4 Flashcards
Alumina
Aluminum oxide (Al2O3 )
➢ Exists in nature, and can be manufactured totally synthetically.
➢ Synthetic alumina can have better properties as impurities are eliminated.
➢ The most common material used for structural ceramic components.
➢ It is used in applications from aerospace to medical, to bearings to analytical instrumentation.
➢ High purity alumina lends itself well to corrosive environments.
Zirconia
Zirconium oxide (ZiO2 )
➢ Zirconia is often used in wear applications requiring improved fracture toughness and stiffness over alumina.
➢ Its high material cost usually warrants its usage only where alumina fails.
Carbides
Tungsten carbide (WC): Two times stiffer than steel (E = 550 GPa)!
➢ Titanium (TiC): Extremely hard (Mohs 9 – 9.5, Diamond is 10!)
➢ Silicon carbide (SiC): Silica sand + coke + sodium chloride + sawdust.
carbon
➢ Commonly used for cutting tools
(requires hardness and high-temprature strength).
Nitrides
Cubic boron nitride (CBN): The second hardest known substance after diamond.
➢ Titanium nitride (TiN): Used as a coating on cutting tools.
➢ Silicon nitride (Si3N4 ): High resistance to creep at elevated temperatures.
Cermets
Is a composite material composed of ceramics (cer) and metals(met). ➢The metal is used as a binder for an oxide, boride, or carbide.
➢ A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation.
➢ High-temperature oxidation resistance of ceramics and toughness of metals. ➢ High-temperature applications such as nozzles of jet engines.
Silica
Compound of the elements silicon and oxygen whose forms are as diverse as its uses.
➢ Most glasses contain more than 50% silica. ➢ Abundant in nature.
➢ Most common form of silica is quartz:
➢ Hard and abrasive,
➢ Hexagonal crystal,
➢ Used in communications applications as an oscillating crystal with fixed frequency (as it exhibits piezoelectric effect).
➢ Silicates are products of the reaction of silica with oxides of aluminum, magnesium, calcium, potassium, sodium, and iron.
➢ Examples of silicates:
➢ clay, asbestos, mica, silicate glasses.
General Properties and Applications of Ceramics
Compared to metals, ceramics have the following relative properties: ➢ brittle,
➢ low toughness,
➢ low density,
➢ low thermal expansion,
➢ low thermal and electrical conductivity,
➢ high strength and hardness at elevated temperatures, ➢ high elastic modulus, E.
Also, ceramics can have a wide range of properties, because of their sensitivity to:
defects,
➢ porosity,
➢ surface or internal cracks, ➢ presence of impurities.
Mechanical Properties of Ceramics
Strength in tension
ack impact toughness. This is due to their inh Ceramics with low thermal conductivity and low thermal expansion areerent lack of ductility. Once a crack is initiated, it propagates rapidly.
Although ceramics are basically resistors, they can be made electrically conductors by alloying them with certain elements, in order to make ceramics act like a semiconductor or even like a superconductor.
Applications of Ceramics
Commonly used as insulators in electrical
The capability of maintaining strength and stiffness at elevated temperatures makes
ceramics very attractive for high-temperature applications.
High-resistance to wear makes them suitable for applications such as cylinder liner,
seals, and bearings.
Glasses
All glasses contain at least 50% silica.
Properties (mechanical [except strength], electrical, high-temperature and anti-chemical corrosion, and optical) of glasses can be modified by addition of oxides of aluminum, sodium, calcium, boron, magnesium, titanium, lithium,…
Mechanical Properties of Glasses
Like ceramics, glasses are also regarded as perfectly elastic and brittle.
This relatively low strength is due to the presence of small flaws and microcracks on its surface during the operation of glass.
➢ These defects reduce the strength of glass by two to three orders of magnitude (compared to its ideal, defect-free form)!!!
Graphite
A crystalline form of carbon having layered structure: → Weak when sheared along the layers.
→ Low frictional properties → used as a solid lubricant!
➢ In vacuum, graphite is abrasive and is a poor lubricant.
➢ Unlike in other materials, strength and stiffness of graphite increase with temperature!
➢ Graphite is very good resistive to chemicals:
→ used as filter for corrosive fluids.
Diamond
It is THE HARDEST substance known! (7000-8000 hk)
➢ Brittle.
➢ Synthetic diamonds are extensively used in industrial applications.
Synthetic diamond is identical to natural diamonds, actually they have superior properties because of their lack of impurities.
➢ Diamond particles can be coated with nickel or titanium for improved properties in grinding operations.
Differences and Similarities of Graphite and Diamond:
The two share the same chemistry, carbon, but have very different structures and properties.
➢ Diamond is hard, graphite is soft (the “lead” of a pencil).
➢ Diamond is an excellent electrical insulator, graphite is a good conductor of electricity.
➢ Diamond is the ultimate abrasive, graphite is a very good lubricant.
➢ Diamond is transparent, graphite is opaque.
➢ Somewhat of a surprise is that at surface temperatures and pressures, graphite is the stable form of carbon. In fact, all diamonds at or near the surface of the Earth are currently undergoing a transformation into graphite. This reaction, fortunately, is extremely slow.
