Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
such as oxides, carbides and nitrides in the pure crystalline state with
very low, sometimes negligible porosity. In comparison with the traditional
ceramics described above, they contain smaller microcracks, so their
strength and toughness is improved, giving properties competitive with
metals for applications such as cutting tools, dies and engine parts.
4.3.1 Processing of modern ceramics
Most ceramic fabrication processes begin with finely ground powder. Oxides
such as alumina (Al 2 O 3 ), magnesia (MgO) and zirconia (ZrO 2 ) occur naturally,
but have to be purified by chemical processing before use as engineering
ceramics. Silicon carbide (SiC) is manufactured by reacting SiO 2 sand with
coke (C) at high temperature and silicon nitride is also synthesized industrially,
usually by reacting silicon powder with nitrogen at 1250 to 1400°C. Before
consolidation, the powders are milled and graded into size (diameter of the
order of 1 µm). They are then blended so that the subsequent shaping operation
leads to material of optimum properties. The next stage is one of shapeforming,
for which there are a number of possible processes.
Pressing requires the powder to be premixed with suitable organic binders
and lubricants and preconsolidated so that it is free flowing. It is then compacted
in a die to form small shapes such as crucibles and insulating ceramics for
electrical devices.
Slip casting is effected by suspending the ceramic particles in a liquid
(usually water) and pouring the mixture into a porous mould (usually plaster)
which removes the liquid and leaves a particulate compact in the mould. An
organic binder is usually present in order that the casting has sufficient
strength to permit its removal from the mould before the firing operation.
Plastic forming is possible if sufficient (25 to 50 vol%) organic additive
is present to achieve adequate plasticity. Injection moulding and extrusion
may then be employed.
Strong, useful ceramic products are produced after the final densification
by sintering at high temperature. Sintering brings about the removal of pores
between the starting particles (accompanied by shrinkage of the component),
combined with strong bonding between the adjacent particles. The primary
mechanisms for transport are atomic diffusion and viscous flow. In some
cases, hot die pressing is employed, whereby pressure and temperature are
applied simultaneously to accelerate the kinetics of densification. Only a
limited number of shapes can be produced by this technique, however.
The thermodynamic driving force for sintering is the reduction in surface
energy (γ) by the elimination of voids. A spherical void of radius 2r will
experience a closure pressure P given by:
P = –2γ /r