Engineering Ceramics

G.GopinathAssistant Prof - Mechanical

Ceramics

• To be most frequently silicates, oxides, nitrides and carbides

• Typically insulative to the passage of electricity and heat

• More resistant to high temperatures and harsh environments than metals and polymers

• Hard but very brittle

Ceramic products

Glasses Clay products

Refractories Abrasives Cements Advanced ceramics

-optical - composite reinforce

- containers/ household

-whiteware - bricks

-bricks for high T (furnaces)

-sandpaper - cutting - polishing

-composites - structural

engine - rotors - valves - bearings

-sensors

Properties of Ceramics

• Extreme hardness– High wear resistance

• Extreme hardness can reduce wear caused by friction

• Corrosion resistance• Heat resistance

– Low electrical conductivity– Low thermal conductivity– Low thermal expansion– Poor thermal shock resistance

• Low ductility– Very brittle– High elastic modulus

• Low toughness– Low fracture toughness– Indicates the ability of a crack or flaw to produce a catastrophic failure

• Low density– Porosity affects properties

• High strength at elevated temperatures

Properties of Ceramics

• Oxides: – Alumina, zirconia

• Non-oxides: – Carbides, borides, nitrides, silicides

• Composites: – Particulate reinforced, combinations of oxides and

non-oxides

Classification of Ceramics

• Al2O3 is an electrical insulator but has a relatively high thermal conductivity for a ceramic material.

• Aluminium oxide is insoluble in water.• In its most commonly occurring crystalline form,

called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.

• Aluminium oxide is used for its hardness and strength. • It is widely used as an abrasive, including as a much less

expensive substitute for industrial diamond. • Many types of sandpaper use aluminium oxide crystals.

Al2O3 ( Alumina)

• In addition, its low heat retention and low specific heat make it widely used in grinding operations, particularly cutoff tools.

• As the powdery abrasive mineral Al oxite, it is a major component, along with silica, of the cue tip "chalk" used in billiards.

• Aluminium oxide powder is used in someCD/DVD polishing and scratch-repair kits.

• Its polishing qualities are also behind its use in toothpaste. • Aluminium oxide can be grown as a coating on aluminium

by anodising or by plasma electrolytic oxidation

Al2O3 ( Alumina)

• Both its strength and abrasive characteristics originate from the high hardness (9 on the Mohs scale of mineral hardness) of aluminium oxide.

• Aluminium oxide flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.

• Health and medical applications include it as a material in hip replacements.

Al2O3 ( Alumina)

• Silicon carbide (SiC), also known as carborundum.• Grains of silicon carbide can be bonded together

by sintering to form very hard ceramics that are widely used in applications requiring high endurance, such as car brakes, car clutches andceramic plates in bulletproof vests.

• Electronic applications of silicon carbide as light-emitting diodes (LEDs) and detectors in early radios were first demonstrated around 1907, and today SiC is used in semiconductor electronics applications that are high-temperature, or high-voltage, or both.

SiC (Silicon Carbide)

• it is used for its hardness inabrasive machining processes such as grinding, honing, water-jet cutting and sandblasting.

• Silicon carbide is used in composite armor.• The low thermal expansion coefficient, high hardness, rigidity

and thermal conductivity make silicon carbide a desirable mirror material for astronomical telescopes.

• Silicon carbide fibers are used to measure gas temperatures in an optical technique called thin filament pyrometry.

• Silicon carbide elements are used today in the melting of glass and non-ferrous metal, heat treatment of metals, float glass production, production of ceramics and electronics components, igniters in pilot lights for gas heaters, etc

SiC (Silicon Carbide)

• Potential material for structural applications. • The disadvantage of materail is porosity. • No loss of strength in air at temperatures to 1000C. • Greater thermal shock resistance than many other

ceramics. • Lower density than other ceramics. • Lower thermal expansion• Better toughness than silicon carbide and aluminum oxide. • High chemical and wear resistance • High fatiuge resistance.

Si3N4 ( Silicon Nitride)

Applications• Cutting tool material. • Gas tubine parts • Material for rolling element bearing applications. • Balls, rollers, races for bearings. • Parts of diesel engines• Hot extrusion dies • Pump parts• Rocket engine parts• Medical – spinal fusion device. • It is specially recommended for high speed machining of cast iron. • Face milling of cast iron • Used in atomic force microscope.• Silicon nitride is often used as an insulator and chemical barrier in manufacturing

integrated circuits, to electrically isolate different structures or as an etch mask in bulk micromachining.

PSZ ( Partially Stabilized Zirconia)

• PSZ is really ZrO2, that has been blended and sintered with some other oxide such as MgO, Calcium oxide or Yttria to control the crystal structure transformation.

• The cubic phase can be stabilized by doping with MgO, CaO or Y2O3

• The tetragonal - monoclinic phase transformation involves a 4.7% volume increase.

• This volume increase is the basis for transformation toughening.

PSZ - Properties

• Better fracutre toughness than other high performance ceramics.

• Very hard• Wear resistance also high . • Tensile streght is better than Alumina. • Good thermal insulator. • Thermal expansion is similar to that of steel.

PSZ - Applications

• Replacement of steels in IC engines. • die material in the metall industry• Thermal barrier coatings• Piston caps• cutting tools

SIALON• SiAlON ceramics are a specialist class of high-temperature refractory

materials, with high strength (including at high temperature), good thermal shock resistance and exceptional resistance to wetting or corrosion by molten non-ferrous metals, compared to other refractory materials such as, for example, alumina.

• SiAlONs are produced by first combining a mixture of raw materials including silicon nitride, alumina, aluminum nitride, silica and the oxide of a rare earth element such as yttrium.

• The powder mix is fabricated into a 'green' compact by isostatic powder compaction or slip casting, for example.

• Then the shaped form is then densified, typically by pressure less sintering or hot isostatic pressing.

• The sintered part may then need to by machined by diamond grinding (abrasive cutting).

SIALON – Applications • SiAlON ceramics have found extensive use in non-ferrous

molten metal handling, particularly aluminium and its alloys, including metal feed tubes for aluminum die casting, burner and immersion heater tubes, injector and degassing for nonferrous metals, thermocouple protection tubes, crucibles and ladles.

• In metal forming, sialon is used as a cutting tool for machining chill cast iron and as brazing and welding fixtures and pins, particularly for resistance welding.

• Other applications include in the chemical and process industries and the oil and gas industries, due to sialons excellent chemical stability and corrosion resistance and wear resistance properties.