Unit 2: Classes of Engineering Materials – *What Do We Have?* The scientist distinguishes materials by the types of bonding between the atomic particles. This in turn is a study of what the valence electrons of the atoms are doing.
With metallic bonding, the valence electrons disassociate from their parent atoms. They form a cloud that permeates the sample, leaving behind roughly spherical positive ion cores. This electron cloud contributes to both the high electrical and the high thermal conductivity of metals. The spherical ions pack densely into symmetrical crystal lattices. Close-packed planes of ions can slide (shear) over each other for ductile, plastic deformation. The details of this sliding involve defects known as dislocations.
The defining link in polymers is the covalent bond, also known simply as the chemical bond. This is the bond that defines molecules such as water, carbon dioxide, and ammonia. Covalent bonds require specific angles, resulting in lower density materials. Often, rotation can occur around a covalent bond, leading to rubber-like elasticity. Some polymers, called thermoplastic plastics, consist of long chains, somewhat analogous to a bowl of cooked spaghetti. Other plastics, called thermosetting plastics, are a 3D network of strong covalent bonds.
Many ceramic materials are again made up of roughly spherical ions, but of differing sizes. Here, a valence electron has escaped from one type of atom and been captured by a different type of atom, resulting in ionic bonding. The smaller positive ions and larger negative ions again pack into symmetrical crystal lattices. Any shearing of planes would bring similar charges into proximity, a condition of strong repulsive force, but this doesn’t happen. Ceramics are brittle. Concrete is considered a ceramic. Other common ceramics include oxides, nitrides, and carbides, which have covalent bonds.
As the name suggests, composite materials are combinations of the above. Common engineering composites are GFRPs (graphite fiber reinforced polymers) and CFRPs (carbon fiber reinforced polymers). Cermets are combinations of ceramics and metals. A naturally occurring composite material is wood.
Unit 2 Time Advisory
This unit should take you approximately 18 hours to complete.
☐ Subunit 2.1: 5 hours
☐ Subunit 2.2: 5 hours
☐ Subunit 2.3: 4 hours
☐ Subunit 2.4: 4 hours
Unit2 Learning Outcomes
Upon successful completion of this unit, you will be able to: - describe the general internal structure of each major class of engineering material: metals, plastics (also know as polymers), and ceramics; - compare the strengths and weakness of the major materials classes of metals, polymers, and ceramics; and - identify examples of combining materials from different classes to fabricate composite materials, often with unique properties.
2.1 Metals Advantages
- Most metals are strong, easy to shape, and relatively inexpensive.
- Metals can be alloyed to be serviceable at extremes of low and high temperatures.
- Many metals can be alloyed to be heat treatable – ductile in one state and strong in another.
- Many metals are excellent conductors of both heat and electricity.
Limitations - Metals are relatively heavy. - Many metals chemically react with gases and liquids.
Each class of material has many, often hundreds, of entries. From the metals, we will select four to follow through our comparisons in Unit 3. Two are familiar – steels and aluminum alloys. Two are a little more exotic – magnesium alloys and titanium alloys. We will also look at some other metals in a less comprehensive way. 2.1.1 Steels - Reading: The Saylor Foundation’s “Steels” Link: The Saylor Foundation’s “Steels” (PDF)
Instructions: Please read this short article, which introduces steels.
2.1.2 Aluminum and Its Alloys - Reading: The Saylor Foundation’s “Aluminum and Its Alloys” Link: The Saylor Foundation’s “Aluminum and Its Alloys” (PDF)
Instructions: Please read this short article, which introduces aluminum and its alloys.
2.1.3 Some Other Metals - Reading: The Saylor Foundation’s “Some Other Metals” Link: The Saylor Foundation’s “Some Other Metals” (PDF)
Instructions: Please read this short article, which introduces other metals.
2.2 Polymers Advantages
- Polymers are light in weight.
- Most polymers are chemically inert.
- Polymers can be colored throughout, not just surface coated.
- Elastomers and foamed polymers have unique properties.
Limitations - Most polymers are essentially room-temperature materials, breaking down at either low or high temperatures. - Many polymers are soft and scratch easily.
From the class of polymers, we will select six engineering plastics to follow through our comparisons in Unit 3. Each has a wide variety of applications. We will identify a few uses to establish familiarity. 2.2.1 Engineering Polymers (Plastics) - Reading: The Saylor Foundation’s “Engineering Polymers (Plastics)” Link: The Saylor Foundation’s “Engineering Polymers (Plastics)” (PDF)
Instructions: Please read this short article, which introduces plastics.
2.2.2 Elastomers Elastomer is the scientific name for the familiar rubber-like polymers. These include natural rubber, silicone rubber, and hard and soft butyl rubbers. Whereas the elastic limit of metals and ceramics is about 0.1% strain, rubbery materials can be elastically deformed several hundred percent. Elastomers have an available deformation mechanism that the other materials do not. Like thermoplastics, elastomers are comprised of long molecular chains. In elastomers, these chains tend to be curled up like a ball of yarn in the unstressed condition. Application of a force tends to uncurl the molecular chains, but this is an elastic, reversible deformation.
2.2.3 Foamed Polymers Foamed polymers, like Styrofoam® coffee cups and polyurethane building insulation, consist mainly of closed cells of trapped air. This makes them lightweight and mechanically weak, but also gives them unique properties. It is the trapped dead air spaces that provide thermal insulation, not the polymers themselves. Foamed polymers are useful as packing materials, as they can absorb energy by the crushing of cells, rather than by deformation of a solid material.
2.3 Ceramics Advantages
- Ceramics are very hard, good for saw blades and drill bits.
- Ceramics are stable to very high temperatures.
- Ceramics are inert to corrosion.
- Ceramics are brittle. - Ceramics can require special processing to form.
2.3.1 Engineering Ceramics - Reading: The Saylor Foundation’s “Engineering Ceramics” Link: The Saylor Foundation’s “Engineering Ceramics” (PDF)
Instructions: Please read this short article, which introduces various engineering ceramics.
Web Media: YouTube: Triwood1973’s “Making Super Sharp Ceramic Knives” Link: YouTube: Triwood1973’s “Making Super Sharp Ceramic Knives” (YouTube)
Instructions: Watch this video to see an application of zirconia.
Watching this video and pausing to take notes should take approximately 30 minutes.
2.3.2 Porous Ceramics These materials are loosely defined and frequently contain several chemical constituents. They become solid ceramics by firing (as with brick, pottery, porcelain) or by chemical reaction, often involving water (as with mortar and concrete).
2.3.3 Glasses - Reading: The Saylor Foundation’s “Glasses” Link: The Saylor Foundation’s “Glasses” (PDF)
Instructions: Please read this short article, which introduces glasses.
2.4 Composite Materials Advantages
- Composites can utilize the best of different materials groups.
- The internal structure of composites may sometimes be controlled in unique ways – to obtain highly directional properties, for example.
Limitations - Special processing can lead to expensive products. - Compatibility of using different materials can cause special problems. Will they successfully bond together, for example?
2.4.1 Advantages of Composite Materials A composite seeks to take advantage of the strengths of two different materials by bonding them together. A familiar example is reinforced concrete. A steel rod is strong in tension, but will buckle under compression. Concrete, like most brittle materials, is about ten times stronger in compression than in tension. By embedding steel reinforcing bars (rebar) within the concrete, we obtain a product that has better combined tensile and compressive strengths than either material alone.
Another familiar composite is steel-belted automobile tires. We obtain the desired friction with the rubber, together with the stiffening of steel for increased fuel efficiency.
2.4.2 Engineering Composites We will look at three types of engineering composites in our comparisons in Unit 3: glass fiber reinforced polymers (GFRP), more commonly known as fiberglass; carbon fiber reinforced polymers (CFRP), found in sports products; and ceramic/metal combinations. The polymers are generally epoxy resins, making GFRP and CFRP lighter than most metals.
In addition to combining physical properties, fiber composites can be layered with fibers in alternative layers and orientated in different directions to give desired spatial distribution of strength.
Reading: YouTube: NASAeClips’ “NASA 360 - Composite Materials” Link: YouTube: NASAeClips’ “NASA 360 - Composite Materials” (YouTube)
Instructions: Watch this video as an introduction to composite materials. NASA’s interest in composites is primarily for improved strength-to-weight materials.
Watching this video and pausing to take notes should take approximately 1 hour.
2.4.3 Wood: Nature’s Composite Material Similar to aligned polymer molecules, common woods such as pine, oak, maple, and others have a grain structure. Properties measured parallel to the grain are generally different from properties measured perpendicular to the grain.