Engineering Materials - Classification

Engineering materials

Basically, they are of four types

Metals: Elements with a valence of 1, 2 or 3. They are crystalline solids composed of atoms held together by a matrix of electrons. The “Electron Gas” that surrounds the “Lattice of atomic nuclei” is responsible for most of the properties.

1. General properties: High electrical conductivity, high thermal conductivity, ductile and relatively high stiffness, toughness and strength. They are ready to machining, casting, forming, stamping and welding. Nevertheless, they are susceptible to corrosion.

2. Further description: Engineering metals are generally Alloys. Alloys are metallic materials formed by mixing two or more elements, e.g.

a.    Mild steel    Fe + C

b.    Stainless steel    Fe + C + Cr + Mn …etc.

C improves strength

Cr improves the corrosion resistance…etc.

3. Classification: of metals and alloys:

Ferrous: Plain carbon steel, Alloy steel, Cast iron,

                  Nonferrous: Light Alloys (Al, Mg, Ti, Zn), Heavy Alloys (Cu, Pb, Ni), Refractory Metals (Mo, Ta, W), Precious metals (Au, Ag, Pt)

4. Applications:

Ø    Electrical wiring

Ø    Structures: buildings, bridges, etc.

Ø    Automobiles: body, chassis, springs, engine block, etc.

Ø    Airplanes: engine components, fuselage, landing gear assembly, etc.

Ø    Trains: rails, engine components, body, wheels

Ø    Machine tools: drill bits, hammers, screwdrivers, saw blades, etc.

Ø    Magnets

Ø    Catalysts

5. Examples:

Ø    Pure metal elements (Cu, Fe, Zn, Ag, etc.)

Ø    Alloys (Cu-Sn=bronze, Cu-Zn=brass, Fe-C=steel, Pb-Sn=solder)

Ø    Intermetallic compound (e.g. Ni3Al)

Ceramics: Inorganic, non-metallic crystalline compounds, usually oxides (SiO2, Al2O3, MgO, TiO2, BaO), Carbides (SiC), Nitrides (Si3N4), Borides (TiB2), Silicides (WSi2, MoSi2). Some literature includes glasses in the same category, however; glasses are amorphous (non-crystalline) compounds i.e. they possess “short range” order of atoms.

1.   General properties: Light weight, Hard, High strength, stronger in compression than tension, tend to be brittle, low electrical conductivity, High temperature resistance and corrosion resistance.

2.   Further description: Ceramics also includes ferrites (ZnFe2O4), semiconductors (ZnO, TiO2, CuO, SiC, AlN, BN, C, Si, Ge, SiGe), piezoelectric and ferroelectric ceramic (BaTiO3, PZT=PbZrTiO3) and superconducting ceramics (YBa2Cu3O7).

3.   Classification: of ceramics:

Ø Traditional Ceramics: Includes pottery, china, porcelain products…etc, these products utilizes natural ceramic ores.

Ø  Advanced Ceramics: Alumina, magnesia, Carbides, Nitrides, Borides, Silicides …etc, they are synthetic materials, usually of better mechanical properties. Electronic ceramics falls in the same category.

Ø  Glass, Glass Ceramic and Vitro Ceramic: Glasses are essentially vitreous (amorphous, non crystalline), Glass ceramics are mostly re-crystallized from glassy medium and, Vitro Ceramics have crystalline microstructure which are partially vitreous at the grain boundaries.

4.    Applications:

Ø  Electrical insulators

Ø  Abrasives

Ø  Thermal insulation and coatings

Ø  Windows, television screens, optical fibers (glass)

Ø  Corrosion resistant applications

Ø  Electrical devices: capacitors, varistors, transducers, etc.

Ø  Highways and roads (concrete)

Ø  Biocompatible coatings (fusion to bone)

Ø  Self-lubricating bearings

Ø  Magnetic materials (audio/video tapes, hard disks, etc.)

Ø  Optical wave guides

Ø  Night-vision

5.    Examples of technical ceramics

Ø  Barium titanate (often mixed with strontium titanate) displays ferroelectricity, meaning that its mechanical, electrical, and thermal responses are coupled to one another and also history-dependent. It is widely used in electromechanical transducers, ceramic capacitors, and data storage elements.

Ø  Bismuth strontium calcium copper oxide, a high-temperature superconductor

Ø  Boron carbide (B₄C), which is used in ceramic plates in some personnel, helicopter and tank armor.

Ø  Boron nitride is structurally isoelectronic to carbon and takes on similar physical forms: a graphite-like one used as a lubricant, and a diamond-like one used as an abrasive

Ø  Ferrite (Fe₃O₄), which is ferrimagnetic and is used in the magnetic cores of electrical transformers and magnetic core memory.

Ø  Lead zirconate titanate is another ferroelectric material.

Ø  Magnesium diboride (MgB₂), which is an unconventional superconductor.

Ø  Sialons / Silicon Aluminium Oxynitrides, high strength, high thermal shock / chemical / wear resistance, low density ceramics used in non-ferrous molten metal handling, weld pins and the chemical industry.

Ø  Silicon carbide (SiC), which is used as a susceptor in microwave furnaces, a commonly used abrasive, and as a refractory material.

Ø  Silicon nitride (Si₃N₄), which is used as an abrasive powder.

Ø  Steatite (MgSiO₃), used as an electrical insulator.

Ø  Uranium oxide (UO₂), used as fuel in nuclear reactors.

Ø  Yttrium barium copper oxide (YBa₂Cu₃O_7-x), another high temperature superconductor.

Ø  Zinc oxide (ZnO), which is a semiconductor, and used in the construction of varistors

Ø  Zirconium dioxide (zirconia), its high oxygen ion conductivity recommends it for use in fuel cells. In another variant, metastable structures can impart transformation toughening for mechanical applications; most ceramic knife blades are made of this material.

6.    Semiconductors Applications and Examples

Ø  Computer CPUs

Ø  Electrical components (transistors, diodes, etc.)

Ø  Solid-state lasers

Ø  Light-emitting diodes (LEDs)

Ø  Flat panel displays

Ø  Solar cells

Ø  Radiation detectors

Ø  Microelectromechanical devices (MEMS)

Ø  Examples: Si, Ge, GaAs, and InSb

Polymers: High molecular weight organic substance made up of a large number of repeat (monomer) units. Their properties are linked directly to their structure, which is dictated mostly by intermolecular bonds.

1.     General properties: compared with metals, polymers have lower density, lower stiffness and tend to creep. They have higher thermal expansion and corrosion resistance. Furthermore, polymers have low electrical conductivity and low thermal conductivity. The prime weakness is that polymers do not withstand high temperatures

2.  Further description: Polymers generally formed via a “Polymerization Process”, in which the polymer chain builds up from monomers with the aid of heat and/or chemical agents. The C-C bonds form the backbone of the polymer chain; when the chains grow very long, they get tangled (twisted) and loose their lattice order, thus, changing increasingly to the amorphous state. Consequently, polymers are semi-crystalline to some “degree of crystallinity” that can be measured by X-ray Diffraction.

3.   Classification: according to their properties:

i.     Plastics: (Hard), they can be semi-crystalline or amorphous (glassy).

1.    Thermoplastics: Such as Polyethylene (PE) and Polymethylmethacrylate (Acrylic and PMMA) are composed of “linear” polymer chains. They flow under shear when heated. They can be compression- or injection- molded.

2.    Thermosets: Such as Polystyrene (PS) and Polyvinylchloride (PVC) are composed of “branched” polymer chains. They not flow when heated. The monomers are ‘cured’ in a mold (‘RIM’).

ii.     Elastomers: (Soft) Rubbery cross-linked solids that will deform elastically under stress, e.g. natural rubber

iii.     Solutions: Viscosity modifiers, polymeric surfactants, lubricants.

4.   Applications and Examples

Ø  Adhesives and glues

Ø  Containers

Ø  Moldable products (computer casings, telephone handsets, disposable razors)

Ø  Clothing and upholstery material (vinyls, polyesters, nylon)

Ø  Water-resistant coatings (latex)

Ø  Biodegradable products (corn-starch packing “peanuts”)

Ø  Biomaterials (organic/inorganic interfaces)

Ø  Liquid crystals

Ø  Low-friction materials (Teflon)

Ø  Synthetic oils and greases

Ø  Gaskets and O-rings (rubber)

Ø  Soaps and surfactants

Composite: A combination of two or more materials to achieve better properties than that of the original materials. These materials are usually composed of a “Matrix” and one or more of “Filler” material. Wood is a natural composite of cellulose fibers in a matrix of polymer called lignin. The primary objective of engineering composites is to increase strength to weight ratio. Composite material properties are not necessarily isotropic, i.e., directional properties can be synthesized according to the type of filler materials and the method of fabrication

1.   General properties: Low weight, high stiffness, brittle, low thermal conductivity and high fatigue resistance. Their properties can be tailored according to the component materials.

2.   Further description:

3.   Classification:

i.    Particulate composites (small particles embedded in a different material): e.g. Cermets (Ceramic particle embedded in metal matrix) and Filled polymers.

ii.     Laminate composites (golf club shafts, tennis rackets, Shield Glass)

iii.     Fiber reinforced composites: e.g. Fiber glass (GFRP) and Carbon-fiber reinforced polymers (CFRP)

4.   Applications:

Ø  Sports equipment (golf club shafts, tennis rackets, bicycle frames)

Ø  Aerospace materials

Ø  Concrete

Ø  "Smart" materials (sensing and responding)

Ø  Brake materials

5.   Examples:

Ø  Fiberglass (glass fibers in a polymer)

Ø  Space shuttle heat shields (interwoven ceramic fibers)

Ø  Paints (ceramic particles in latex)

Ø  Tank armor (ceramic particles in metal)