0.6 Chapter 2. solid state of matter.  (Page 2/7)

Section 2.2. General Material Classification.

Based on atomic bonding forces, matter is classified in three classes namely: metallic, ceramic and polymeric. These three classes can be further combined together to form composites. In Figure 2.2. we illustrate the three classes and their composites.

Section 2.2.1. Metals.

Metals account for about two thirds of all the elements and about 24% of the mass of the planet. Metals have useful properties including strength, ductility, high melting points, thermal and electrical conductivity, and toughness. From the periodic table, it can be seen that a large number of the elements are classified as being a metal. A few of the common metals and their typical uses are presented below.

Common Metallic Materials:

(1)Iron/Steel - Steel alloys are used for strength critical applications

(2)Aluminum - Aluminum and its alloys are used because they are easy to form, readily available, inexpensive, and recyclable.

(3)Copper - Copper and copper alloys have a number of properties that make them useful, including high electrical and thermal conductivity, high ductility, and good corrosion resistance.

(4)Titanium - Titanium alloys are used for strength in higher temperature (~1000° F) application, when component weight is a concern, or when good corrosion resistance is required

(5)Nickel - Nickel alloys are used for still higher temperatures (~1500-2000° F) applications or when good corrosion resistance is required.

(6)Refractory materials are used for the highest temperature (>2000° F) applications.

In metal, the lattice centers are immersed in a sea of conducting electrons. These conducting electrons give metallic bonding which is malleable and ductile. It does not have brittleness. Plus it is conductive.

Section 2.2.2. Ceramics.

A ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared from powdered materials, is fabricated into products through the application of heat, and displays such characteristic properties as hardness, strength, low electrical conductivity, and brittleness." The word ceramic comes the from Greek word "keramikos", which means "pottery." They are typically crystalline in nature and are compounds formed between metallic and nonmetallic elements such as aluminum and oxygen (alumina-Al ₂ O ₃ ), calcium and oxygen (calcia - CaO), and silicon and nitrogen (silicon nitride-Si ₃ N ₄ ).

The two most common chemical bonds for ceramic materials are covalent and ionic. Covalent and ionic bonds are much stronger than in metallic bonds and, generally speaking, this is why ceramics are brittle and metals are ductile.

Section 2.2.3. Polymers.

A polymeric solid can be thought of as a material that contains many chemically bonded parts or units which themselves are bonded together to form a solid. The word polymer literally means "many parts." Two industrially important polymeric materials are plastics and elastomers. Plastics are a large and varied group of synthetic materials which are processed by forming or molding into shape. Just as there are many types of metals such as aluminum and copper, there are many types of plastics, such as polyethylene and nylon. Elastomers or rubbers can be elastically deformed a large amount when a force is applied to them and can return to their original shape (or almost) when the force is released.