Chapter 1

The Atomic Structures of Materials

Figure 1.1 Outline of the topics described in Chapter 7

Materials

FIGURE 1.2 An outline of engineering materials

Engineering Materials

FIGURE 1.3 Section of an automotive engine - the Duratec V-6 - showing various components and the materials used in making them. Source: Courtesy Ford Motor Company. Illustration by David Kimball.

Duratec Engine

FIGURE 1.4 (a) Chart showing varous steps involved in designing and manufacturing a product. Depending on the complexity of the product and the type of materials used, the time span between the original concept and the marketing of a product may range from a few months to many years. (b) Chart showing general product flow, from market analysis to selling the product, and depicting concurrent engineering. Source: After S. Pugh, Total Design. Addison-Wesley, 1991.

Design Steps

Metal-

• Those materials that occupy the left side of the periodic table are characterized by having one, two, or three valence electrons, and bond with the metallic bond.

Periodic Table

Nucleus-

• The positively charged central part of an atom containing the protons and neutrons, and therefore most of the atom's mass.

Atomic bonding-

• The joining together of atoms with ionic, covalent, or metallic bonds; the molecular or van der Waals bond is a weak bond seen in inert gases, or a secondary bond in polymers.

Metallic bond-

• In metals, the attractive force between their positive nuclei and inner electron shells (with a net positive charge), and a negatively charged cloud of valence electrons.

• This type of bonding provides free electrons for electrical and thermal conductivity and permits plastic deformation or cold working.

Metallic Bond

Ionic bond-• A type of atomic bonding in which atoms with

one or more valence electrons donate or give away their valence electrons to elements that lack one or more electrons to fill their valence shell; each atom thus becomes an ion; the one donating becomes positive, the one accepting becomes negative.

• The ionic bond commonly forms compounds between elements widely separated on the periodic table, e.g., Na + and CI- combine to form NaCI.

Ionic bonding

Covalent bond-

• A type of atomic bonding that requires the sharing of valence electrons to complete the outer shell; appears principally in gases, liquids, and polymers.

Covalent Bond

van der Waals bond-

• A relatively weak bond between molecules, for example, in inert gases.

• It is a weak secondary bond between adjacent chains of a polymer.

• The strength of the bond is directly related to the size of the molecules, inversely related to the distance between the molecules, and easily weakened by heat.

Valence-

• The capacity of an atom to combine with other atoms to form a molecule.

• The inert gases have zero valence.

• Valence considers positive and negative properties of atoms, as determined by the gain or loss of valence electrons.

Lattice-

• A term that is used to denote a regular array of points in space.

• The points of the three-dimensional space lattice are constructed by the repeated application of the basic translations that carry a unit cell into its neighbor.

Lattice

Ceramics-

• A family of materials that are compounds, traditionally consisting of a metal and an oxide, but they may also be carbides, sulfides, nitrides, and intermetallic compounds.

• Ceramics generally have an ionic bond, are very hard and brittle, and can withstand high temperatures.

(a) (b)

Figure 1.5 A variety of ceramic components. (a) High-strength alumina for high-temperature applications. (b) Gas-turbine rotors made of silicon nitride. Source: Wesgo Div., GTE.

Examples of Ceramics

TABLE 1.1 Type General Characteristics Oxide ceramics

Alumina High hardness, moderate strength; most widely used ceramic; cutting tools, abrasives, electrical and thermal insulation.

Zirconia High strength and toughness; thermal expansion close to cast iron; suitable for heat engine components.

Carbides Tungsten carbide Hardness, strength, and wear resistance depend on cobalt binder

content; commonly used for dies and cutting tools. Titanium carbide Not as tough as tungsten carbide; has nickel and molybdenum as

the binder; used as cutting tools. Silicon carbide High-temperature strength and wear resistance; used for heat

engines and as abrasives. Nitrides

Cubic boron nitride Second-hardest substance known, after diamond; used as abrasives and cutting tools.

Titanium nitride Gold in color; used as coatings because of low frictional characteristics.

Silicon nitride High resistance to creep and thermal shock; used in heat engines. Sialon Consists of silicon nitrides and other oxides and carbides; used as

cutting tools. Cermets Consist of oxides, carbides, and nitrides; used in high-temperature

applications. Silica High temperature resistance; quartz exhibits piezoelectric effect;

silicates containing various oxides are used in high-temperature nonstructural applications.

Glasses Contain at least 50 percent silica; amorphous structures; several types available with a range of mechanical and physical properties.

Glass ceramics Have a high crystalline component to their structure; good thermal-shock resistance and strong.

Graphite Crystalline form of carbon; high electrical and thermal conductivity; good thermal shock resistance.

Diamond Hardest substance known; available as single crystal or polycrystalline form; used as cutting tools and abrasives and as dies for fine wire drawing.

Types and General

Characteristics of Ceramics

Thermoplastic-

• Capable of softening or fusing when heated and of hardening again when cooled.

Thermosetting-

• Capable of becoming permanently rigid when cured by heating; will not soften by reheating.

Polymer-

• A compound or compounds, usually hydrocarbons, that have been polymerized to form a long chain repeating unit structures.

Range of Mechanical Properties for Various Engineering Plastics

TABLE 1.2 Material

UTS (MPa)

E (GPa)

Elongation (%)

Poisson’s ratio ()

ABS ABS, reinforced Acetal Acetal, reinforced Acrylic Cellulosic Epoxy Epoxy, reinforced Fluorocarbon Nylon Nylon, reinforced Phenolic Polycarbonate Polycarbonate, reinforced Polyester Polyester, reinforced Polyethylene Polypropylene Polypropylene, reinforced Polystyrene Polyvinyl chloride

28–55 100

55–70 135

40–75 10–48

35–140 70–1400

7–48 55–83

70–210 28–70 55–70

110 55

110–160 7–40

20–35 40–100 14–83 7–55

1.4–2.8 7.5

1.4–3.5 10

1.4–3.5 0.4–1.4 3.5–17 21–52 0.7–2

1.4–2.8 2–10

2.8–21 2.5–3

6 2

8.3–12 0.1–1.4 0.7–1.2 3.5–6 1.4–4

0.014–4

75–5 —

75–25 —

50–5 100–5 10–1 4–2

300–100 200–60 10–1 2–0

125–10 6–4

300–5 3–1

1000–15 500–10

4–2 60–1

450–40

— 0.35 —

0.35–0.40 — — — —

0.46–0.48 0.32–0.40

— —

0.38 —

0.38 —

0.46 — —

0.35 —

Structure of

Polymer Molecules

Figure 1.6 Basic structure of polymer molecules: (a) ethylene molecule; (b) polyethylene, a linear chain of many ethylene molecules; © molecular structure of various polymers. These are examples of the basic building blocks for plastics

Molecular Weight and Degree of Polymerization

Figure 1.7 Effect of molecular weight and degree of polymerization on the strength and viscosity of polymers.

Polymer ChainsFigure 1.8 Schematic illustration of polymer chains. (a) Linear structure--thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures. (b) Branched structure, such as in polyethylene. (c) Cross-linked structure--many rubbers or elastomers have this structure, and the vulcanization of rubber produces this structure. (d) Network structure, which is basically highly cross-linked--examples are thermosetting plastics, such as epoxies and phenolics.

Polymer Behavior

Figure 1.9 Behavior of polymers as a function of temperature and (a) degree of crystallinity and (b) cross-linking. The combined elastic and viscous behavior of polymers is known as viscoelasticity.

Polymerization-

• A chemical reaction in which two or more small molecules combine to form larger molecules that contain repeating structural units of the original molecules.

Polycrystalline-

• The term used to describe the crystalline nature of most metals encountered, i.e., they are made up of more than one metallic crystal, as opposed to being single crystals.

Grain

• In metals, a structure containing atoms of one crystalline orientation.

• Grains form during the solidification (or crystallization) of the metal; they may be re-formed during recrystallization.

Allotropy-

• The ability of a material to exist in several crystalline forms.

Solidification-

• The process in which a liquid metal changes to a solid; in this process heat is removed and the atoms have to fit into an atom lattice.

Stages During Solidification

FIGURE 1.10 Schematic illustration of the various stages during solidification of molten metal. Each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal. Note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) solidified metal, showing individual grains and grain boundaries. Note the different angles at which neighboring grains meet each other. Source: W. Rosenhain.

Figure 1.11 Schematic illustration of the stages during solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other. Source: W. Rosenhain.

Solidification

Dendrite-

• A crystal characterized by a treelike pattern that is usually formed by the solidification of a metal.

• Dendrites generally grow inward from the surface of a mold.

Grain-boundary-

• The outer perimeter of a single grain where it is in contact with an adjoining grain; because atoms are not at their ideal distance apart (and therefore at their lowest energy level), it is a region of higher energy.

Isomerism-

• Compounds are said to be isomeric when they have the same elementary composition (i.e., their molecules contain the same numbers and kinds of atoms) but different structures, and hence, properties.

• It is believed that differences are due to the arrangement of atoms in each molecule.

Cross-linking-

• Primary, i.e., ionic or covalent, bonds between chains of a polymer, so that the polymer takes on a three-dimensional structure; this normally occurs in thermoset polymers.

Crystalline, Crystallinity-

• In solid metals, having a repeating geometric arrangement of atoms.

• Polymers are said to be crystalline, or exhibit crystallinity, when their chains become aligned into a pattern.

• The opposite of amorphous, having a random structure.

CrystallinityFigure 1.12 Amorphous and crystalline regions in a polymer. The crystalline region (crystallite) has an orderly arrangement of molecules. The higher the crystallinity, the harder, stiffer, and less ductile the polymer.

Vulcanization-

• The process of treating crude or synthetic rubber or similar plastic material chemically to give it useful properties, such as elasticity, strength, and stability.