Materials_Science_Ceramics

Ceramics Basics

Definition of a Ceramic Material. Ceramic encompass such a vast array of materials that a concise definition is almost impossible. However, one workable definition of ceramics can be expressed as follows: a ceramic is a refractory (high-temperature, heat-resistant), inorganic and nonmetallic material.

Classes of Ceramics. Ceramics are subdivided into two basic classes, namely, traditional ceramics and advanced ceramics. Traditional ceramics include clay products and silicates (silicon- and oxygen-containing glass and cement). Advanced ceramics consist of carbides (such as silicon carbide [SiC]), pure oxides (such as aluminum oxide [Al2O3]), nitrides (such as silicon nitride [Si3N4]), nonsilicate glasses and many others.

Special Features of Ceramics. Ceramics offer many advantages to other materials. Firstly, they are harder and stiffer than steel. Secondly, they are more heat- and corrosion-resistant than metals as well as polymers. Thirdly, they are less dense than most metals and their alloys. Fourthly, their raw materials are both plentiful and inexpensive. Furthermore, ceramics display a wide range of properties which facilitate their use in many different product areas, such as in the aerospace, automotive, medical, military, microelectronic and communications fields.

Physical and Chemical Properties of Ceramics. The wide-range of applications in which ceramics have been used is due to their remarkable range of physical and chemical behavior. The following is a brief listing of such behavior:

1. The capacity to withstand enormous amounts of heat energy.

2. The ability to store enormous amounts of heat energy.

3. The ability to insulate from electric current.

4. The ability to act as a semiconductor to electric current.

5. The ability to act as a superconductor to electric current.

6. The ability to be magnetic.

7. The ability to be nonmagnetic.

8. High hardness (resistance to abrasion).

9. High strength.

The diversity in their properties stems from the bonding of their component atoms and molecules and their crystalline structure.

Bonding of Atoms and Molecules in Ceramics. Two types of bonding mechanisms occur in ceramic materials, namely, ionic and covalent. Often both these mechanisms coexist in the same ceramic material. Each type of bond leads to different physical and chemical characteristics. Ionic bonds for example most often occur between metallic and nonmetallic elements. The resulting compound has a very high melting point since these types of bonds are strong. Compounds that are either mostly ionic or mostly covalent generally have higher melting points than compounds in which neither kind of bonding predominates.

Classification of Ceramic Materials. Ceramic materials can be divided into two classes, namely, crystalline and amorphous (noncrystalline). Crystalline ceramics reveal a repetitive three-dimensional arrangement of component atoms and are created as a result of slow cooling of the ceramic during its initial formation. Noncrystalline ceramics do not possess a repetitive internal atomic arrangement and are developed as a result of the rapid cooling of the parent material.

Questions.

1. Briefly define ceramic material and how it differs from other types of materials.

2. Briefly explain the meaning of the term refractory in the materials science context

3. Explain the basis of the classification scheme of ceramic materials and its relevance to the history of their development.

4. Briefly list the four basic advantages which ceramic materials possess over all other types of materials.

5. Briefly summarize the physical and chemical behavior of a ceramic material.

6. How does atomic and molecular bond type influence the thermal behavior of a compound?

7. Briefly summarize the principal characteristics upon which the principal classes of ceramic materials is based.

8. Briefly explain the physical conditions under which each class of ceramic material is created.