CLASSIFICATION OF MATERIALS
Solid materials have been conveniently grouped into three
basic classifications - metals, ceramics, and polymers. This scheme is based
primarily on chemical makeup and atomic structure, and most materials fall into
one distinct grouping or another, although there are some intermediates. In
addition, there are three other groups of important engineering
materials—composites, semiconductors, and biomaterials.
A
brief explanation of the material types and representative characteristics can be
given as ---
METALS
Metallic materials are normally combinations of metallic
elements. They have large numbers of non-localized electrons; that is, these
electrons are not bound to particular 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 visible light; a polished
metal surface has a lustrous appearance. Furthermore, metals are quite strong,
yet deformable, which accounts for their extensive use in structural
applications.
CERAMICS
Ceramics are compounds between metallic and nonmetallic
elements; they are most frequently oxides, nitrides, and carbides. The wide
range of materials that falls within this classification includes ceramics that
are composed of clay minerals, cement, and glass. These materials are typically
insulative to the passage of electricity and heat, and are more resistant to high
temperatures and harsh environments than metals and polymers. With regard to
mechanical behavior, ceramics are hard but very brittle.
POLYMERS
Polymers include the familiar plastic and rubber materials.
Many of them are organic compounds that are chemically based on carbon,
hydrogen, and other nonmetallic elements; furthermore, they have very large
molecular structures. These materials typically have low densities and may be
extremely flexible.
COMPOSITES
A number of composite materials have been engineered that
consist of more than one material type. Fiberglass is a familiar example, in
which glass fibers are embedded within a polymeric material. A composite is
designed to display a combination of the best characteristics of each of the
component materials. Fiberglass acquires strength from the glass and
flexibility from the polymer. Many of the recent material developments have
involved composite materials.
SEMICONDUCTORS
Semiconductors have electrical properties that are
intermediate between the electrical conductors and insulators. Furthermore, the
electrical characteristics of these materials are extremely sensitive to the
presence of minute concentrations of impurity atoms, which concentrations may
be controlled over very small spatial regions. The semiconductors have made
possible the advent of integrated circuitry that has totally revolutionized the
electronics and computer industries (not to mention our lives) over the past
two decades.
BIOMATERIALS
Biomaterials are employed in components implanted into the
human body forreplacement of diseased or damaged body parts. These
materials must not produce toxic substances and must be compatible with body
tissues (i.e., must not cause adverse biological reactions).
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