Ceramic materials are inorganic, nonmetallic materials consisting of metallic and nonmetallic
elements bonded together primarily by ionic and/or covalent bonds. As a result, the chemical
compositions and structures of ceramic materials vary considerably. They may consist
of a single compound such as, for example, pure aluminum oxide, or they may be composed
of a mixture of many complex phases such as the mixture of clay, silica, and feldspar in
electrical porcelain.
The properties of ceramic materials also vary greatly due to differences in bonding. In general, most ceramic materials are typically hard and brittle with low impact resistance and ductility. Consequently, in most engineering designs, high stresses in ceramic materials are usually avoided, especially if they are tensile stresses. Ceramic materials are usually good electrical and thermal insulators due to the absence of conduction electrons, and thus many ceramics are used for electrical insulation and refractories. Some ceramic materials can be highly polarized with electric charge and are used for dielectric materials for capacitors. Permanent polarization of some ceramic materials produces piezoelectric properties that permit these materials to be used as electromechanical transducers. Other ceramic materials, for example, Fe304 are semiconductors and find application for
thermistors for temperature measurement. Graphite, diamond, buckyballs, & buckytubes are all allotropes of carbon and are disscussed in this chapter because Graphite is sometimes
considered a ceramic material. These allotropes have significantly different properties that are directly related to differences in atomic structure and positioning. Buckyballs & buckytubes are becoming more important in nanotechnology applications.
The processing of ceramic materials usually involves the agglomeration of small particles by a variety of methods in the dry, plastic, or liquid states. Cold-forming processes predominate in the ceramics industry, but hot-forming processes are also used. Pressing, slip casting, and extrusion are commonly used ceramic-forming processes. After forming, ceramic materials are usually given a thermal treatment such as sintering or vitrification. During sintering, the small particles of a formed article are bonded together by solid-state diffusion at high temperatures. In vitrification, a glass phase serves as a reaction medium to bond the unmelted particles together.
Glasses are inorganic ceramic products of fusion that are cooled to a rigid solid without
crystallization. Most inorganic glasses are based on a network of ionically covalently bonded
silica (SiO2) tetrahedra. Additions of other oxides such as Na2O and CaO modify the silica network to provide a more workable glass. Other additions to glasses create a spectrum of properties. Glasses have special properties such as transparency, hardness at room temperature, and excellent resistance to most environments that make them important for many engineering designs. One of the important applications of ceramics is in coating of component surface to protect it from corrosion or wear. Glasses, oxides, and carbides are all used as coating materials for various applications. Ceramics are also being applied in the biomedical field as implant materials. Their chemical stability and biocompatibility is perfectly suitable for the harsh environment of the human body in joint replacement and other orthopedic applications. Nanotechnology research is promising to improve on the major drawback of ceramic materials: their brittleness. Early research shows that nanocrystalline ceramics possess higher ductility. This may allow for cheaper production of more complex ceramic parts.
