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About Ceramics
 When the average person hears the word ceramics, usually an image of pottery (or at the other extreme even space shuttle tiles) appears. But what many people don’t realize is that ceramics play an important role almost everywhere you look and sometimes where you can’t. Besides the everyday objects of dinnerware, glassware, floor and wall tile, and other consumer products, ceramics are helping computers and other electronic devices operate, improving people’s health in various ways, providing global telecommunications, and protecting soldiers during combat. And the list goes on.
Definition of Ceramic
So what is a ceramic? Merriam-Webster’s online dictionary defines it as a clay material that is fired at a high temperature to form such products as earthenware, porcelain or brick. The word itself can be traced back to the Greek term keramos, meaning potter’s clay or pottery. Keramos in turn is related to an older Sanskrit root meaning “to burn.” Ceramus or Keramos was also an ancient city on the north coast of the Aegean Sea in what is currently Turkey. (The word Keramos lives on as the name for the national professional ceramic engineering fraternity. Find out more about that here.)
In the most simple of terms, ceramics can be defined as inorganic, nonmetallic materials. They are typically crystalline in nature (have an ordered structure) and are compounds formed between metallic and nonmetallic elements such as aluminum and oxygen (alumina, Al2O3), calcium and oxygen (calcia, CaO), and silicon and nitrogen (silicon nitride, Si3N4).
In broader terms ceramics also include glass (which has a non-crystalline or amorphous random structure), enamel (a type of glassy coating), glass-ceramics (a glass containing ceramic crystals), and inorganic cement-type materials (cement, plaster and lime). However, as ceramic technology has developed over time, the definition has expanded to include a much wider range of other compositions used in a variety of applications.
Structure and Properties The properties of ceramic materials, like all materials, are dictated by the types of atoms present, the types of bonding between the atoms, and the way the atoms are packed together. The type of bonding and structure helps determine what type of properties a material will have.
Ceramics usually have a combination of stronger bonds called ionic (occurs between a metal and nonmetal and involves the attraction of opposite charges when electrons are transferred from the metal to the nonmetal); and covalent (occurs between two nonmetals and involves sharing of atoms). The strength of an ionic bond depends on the size of the charge on each ion and on the radius of each ion. The greater the number of electrons being shared, is the greater the force of attraction, or the stronger the covalent bond.
These types of bonds result in high elastic modulus and hardness, high melting points, low thermal expansion, and good chemical resistance. On the other hand, ceramics are also hard and often brittle (unless the material is toughened by reinforcements or other means), which leads to fracture. In general, metals have weaker bonds than ceramics, which allows the electrons to move freely between atoms. Think of a box containing marbles surrounded by water. The marbles can be pushed anywhere within the box and the water will follow them, always surrounding the marbles. This type of bond results in the property called ductility, where the metal can be easily bent without breaking, allowing it to be drawn into wire. The free movement of electrons also explains why metals tend to be conductors of electricity and heat.
Plastics or polymers of the organic type consist of long chains of molecules which are either tangled or ordered at room temperature. Because the forces (known as van der Waals) between the molecules are very weak, polymers are very elastic (like a rubber band), can be easily melted, and have low strength. Like ceramics, polymers have good chemical resistance, electrical and thermal insulation properties. They are also brittle at low temperatures. The following table provides a general comparison of the properties between the three types of materials.
General Comparison of Materials
Note: For general comparison only; specific properties depend on the material’s specific composition and how it is made.
These three material types can also be combined in various ways to form composites to take advantage of each material’s properties. For instance, ceramic particles or fibers can be added to a ceramic or metal matrix to improve the mechanical properties and/or produce a special property the matrix by itself generally would not have. Polymers are also reinforced with glass fibers for a wide range of construction and structural applications.
Microstructure
Another characteristic that plays an important factor in the final property of a material is called microstructure. The microstructure of a material is usually too small to be seen with the naked eye. For ceramics, the microstructure is made up of small crystals called grains. In general, the smaller the grain size, the stronger and denser is the ceramic material. In the case of a glass material, the microstructure is non-crystalline. When these two materials are combined (glass-ceramics), the glassy phase usually surrounds small crystals, bonding them together.
Depending on the composition and the processing of the raw materials, as well as the fabrication and firing conditions, the properties of the material can often be closely tailored to the desired application.
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