materials, whereas semiconductors are utilized becauseof their unusual electrical characteristics; biomaterials are implanted into the human body. A brief explanation of the material types and representative characteristics is offerednext.
Metallic materials are normally combinations of metallic elements. They have large numbers of nonlocalized electrons; that is, these electrons are not bound toparticular atoms. Many properties of metals are directly attributable to these electrons. Metals are extremely good conductors of electricity and heat and are not transparent to visiblelight; a polished metal surface has a lustrous appearance. Furthermore, metals are quite strong, yet deformable, which accounts for their extensive use in structural applications.