Production of various ceramics

1. Using chemical composition as a basis, it is possible
to classify ceramics into five main categories:
1. Oxides — alumina, Al2O3 (spark plug insulators, grinding wheel
grits), magnesia, MgO (refractory linings of furnaces, crucibles),
zirconia, ZrO2 (piston caps, refractory lining of glass
tank furnaces), zirconia/alumina (grinding media), spinels, M2+O-
M^+O3 (ferrites, magnets, transistors, recording tape), 'fused' silica
glass (laboratory ware),
2. Carbides — silicon carbide, SiC (chemical plant, crucibles, ceramic
armour), silicon nitride, Si3N4 (spouts for molten aluminium, high-
temperature bearings), boron nitride, BN (crucibles, grinding wheels for
high-strength steels).
3. Silicates —porcelain (electrical components), steatites (insulators),
mullite (refractories).
4. Sialons — based on Si-Al-O-N and M-Si-Al-O-N where M = Li, Be,
Mg, Ca, Sc, Y, rare earths
(tool inserts for high-speed cutting, extrusion dies, turbine blades).
5. Glass-ceramics — Pyroceram, Cercor, Pyrosil (recuperator discs for
heat exchangers).

2. It is appropriate to classify ceramic materials in microstructural
terms, in the following manner:
1. Single crystals of appreciable size (e.g. ruby laser crystal)
2. Glass (non-crystalline) of appreciable size (e.g. sheets of 'float' glass)
3. Crystalline or glassy filaments (e.g. E-glass for glass-reinforced
polymers, single-crystal 'whiskers', silica glass in Space Shuttle tiles)
4. Polycrystalline aggregates bonded by a glassy matrix (e.g. porcelain
pottery, silica refractories, hot-pressed silicon nitride)
5. Glass-free polycrystalline aggregates (e.g. ultrapure, fine-grained,
'zero-porosity' forms of alumina, magnesia and beryllia)
6. Polycrystalline aggregates produced by heattreating glasses of special
composition (e.g. glassceramics)
7. Composites (e.g. silicon carbide or carbon filaments in a matrix of
glass or glass-ceramic, magnesiagraphite refractories, concrete).
This approach to classifying ceramics places the necessary emphasis
upon the crystalline and noncrystalline (glassy) attributes of the ceramic
body, the significance of introducing grain boundary surfaces and the
scope for deliberately mixing two phases with very different properties.

3. Alumina
General properties and applications of alumina
Alumina is the most widely used ceramics. The actual content of
alumina, reported as Al2O3, ranges from 85% to 99.9%, depending upon
the demands of the application.
Alumina-based refractories of coarse grain size are used in relatively
massive forms such as slabs, shapes and bricks for furnace construction.
Alumina has a high melting point (20500C) and its heat resistance, or
refractoriness,. alumino silicate refractories (based upon clays) to be
replaced by more costly high-alumina materials and high-purity alumina.

4. Interatomic bonding forces, partly ionic and partly covalent, are
extremely strong and the crystal structure of alumina is physically
stable up to temperatures of 1500-1700°C.
It is used for protective sheaths for temperature-measuring
thermocouples which have to withstand hot and for filters remove
foreign particles and oxide dross from fast-moving streams of molten
aluminium prior to casting.
Large refractory blocks cast from fused alumina are used to line
furnaces for melting glass. However, although alumina is a heat
resisting material with useful chemical stability, it is more sensitive
to thermal shock than silicon carbide and silicon nitride.
 A contributory factor is its relatively high linear coefficient of
thermal expansion (α).
for use as engineering components at lower temperatures, alumina
ceramics usually have a fine grain size (0.5-20 urn) and virtually zero
porosity.

5. The chemical inertness of alumina and its biocompatibility with
human tissue have led to its use for hip prostheses.
insulating body of the spark-ignition plug for petrol-fuelled engines
in the electrical and electronics industries (e.g. substrates for
circuitry, sealed packaging for semiconductor microcircuits).
Unlike metals, there are no 'free' electrons available in the structure
to form a flow of current. The dielectric strength, which is a measure of
the ability of a material to withstand a gradient of electric potential
without breakdown or discharge, is very high.
Even at temperatures approaching 1000°C, when the atoms tend to
become mobile and transport some electrical charge, the resistivity is
still significantly high.
Electrical properties usually benefit when the purity of alumina is
improved.

6. Many mass-produced engineering components take advantage of
the excellent compressive strength, hardness and wear resistance
of alumina, machine jigs and cutting tools, agricultural equipment,
shaft bearings in watches and tape-recording machines,
Emery, the well-known abrasive, is an impure anhydrous form
of alumina which contains as much as 20% SiO2 + Fe2O;
low density (3800 kg m~3) is often advantageous.
However, like most ceramics, alumina
is brittle and should not be subjected to or excessive tensile
stresses during service.

7. Preparation and shaping of alumina powders
The principal raw material for alumina production is bauxite
Al2O(OH)4,

8. Bayer process
In this process, prepared bauxitic ore is digested under pressure in a
hot aqueous solution of sodium hydroxide and then 'seeded' to induce
precipitation of Al(OH)3 crystals, usually referred 'gibbsite'. (The
conditions of time, temperature, agitation, etc. during this stage greatly
influence the quality of the Bayer product.)
Gibbsite is chemically decomposed by heating (calcined) at a
temperature of 1200°C. Bayer calcine, which consists of a-alumina
(>99% Al2O3), is graded according to the nature and amount of
impurities.
The calcine consists of agglomerates of a-alumina crystallites which
can be varied in average size from 0.5 to 100 um by careful selection
of calcining conditions.
Bayer calcine is commonly used by manufacturers to produce high-
purity alumina components as well as numerous varieties of lower-
grade components containing 85-95% Al2O3.

9. The powder can be shaped by a variety of methods (e.g. dry,
isostatic-or hot-pressing, slip- or tape-casting, roll-forming, extrusion,
injection moulding).
Extremely high production rates are often possible; for instance, a
machine using air pressure to compress dry powder isostatically in
flexible rubber moulds ('bags') can produce 300-400 spark plug bodies
per hour.
In some processes, binders are incorporated with the powder; for
instance, a thermoplastic can be hot-mixed with alumina powder to
facilitate injection-moulding and later burned off.
In tape-casting, which produces thin substrates for microelectronic
circuits, alumina powder is suspended in an organic liquid.

10. Densification by sintering
The fragile and porous 'green' shapes are finally fired in kilns
(continuous or intermittent).
Firing is a costly process and, wherever possible, there has been a
natural tendency to reduce the length of the time cycle for small
components.
 Faster rates of cooling after 'soaking‘ at the maximum temperature
have been found to give a finer, more desirable grain structure.
In general, an increase in alumina content from 88% to 99.8% requires
a corresponding increase in firing temperature from 1450°C to 1750°C.
'Harder' firing incurs heavier energy costs and has led to the development
of reactive alumina which has an extremely small particle size (1 um)
and a large specific surface.
'Softer' firing temperatures became possible with this grade of alumina.

11. Silicon nitride
Silicon nitride exists in two crystalline forms (α, ß): both belong to
the hexagonal system. Bonding is predominantly covalent.
SYNTHESIS reaction-bonding process
First, a fragile pre-form of silicon powder (mainly α-Si3N4) is
prepared,
methods(e.g die-pressing, isostatic-pressing, slip-casting, flame-
spraying, polymer-assisted injection-moulding, extrusion).
In the first stage of a reaction-bonding process, this pre-form is
heated in a nitrogen atmosphere and the following chemical reaction
takes place:
3Si + 2N2 = Si3N4
A reticular network of reaction product forms throughout the mass,
bonding the particles together without liquefaction.
Single crystal 'whiskers' of a-silicon nitride also nucleate and grow
into pore space.

12. Reaction is strongly exothermic and close temperature control is
necessary in order to prevent degradation of the silicon.
The resultant nitrided compact is strong enough to withstand
conventional machining.
In the second and final stage of nitridation, the component is heated
in nitrogen at a temperature of 1400°C, forming more silicon nitride in
situ and producing a slight additional change in dimensions of less than
1%.
The final microstructure consists of α-Si3N4 (60-90%), ß-Si3N4 (10-
40%), unreacted silicon and porosity (15-30%). As with most ceramics,
firing is the most costly stage of production.
The final product, reaction-bonded silicon nitride (RBSN), has a bulk
density of 2400-2600 kg m~3. It is strong, hard and has excellent
resistance to wear, thermal shock and attack by many destructive
fluids (molten salts, slags, aluminium, lead, tin, zinc, etc.). Its modulus
of elasticity is high.

13. Hot-pressed forms of silicon nitride
Silicon nitride powder, is mixed with one or more fluxing oxides
(magnesia,yttria, alumina) and compressed at a pressure of 23 MN m~2 in
radio-frequency induction-heated graphite dies at temperatures up to
1850°C for about 1 h.
The thin film of silica that is usually present on silicon nitride particles
combines with the additive(s) and forms a molten phase.
Densification and mass transport then take place at the high
temperature in a typical 'liquid-phase' sintering process.
As this intergranular phase cools, it forms a siliceous glass which
crystallize (devitrify) by slow cooling
This HP route produces a limited amount of second phase as a means
of bonding the refractory particles; however, this bonding phase has
different properties to silicon nitride and can have a weakening effect,
particularly if service temperatures are high.
.

14. Thus, with 3-5% added magnesia, at temperatures below the
softening point of the residual glassy phase, say 1000°C, silicon nitride
behaves as a brittle and stiff material;
at higher temperatures, there is a fairly abrupt loss in strength, as
expressed by modulus of rupture (MoR) values, and (creep) becomes
evident.
For these reasons, controlled modification of the structure of the
inter-granular residual phase is impt.
Yttria has been used as an alternative densifier to magnesia. Its
general effect is to raise the softening point of intergranular phase
significantly. More specifically, it yields crystalline oxynitrides (e.g.
Y2Si3O3N4) which dissolve impurities (e.g. CaO) and form refractory
solid solutions ('mixed crystals').

15. , hot-pressing increases the bulk density and improves strength and corrosion
resistance.
The combination of strength and a low coefficient of thermal expansion in hot-
pressed silicon nitride confer excellent resistance to thermal shock.
Small samples of HPSN are capable of surviving 100 thermal cycles.
Silicon nitride powder, together with a relatively small amount of the oxide
additive(s) that promote liquid-phase sintering, is formed into a compact. This
compact is encapsulated in glass (silica or borosilicate).
The capsule is evacuated at a high temperature, sealed and then HIPed, with gas as the
pressurizing medium, at pressures up to 300 MN m~2 for a period of 1 h.
Finally, the glass envelope is removed from the isotropic HIPSN component by sand-
blasting.
Like HPSN, its microstructure consists of β-Si3N4 (>90%) and a small amount of
intergranular residue (mainly siliceous glass).

16. Automobile industry
One of the major applications of sintered silicon nitride is in automobile industry as a
material for engine parts. Those include, in
diesel engines,
glowplugs for faster start-up;
precombustion chambers (swirl chambers) for lower emissions, faster start-up and
lower noise; turbocharger for reduced engine lag and emissions.
Bearings
Silicon nitride bearings are both full ceramic bearings and ceramic hybrid bearings
with balls in ceramics and races in steel. Silicon nitride ceramics have good shock
resistance compared to other ceramics. Therefore, ball bearings made of silicon nitride
ceramic are used in performance bearings

17. Si3N4 ball bearings are harder than metal,. This results in 80% less
friction, 3 to 10 times longer lifetime, 80% higher speed, 60% less
weight, the ability to operate with lubrication starvation, higher corrosion
resistance and higher operation temperature, as compared to traditional
metal bearings.
Si3N4 balls weigh 79% less than tungsten carbide balls. Si3N4 ball
bearings can be found in high end automotive bearings, industrial
bearings, wind turbines, motorsports, bicycles, rollerblades and
skateboards. Si3N4 bearings are especially useful in applications where
corrosion, electric or magnetic fields prohibit the use of metals.

18. High-temperature material
Silicon nitride has long been used in high-temperature applications.
capable of surviving the severe thermal shock and thermal gradients
generated in hydrogen/oxygen rocket engines.

19. Medical
Silicon nitride has many orthopedic applications. The material is also an
alternative to PEEK (polyether ether ketone) and titanium, which are
used for spinal fusion devices. It is silicon nitride’s hydrophilic,
microtextured surface that contributes to the materials strength,
durability and reliability compared to PEEK and titanium.
Metal working and cutting tools
The first major application of Si3N4 was abrasive and cutting tools.
Bulk, monolithic silicon nitride is used as a material for cutting
tools, due to its hardness, thermal stability, and resistance to wear. It
is especially recommended for high speed machining of cast iron. Hot
hardness, fracture toughness and thermal shock resistance mean that
sintered silicon nitride can cut cast iron, hard steel and nickel based
alloys with surface speeds up to 25 times quicker than those
obtained with conventional materials such as tungsten

20. Electronics
Silicon nitride is often used as an
insulator and chemical barrier in
manufacturing integrated circuits,
to electrically isolate different
structures.
it is a better diffusion barrier
against water molecules and
sodium ions, two major sources of
corrosion and instability in
microelectronics.
.
Example of local silicon oxidation through
a Si3N4 mask

21. Zirconia
Zirconium oxide (ZrO2) has a very high melting point (2680°C),
chemical durability and is hard and strong; because of these properties,
it has long been used for refractory containers and as an abrasive
medium. At temperatures above 1200°C, it becomes electrically
conductive and is used for heating elements in furnaces operating with
oxidizing atmospheres.
Zirconia is chemically unreactive. It is slowly attacked by concentrated
hydrofluoric acid and sulfuric acid. When heated with carbon, it
converts to zirconium carbide. When heated with carbon in the
presence of chlorine, it converts to zirconium tetrachloride. This
conversion is the basis for the purification of zirconium metal and is
called the Kroll process.

22. Zirconia-based materials have similar thermal expansion
characteristics to metallic alloys and can be usefully integrated with
metallic components in heat engines
In addition to these established applications, reducing notch-
sensitivity and raising fracture toughness values into the 15-20 MN
m~3/2 band, thus providing a new class of toughened ceramics.
an alternative to increasing the toughness of a ceramic by either (1)
adding filaments or (2) introducing microcracks that will blunt the tip
of a propagating crack.

23. Zirconia exists in three crystalline forms; their interrelation, in order
of decreasing temperature, is as follows:
Melt Cubic Tetragonal
2680°C 2370°C
1150°C
Monoclinic

24. .
The volume expansion caused by the cubic to tetragonal to
monoclinic transformation induces large stresses, and these stresses
cause ZrO2 to crack upon cooling from high temperatures.
When the zirconia is blended with some other oxides, the tetragonal
and/or cubic phases are stabilized.
Effective dopants include magnesium oxide (MgO), yttrium oxide
(Y2O3, yttria), calcium oxide (CaO), and cerium(III) oxide (Ce2O3)
Zirconia is often more useful in its phase 'stabilized' state.
Upon heating, zirconia undergoes disruptive phase changes. By
adding small percentages of yttria, these phase changes are
eliminated, and the resulting material has superior thermal,
mechanical, and electrical properties.

25. TRANSFORMATION TOUGHENING
In some cases, the tetragonal phase can be metastable. If sufficient
quantities of the metastable tetragonal phase is present, then an applied
stress, magnified by the stress concentration at a crack tip, can cause
the tetragonal phase to convert to monoclinic, with the associated
volume expansion.
This phase transformation can then put the crack into compression,
retarding its growth, and enhancing the fracture toughness.
 This mechanism is known as transformation toughening, and
significantly extends the reliability and lifetime of products made
with stabilized zirconia

26. In the event of a propagating crack passing into or near metastable
regions, the concentrated stress field at the crack tip enables the t-
crystals of Zirconia rich solid solution transform into stable but less
dense m-zirconia. The transformation is martensitic behavior in
character
The related volumetric expansion (3-5%) tends to close the crack
and relive stresses at t its tip.
After cooling to room temperature, the structure is essentially
single-phase, consisting of very fine grains of t-ZrO2 which make this
material several times stronger than other types of zirconia-toughened
ceramics.

28. Schematic phase diagram for ZrO2 –Y2O3 system: all phases depicted are solid solutions. TZP
=tetragonal zirconia polycrystal, PSZ = partially-stabilized zirconia, CSZ = cubic-stabilized
zirconia.

29. Three zirconia-based types of ceramic have been superimposed upon
the diagram; CSZ, TZP and PSZ.
The term CSZ (Cubic stabilized zirconia) refers to material with a
fully-stabilized cubic (not tetragonal) crystal structure which cannot
take advantage of the toughening transformation. It is used for furnace
refractories and crucibles.
The version known as tetragonal zirconia polycrystal (TZP)
contains the least amount of oxide additive (e.g. 2-4 mol% Y2O3) and
is produced in a fine-grained form by sintering and densifying ultra-
fine powder in the temperature range 1350-1500°C;.
- After cooling to room temperature, the structure is essentially
single-phase, consisting of very fine grains (0.2—1µm) of t-ZrO2
which make this material several times stronger than other types of
zirconia-toughened ceramics

30. In partially-stabilized zirconia (PSZ), small t-crystals are dispersed
as a precipitate throughout a matrix of coarser cubic grains.
Zirconia is mixed with 8-10 mol% additive (MgO, CaO or Y2O3) and
heat treated in two stages. Sintering in the temperature range 1650-
1850°C produces a parent solid solution with a cubic structure which is
then modified by heating in the range 1100-1450°C.
 This second treatment induces a precipitation of coherent t-crystals
(~200 nm in size) within the c-grains. The morphology of the precipitate
depends upon the nature of the added solute (e.g. ZrO2-MgO, and ZrO2-
Y2O3 solid solutions produce lenticular, cuboid and ZrO2-CaO platey
crystals, respectively). The average size of precipitate crystals is
determined by the conditions of temperature and time adopted
during heat-treatment in the crucial 't + c' field of the phase diagram.

31. If the temperature of a zirconia-toughened material is raised to 900-
1000°C, which is close to the t-m transition temperature, the toughening
mechanism tends to become ineffective.

32. Silicon carbide
Structure and properties of silicon carbide
The two principal structural forms of this synthetic compound are α-
SiC (non-cubic; hexagonal, rhombohedral) and β-SiC (cubic). The cubic
β -form begins to transform to α -SiC when the temperature is raised
above 2100C.
).
 Tetrahedral grouping of carbon atoms around a central silicon . This
covalent bonding of two tetravalent elements gives exceptional strength
and hardness and a very high melting point (>2700°C).

33. Despite its carbon content, silicon carbide offers useful resistance to
oxidation. At elevated temperatures, a thin impervious layer of silica
(cristobalite) forms on the grains of carbide.
On cooling, the α/β transition occurs in cristobalite and it can crack,
allowing ingress of oxygen.
 Above a temperature of 1500C, the silica layer is no longer
protective and the carbide degrades, forming SiO and CO.
` Production of silicon carbide powder and products
Silicon carbide is a relatively costly material because its production
is energy-intensive.
The Acheson carbothermic process, which is the principal source of
commercial- quality silicon carbide, requires 6-12 kWh per kg of
silicon carbide.
In this unique process, a charge of pure silica sand (quartz),
petroleum coke (or anthracite coal), sodium chloride and sawdust is
packed around a 15 m long graphite conductor.

34. Silicon carbide shapes for general refractory applications are produced
by firing a mixture of SiC grains and clay at a temperature of 1500C.
The resultant bond forms mullite and a glassy phase, absorbing the thin
layer of silica which encases the grains.
Other bonding media include ethyl silicate, silica and silicon nitride.
The latter is developed in situ by firing a SiC/Si compact in a nitrogen
atmosphere.
This particular bond is strong at high temperatures and helps to
improve thermal shock resistance.
The methods available for forming silicon carbide powders include
dry-pressing, HIPing, slip-casting, extrusion and injection-moulding

35. Hot pressing
The hot-pressing method for producing α-SiC blanks of high density
A small amount of additive (boron carbide (B4C) or a mixture of
alumina and aluminium) plays a key role while the carbide grains are
being heated (>2000°C) and compressed in induction-heated graphite
dies.
in HP SiC (and S SiC) that the boron encourages grain
boundary/surface diffusion and that the carbon breaks down the silica
layers which contaminate grain surfaces.
These additives leave an intergranular residue which determines the
high-temperature service ceiling.
The hot-pressed blanks usually require mechanical finishing (e.g.
diamond machining).
Production of complex shapes by hot-pressing is therefore
expensive.

36. The pressureless-sintering route (for S SiC) uses extremely fine
silicon carbide powders of low oxygen content.
Again, an additive is necessary (B4C or aluminium -f- carbon) in
order to promote densification.
The mixture is cold-pressed and then fired at approximately 2000°C
in an inert atmosphere
The REFEL process for producing siliconized silicon carbide (Si SiC)
was developed by the UKAEA and is an example of reaction-sintering
(reaction bonding).
A mixture of α-SiC, graphite and a plasticizing binder is compacted
and shaped by extrusion, pressing, etc.
This 'green' compact can be machined, after which the binder is
removed by heating in an oven.
The pre-form is then immersed in molten silicon under vacuum at a
temperature of 1700C.
Graphitic carbon and silicon react to form a strong intergranular
bond of β-SiC..

37. A heavy electric current is passed through the conductor and develops
a temperature in excess of 2600°C.
The salt converts impurities into volatile chlorides and the sawdust
provides connected porosity within the charge, allowing gases/vapours
to escape.
The essential reaction is:
SiO2 + 3C —> α-SiC + 2CO
The reaction product form around the electrode is ground and graded
according to the purity and size.
The temperatures involved are generally much lower than those
developed in the Acheson process; consequently, they yield the cubic β-
form of silicon carbide.
Chemical vapour deposition (CVD) has been used to produce
filaments and ultra-fine powders of β-SiC.

38. A substantial amount of 'excess‘ silicon (say, 8-12%) remains in the
structure; the maximum operating temperature is thus set by the melting
point of silicon (i.e. 1400C).
Beyond this temperature there is a rapid fall in strength.
Ideally, neither unreacted graphite nor unfilled voids should be
present in the final structure.
The dimensional changes associated with the HP process are small
and close tolerances can be achieved; the shrinkage of 1-2% is largely
due to the bake-out of the binder

39. Applications of silicon carbide
It is used for metal-machining, refractories and heating elements in
furnaces, chemical plant, heat exchangers, heat engines, etc.
The extreme hardness (2500-2800 kgf mm~2; Knoop indenter) of its
particles and their ability to retain their cutting edges at high contact
temperatures (1000C) quickly established silicon carbide grits as
important grinding media.
The comparatively high cost is justified by their outstanding high
temperature strength, chemical inertness, abrasion resistance and high
thermal conductivity.
In the New Jersey process for producing high-purity zinc, SiC is used
for components such as distillation retorts, trays and the rotating
condensation impellers which have to withstand the action of
molten zinc and zinc vapour.

40. In iron-making, silicon carbide has been used to line the water-
cooled bosh and stack zones of iron-smelting blast furnaces, where
its high thermal conductivity and abrasion-resistance are very relevant.
However, it can be attacked by certain molten slags, particularly those
rich in iron oxides.
Silicon carbide also has a key role in recent designs of radiant tube
heaters in gas-fired furnaces.
Silicon carbide is electrically conductive and care has to be taken
when it is used as a refractory in the structure of electrometallurgical
plant.
However, the combination of electrical conductivity and
refractoriness offers special advantages. For example, silicon
carbide resistor elements have been used since about 1930 in indirect
resistance-heated furnaces throughout industry (e.g. Globars).
 These elements act as energy conversion devices, heating the
furnace charge by radiation and convection