Metals & Ceramics Tutorial

Introduction

Structural materials encompass a wide range of compounds from metals and alloys to ceramics. Some compounds can find very unexpected applications.

Ceramics

Ceramics are nonmetallic, inorganic materials typically formed as powders and often sintered into useful forms. They encompass a wide range of structures from amorphous to polycrystalline or even single crystals. They are typically very hard but rigid, and tend to break under mechanical stress. Since they are nonmetallic, they are usually electrically nonconductive or wide band-gap semiconductors but can be doped to decrease the band-gap to the semiconductor regime. They are often noted for their thermal stability at high temperatures.

Silicon carbide for example has excellent high-temperature characteristics but due to its brittleness has limited utility as a structural material. Micron-thin coatings of alumina (Al2O3) on SiC platelets gives a composite which is 2–3 times stronger than traditional SiC ceramics. The result is a stress- and oxidation-resistant material for high-temperature structural applications.1 Silicon carbide is also used to enhance the properties of other structural materials. Titanium composites inlaid with SiC monofilaments yield a stiff, strong, thermally stable structural material used in aircraft components.2

Alumina is a common and versatile structural ceramic due to its chemical, electrical, mechanical, and thermal properties. It possesses poor fracture resistance, however. The addition of yttria stabilized zirconia to alumina significantly improves the fracture toughness of Al2O3.3

Engineering Polymers

Engineering polymer are materials with exceptional mechanical properties such as stiffness, toughness, and low creep that makes them valuable in the manufacture of structural products like gears, bearing, electronic devices, and auto parts.4-6 Typical engineering plastics include acetals, polyamides, poly(amide-imide)s, polyarylates, poly(ether etherketone)s, poly(ether-imides)s, poly(phenylene oxide)s, poly(phenylene sulfide)s, and polysulfones.7

Polyamides are crystalline and have good impact strength, toughness, and abrasion resistance. Polyesters are often used with fillers like fiberglass, mica, and minerals to increase strength and stiffness. Sulfone-containing polymers show high resistance to acids and alkalis. These thermally stable polymers are used in electronic connectors, circuit boards, sterilizable items, and appliance covers.8 Polyimides have outstanding thermal properties. Additionally, thermoplastics, thermosets, and rubbers have significant applications as engineering polymers.9

 

References

  1. http://www.lbl.gov/msd/PIs/DeJonge/94Hi.html (accessed 3/31/03).
  2. Williams, J.C. The Production, Behavior and Application of Ti Alloys, pp 85-134; In High Performance Materials in Aerospace; Flower, H.M., ed.; Chapman & Hall, London; 1995.
  3. Mangalaraja, R.V. et al. Mat. Sci. Eng. A 2003, 343, 71.
  4. Alger, M.S.M., Ed. Polymer Science Dictionary; Elsevier Science: New York, 1989.
  5. Seymour, R.B.; Carraher, C.E., Jr. Polymer Chemistry: An Introduction, 3rd ed.; Marcel Dekker: New York, 1992; p 243.
  6. Odian, G. Principles of Polymerization, 3rd ed.; John Wiley & Sons, 1991; p100.
  7. Stevens, M.P. Polymer Chemistry: An Introduction, 2nd ed.; Oxford University Press: New York, 1990; p32.
  8. Saunders, K.J. Organic Polymer Chemistry, 2nd ed.; Chapman and Hall: New York, 1988, pp281-285.
  9. Edwards, K.L. Materials and Design 1998, 19, 57.