Materials Applications - Lecture 4 - Material Engineering, Lecture notes of Material Engineering

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Ceramics Materials

http://www.ultramet.com/

Advanced Ceramics: Automobile Engines

Advantages:

• (^) Operate at high temperatures – high efficiencies
• (^) Low frictional losses
• (^) Operate without a cooling system
• (^) Lower weights than current engines Disadvantages: - (^) Ceramic materials are brittle - (^) Difficult to remove internal voids (that weaken structures) - (^) Ceramic parts are difficult to form and machine
• Potential candidate materials:^ Si 3 N 4 (engine valves, ball bearings), SiC (MESFETS), & ZrO 2 (sensors),
• (^) Possible engine parts: engine block & piston coatings

Abrasive Ceramics

• (^) Abrasives are used to wear, grind or cut away other softer materials.
• (^) Diamonds, natural and synthetic, are used as abrasives, though relatively expensive.
• (^) Industrial diamonds are valued mostly for their hardness and heat conductivity.
• (^) They are reportedly 100 million carats unsuitable for use as gemstones, destined for industrial use.
• (^) More common ceramic abrasives are silicon carbide, tungsten carbide, aluminum oxide (corundum) and silica sand.
• (^) Abrasives are either bonded (with a glassy ceramic or an organic resin) to a grinding wheel or made into a powder and used with a cloth or paper (like sandpaper). Al 2 O 3 abrasive grains Diamonds Silicon carbide

Glass

• (^) A glass is an inorganic, non metallic material that does not have a crystalline structure. These materials are said to be amorphous and are virtually solid liquids cooled at such a rate that crystals do not form.
• (^) Typical glasses range from the soda-lime silicate glass for soda bottles to the extremely high purity silica glass for optical fibers.
• (^) Glass is widely used for windows, bottles, glasses for drinking, transfer piping and receptacles for highly corrosive liquids, optical glasses, windows for nuclear applications.
• (^) The main constituent of glass is silicon dioxide (SiO 2 ). The most common form of silica used in glassmaking has always been sand.
• (^) Sand by itself can be fused to produce glass, but it has to be done at 1700 °C. Adding other chemicals to sand can considerably reduce the temperature of the fusion.
• (^) The addition of sodium carbonate (Na 2 CO 3 ), known as soda ash, in a quantity to produce a fused mixture of 75% Silica (SiO 2 ) and 25% of sodium oxide (Na 2 O), will reduce the temperature of fusion to about 800 °C.
• (^) To give glass stability, other chemicals like Calcium Oxide (CaO) and magnesium oxide (MgO) are added. The raw materials used are carbonates: limestone (CaCO 3 ) and dolomite (MgCO 3 ). When subjected to high temperatures they give off CO 2 , leaving the oxides in the glass.

Polymers

• (^) Plastics are materials that have some structural rigidity under load and are used in general purpose applications: Table 13.12, next 2 slides.
• (^) Many natural plastics exist, such as shellac, rubber, asphalt, and cellulose ; however, it is the ability to synthetically create (cheaply) a broad range of materials demonstrating useful properties that has made plastic so prevalent.
• (^) Plastics are used in clothing, housing, automobiles, aircraft, packaging, electronics, signs, recreation items, and medical implants. Molded plastic food replicas

Elastomers

• (^) An elastomer is a polymer with elasticity. The term,

derived from elastic polymer, is often used

interchangeably with the term rubber. Each of the

monomers that link to form the polymer is usually

made of carbon, hydrogen, oxygen and/or silicon.

• (^) Elastomers are amorphous polymers existing above

their glass transition temperature, so that considerable

segmental motion is possible. Their primary uses are

for seals, adhesives and molded flexible parts.

• The Glass transition temperature, T g, is the

temperature at which an amorphous solid, such as

glass or a polymer, becomes brittle on cooling, or soft

on heating.

Liquid Crystal Polymers (LCP)

• (^) The primary use of LCPs are in LCDs (liquid crystal displays) on watches, flat panel computer monitors, televisions and clocks.
• (^) LCDs are common because they are thinner and lighter and draw much less power than cathode ray tubes (CRTs).
• (^) The name "liquid crystal" sounds like a contradiction. A crystal is a solid material like quartz, usually as hard as rock, and a liquid is obviously different.
• (^) There are some substances that can exist in an odd state that is sort of like a liquid and sort of like a solid. It turns out that liquid crystals are closer to a liquid state than a solid. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, and it only takes a little more heat to turn that same liquid crystal into a real liquid.
• (^) One feature of liquid crystals is that they're affected by electric current. A particular sort of nematic liquid crystal, called twisted nematics (TN), is naturally twisted. Applying an electric current to these liquid crystals will untwist them to varying degrees, depending on the current's voltage.
• (^) LCDs use these liquid crystals because they react predictably to electric current in such a way as to control light passage. http://www.howstuffworks.com/lc^16 d.htm

Advanced Polymers

• (^) Molecular weight ca. 4 x 106 g/mol
• (^) Outstanding properties
  • (^) high impact strength
  • (^) resistance to wear/abrasion
  • (^) low coefficient of friction
  • (^) self-lubricating surface
• (^) Important applications
  • (^) bullet-proof vests
  • (^) golf ball covers
  • (^) hip implants (acetabular cup) UHMWPE Ultrahigh Molecular Weight Polyethylene (UHMWPE)